WO2014012141A1

WO2014012141A1 - Measurement system for radiation dose

Info

Publication number: WO2014012141A1
Application number: PCT/AU2013/000793
Authority: WO
Inventors: Natalka Suchowerska; David Robert MACKENZIE; Paul Liu; Pourandokht NASERI
Original assignee: The University Of Sydney
Priority date: 2012-07-17
Filing date: 2013-07-17
Publication date: 2014-01-23

Abstract

There is provided a system for compensating for induced Cerenkov light. The system comprises a first optical fibre (7) that conveys a signal from a source end (12) of the first optical fibre to a measurement end of the first optical fibre and a second optical fibre (18) having a blind end (14) adjacent the source end of the first optical fibre. The system also comprises a measurement system (101a, 101b; 101c) that measures a signal output from the first optical fibre (7) and a partner output from the second optical fibre (18), the partner output indicative of Cerenkov light induced in the second optical fibre. The system further comprises a data processor (105) for subtracting an inferred amount of Cerenkov light from the signal output, wherein the inferred amount is dependent on the measured partner output.

Description

Measurement system for radiation dose

Field of the invention

The present invention relates to dosimeters for measuring the dose from radiation fields and to methods of manufacturing dosimeters. In particular arrangements the invention relates to arrangements for mitigating background noise in dosimeter measurements.

Background of the invention

While a range of devices suitable for detecting radiation fields are known, few if any, are suited to use in a dosimeter that satisfies the demands of contemporary radiation therapy techniques, which employ small modulated fields with high dose gradients. Small modulated fields escalate the dose to the tumour while preserving surrounding healthy tissue.

In order to accommodate contemporary therapy techniques, there is a need for a dosimeter that is effectively water equivalent in its interactions with ionising radiation and can accurately verify the radiation treatment therapy, accommodate time dependent therapy techniques such as intensity modulated radiation therapy (IMRT), and also accommodate precision therapy techniques such as stereotactic radiosurgery (SRS). The dosimeter should be able to provide high spatial resolution, while retaining the ability to integrate the total dose over the whole treatment period. The dosimeter should also provide a frequently updated reading of the current radiation dose.

A further requirement for brachytherapy applications is that the dosimeter should be of very small size. A still further requirement is for the dosimeter to be relatively robust, an advantage for any application, but again particularly so if the application requires insertion into patient cavities, for example the urethra.

The emerging trend in radiation therapy is towards better spatial precision and dosimetric accuracy. A problem is that the development of dose measuring devices (dosimeters), necessary to ensure the prescribed treatment is actually delivered, have not kept pace. There is perceived to be a widening disconnect between radiation delivery capability and radiation measurement capability. Such discrepancies challenge the aims of high precision targeting of tumours with small fields, as in SRS, or with composite small fields as in IMRT. Scintillation dosimeters with a fibre optic readout have a number of characteristics that provide advantages over the alternatives for use with radiation therapy techniques. The scintillator of a fibre optic dosimeter, consisting of a small water-equivalent plastic material, avoids disadvantages associated with energy dependence or perturbation of the radiation beam, which occurs with alternative dosimeters. The impact of detector density and the advantage of water-equivalent detectors are discussed, for example in Scott et al (2012).

United States patent number 5,006,714 describes a scintillator dosimetric probe. A scintillator is positioned in an ionising radiation beam, which creates light output. The light is conducted from the scintillator through a light pipe to a photomultiplier tube, which converts the light into an electric current. The electric current produced by the photomultiplier tube is proportional to the radiation dose-rate incident upon the scintillator. Through a measurement of the electric current, the radiation dose rate may then be displayed or recorded.

An identified problem with fibre optic dosimeters is the generation of Cerenkov (or Cherenkov) radiation in, and transmission of the Cerenkov radiation along, the light pipe. Cerenkov radiation may be generated when relativistic charged particles pass through a medium at speeds greater than the local speed of light. The Cerenkov background presents a problem because it is highly dependent on the angle of the beam relative to the axis of an optical fibre. The intensity of the Cerenkov radiation is dependent on factors other than the radiation dose at the scintillator and therefore the Cerenkov radiation represents noise in the measurement signal. There have been several methods proposed to manage Cerenkov radiation produced in the optical fibre. For example, a neighbouring optical fibre that traverses almost the same path through the radiation field can be used to measure the Cerenkov background (Beddar et al 1992). The delay in scintillation emission can be used to discriminate it from the prompt Cerenkov radiation (Clift et al 2002). Cerenkov radiation can be distinguished spectrally (Deboer et al 1993, Fontbonne et al 2002, Frelin et al 2005) and can be removed with signal processing. However, this implementation requires a colour CCD camera located in the linear accelerator room that is protected by shielding (Lacroix et al 2008).

Co-assigned patent application WO 2007/085060 "Fibre optic dosimeter", filed on 30 January 2007, describes a scintillation dosimeter with a light pipe having a hollow core with a light reflective material about the periphery of the core. The arrangement addresses the Cerenkov problem by limiting the generation of Cerenkov light in the dosimeter.

It is an object of the present invention to provide a dosimeter that satisfies one or more of the aforementioned needs and/or overcomes or alleviates at least some of the problems of existing dosimeters, or at least one that provides the public with a useful alternative.

It is a further or alternate object of the present invention to provide a method of manufacture of a fibre optic dosimeter that results in an improved dosimeter or at least one that provides a useful alternative.

Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in Australia or any other jurisdiction or that this prior art could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art.

Summary of the invention

According to a first aspect of the invention there is provided a dosimeter for radiation fields, comprising a scintillator; a light guide having an input end in optical communication with the scintillator, the light guide comprising a plurality of elongate planar surfaces surrounding a hollow core, the planar surfaces reflecting light within the hollow core; a signal optical fibre having a first end in optical communication with an output end of the light guide; a signal detector to detect light output from a second end of the signal optical fibre and to provide a signal output indicative of the intensity of a first received light signal; a partner optical fibre having a blind end located adjacent the first end of the signal optical fibre; and a partner detector to detect light output from an output end of the partner optical fibre and to provide a partner output indicative of the intensity of a second received light signal.

The partner optical fibre and the signal optical fibre may be twisted together, e.g., one around the other, to form a double helix.

A periodicity of the double helix may vary between a first end and a second end of the signal optical fibre. The signal detector and the partner detector may be a shared detector with the dosimeter further comprises a switch for switching an input to the shared detector between the second end of the signal optical fibre and the output end of the partner optical fibre so that, in use, the shared detector detects light from the signal optical fibre and the partner optical fibre. The dosimeter may further comprise a data processor to adjust the signal output associated with the signal optical fibre based on the partner output associated with the partner optical fibre.

According to a further aspect of the invention there is provided a dosimeter array comprising at least two dosimeters as defined above. According to yet a further aspect of the invention there is provided a system for compensating for induced Cerenkov light, the system comprising a first optical fibre that conveys a signal from a source end of the first optical fibre to a measurement end of the first optical fibre; a second optical fibre having a blind end adjacent the source end of the first optical fibre; a measurement system that measures a signal output from the first optical fibre and a partner output from the second optical fibre, the partner output being indicative of Cerenkov light induced in the second optical fibre; and a data processor for subtracting an inferred amount of Cerenkov light from the signal output, wherein the inferred amount is dependent on the measured partner output.

The second optical fibre may be twisted with the first optical fibre to form a double helix A periodicity of the double helix may vary between the source end and the measurement end of the first optical fibre.

According to a further aspect of the invention there is provided a method of determining a relative sensitivity of the first optical fibre and the second optical fibre in the system as defined above, the method comprising: measuring the signal output and the partner output during a first exposure of the system to a first field intensity with a known measured dose rate; measuring the signal and partner output during a second exposure of the system to a second field intensity with a second known dose rate, wherein the first and second field intensities allow for different dose rates; determining at least one parameter characterising a relationship between the signal output, the partner output and the measured dose rates; and storing the at least one parameter for use in compensating for induced Cerenkov light.

The second known dose rate may be determined by using known beam treatment planning parameters.

The method may further comprise measuring the signal output and the partner output during an exposure of the system where the measurement system is sufficiently shielded or removed from the signal output to assume that the first known dose rate is zero.

As used herein, except where the context requires otherwise, the term "comprise" and variations of the term, such as "comprising", "comprises" and "comprised", are not intended to exclude further additives, components, integers or steps. Further aspects of the present invention and further embodiments of the aspects described in the preceding paragraphs will become apparent from the following description, given by way of example and with reference to the accompanying drawings.

Brief description of the drawings

Fig. 1A is a schematic illustration of a scintillation dosimeter that includes a light guide between a scintillator and an optical fibre to a photodetector;

Fig. IB shows a cross-section of an example of the light-guide of Fig. 1A through section

A-A;

Fig. IB shows a cross-section of an example of the light-guide of Fig. 1 A through section A-A with a light-reflective layer located around a hollow core; Fig. 2A is a schematic cross section of a linear array of scintillation dosimeters each including a light guide between a scintillator and an optical fibre; Fig. 2B is a schematic illustrating alternative versions of the linear array of Fig. 2A having shutters to decouple the scintillators and the light guides;

Fig. 3 A is a perspective view of structural features of a linear array of dosimeters used to describe a method of manufacturing the dosimeters; Fig. 3B is a schematic cross section of an array of dosimeters made using the method described in relation to the linear array of dosimeters of Fig. 3 A;

Fig. 4A is a cross section of a linear array of scintillation dosimeters to illustrate an alternative method of manufacturing a linear array of dosimeters;

Fig. 4B is a cross section of yet another alternative of a linear array of scintillation dosimeters to illustrate a further method of manufacturing a linear array of dosimeters;

Fig. 5A shows a two-dimensional array of dosimeters;

Fig. 5B shows a shutter for use with the two-dimensional array of Fig. 5 A;

Fig. 5C shows a housing accommodating the array and shutter of Figs. 5A and 5B;

Fig. 6A shows a three-dimensional array of dosimeters; Fig. 6B shows the light guides associated with a first row of scintillators in the array of

Fig. 6A;

Fig. 6C shows the light guides associated with a second row of scintillators in the array of Fig. 6A in the same plane as the first row of scintillators;

Fig. 6D shows the light guides associated with a third row of scintillators in the array of Fig. 6A, in a plane orthogonal to the plane of the first and second rows;

Fig. 6E is a schematic illustration of a bend provided in the light guides associated with the third row of scintillators;

Fig. 7 A shows an array of light guides mounted eccentrically in a phantom;

Fig. 7B is an end view of array and phantom of Fig 7A illustrating the eccentric configuration; Fig. 7C is a perspective view of the array and phantom of Fig. 7 A

Figure 8A is a graph that compares the performance of light guides having a circular cross section with light guides as described with reference to Figure 3A;

Figure 8B is a graph showing readings obtained with a linear array of dosimeters and a 5mm radiation beam;

Figure 8C shows a set of results for different beam field sizes measured using a linear array of dosimeters formed by the method illustrated in Figure 3A and 3B;

Figure 8D shows a depth dose curve measured using a linear array of dosimeters formed by the method illustrated in Figure 3A and 3B; Figure 9A illustrates the experimental configuration for Figures 8B and 8C; and

Figure 9B illustrates the experimental configuration for Figure 8D.

Figure 10 shows schematically a dosimeter including a twisted fibre optic pair in which one of the twisted fibres is used to measure the Cerenkov background;

Figures 1 1A and 1 IB show schematically the measurement of signals in each of the fibres in the twisted pair of Figure 10;

Figure 1 1C shows an electronic data processing system that may be used to process data generated in the systems of Figures 1 1 A and B;

Figure 1 ID is a flow chart of a method for subtracting Cerenkov radiation from a measured radiation signal; Figure 12 is a schematic diagram of the twisted pair of Figure 10 used in an array of dosimeters;

Figure 13 illustrates schematically a method of producing the twisted pair of Figure 10; and

Figures 14A-E show experimental results obtained using the twisted pair of Figure 10. Detailed description of the embodiments

Figure 1 is a schematic view of a first embodiment of a dosimeter generally referenced by arrow 100.

The dosimeter 100 includes a scintillator 1 in communication with a first end of a light guide 2. Suitable scintillators for use in the dosimeter include anthracene-doped Polyvinyl Toluene (PVT), Polystyrene (PS) or Poly(methyl)methacrylate (PMMA) based scintillators, or scintillating fibres with a polystyrene-based core and a poly(methyl)methacrylate-based cladding, both available from Saint-Gobain of France and elsewhere. In one arrangement the scintillator 1 has the dimensions 2x2x4 mm³ and is configured to fit into an input end of the light guide 2. Alternatively, a holder may be provided to position the scintillator adjacent to the input end of the light guide 2.

The light guide 2 has a hollow air-core surrounded by a plurality of elongate planar surfaces forming a channel. At the end of the light guide opposite the scintillator 1, an interface 3 couples the light guide to an optical fibre 7 (also referred to as a fibre optic line) such that light travelling in the light guide 2 passes into the optical fibre. The optical fibre 7 is connected to a photodetector 8, which may be any suitable detector, including a photomultiplier or photodiode device. Suitable devices and techniques for converting a light signal to an electronic signal and outputting an indication of the intensity of the light signal are well known and will therefore not be described in detail herein. In one arrangement the photodetector 8 is an array of photomultiplier detectors available from Hamamatsu and having 32 photocathodes in a single glass envelope. The signals from the photomultiplier array may be read, for example by a 32 channel signal processing unit available from Vertilon. The signals may be sampled and integrated to provide an accumulated charge in coulombs, with an integration time of 95 ms, for example. The optical fibre 7 is preferably of a sufficient length, during testing, that the photodetector 8 may be located outside a shielded bunker within which a main radiation beam is activated. In use, e.g. in brachytherapy applications, the optical fibre 7 should be of sufficient length so that the photodetector 8 is located outside the patient's body.

The optical fibre 7 may, for example, use a polymethyl methacrylate (PMMA) optical fibre with a 1 mm core diameter held in place 15 mm inside the light guide 2. The length of the light guide 2 is 20 cm in one arrangement. Consequently, the distance between the end of the optical fibre 7 and the scintillator 1 may be at least 185 mm. This distance is considered sufficient to ensure that the end of the fibre optic is located outside the main beam of radiation that in use causes the scintillator 1 to emit light. This arrangement limits the generation of Cerenkov light.

The length of the light guide is preferably in the range of 5 cm to 100 cm and more preferably in the range of 5 to 20 cm. There is a balance between the need to distance the optical fibre 7 from the main beam and the need to limit attenuation in the light guide 2.

In one arrangement the light guide 2 has a square cross-section, as illustrated schematically in Figure 2, in which four elongate planar surfaces 4a, 4b, 4c and 4d surround a hollow core 5. Uncoated black PMMA may be used, with cast surfaces of low surface roughness and an adequate flatness over a distance of tens of centimetres. Using black PMMA limits interference from stray light. PMMA is available for example from B & M Plastics Pty Ltd of Australia. The cross-section of the light guide 2 may also have other shapes, including a rectangular shape. The cross-section is not limited to having four sides. Other arrangements having a plurality of linear sections may also be used, for example an octagonal cross-section.

Figure 1C shows an arrangement in which the interior surfaces surrounding the hollow core 5 of the light guide 2 are covered in a light-reflecting layer 6. The layer may be a metallised coating, for example formed by passing a silver nitrate solution through the light guide so that the silver precipitates out onto the inner surfaces. In an alternative embodiment, the metallised layer may be replaced by another reflective material or structure, for example a coating of dielectric layers or a microstructure array to create internal reflections.

A layer of silver having a thickness of approximately 1 micron or more may be suitable for most applications using the light-reflecting layer 6. Thicknesses as low as approximately 0.1 micrometers may be used, whereas forming layers at thicknesses above approximately 2 micrometres may create difficulties in maintaining a smooth surface, resulting in excessive losses. The actual thickness required will depend on the manufacturing technique used and the requirements specification for the dosimeter. A reflector, for example a metallised film, may be provided over the distal end of the scintillator 1 from the light guide 2. The reflector redirects light that would otherwise escape from the end of the scintillator 1 and therefore increases the amount of light captured by the guide 2. Linear array of dosimeters

The dosimeter 100 may be used in a modular configuration to provide an array of dosimeters each having an air-cored light guide providing a channel between a scintillator and a respective optical fibre.

An example of a dosimeter array 200 with three dosimeters is illustrated in Figure 2A. It will be understood that arrays with different numbers of dosimeters may also be provided. A light guide unit 210 is shown in cross section. Elongate planar members 22a, 22b, 22c and 22d are positioned in parallel to define three light guides 20a, 20b and 20c. The planar members 22a- 22d are formed from a material having a refractive index greater than that of the medium filling the light guides 20a-c (in this case air) such that in use light is guided along the channels from a scintillator at an input end of each guide 20a-c to an optical fibre 24a, 24b, 24c located at an output end of the light guides 20a, 20b, 20c respectively. The optical fibres 24a, 24b, 24c convey the light to an array of photodetectors to measure the light in each dosimeter.

Additional planar surfaces (not shown in Fig. 2A) enclose each light guide. Uncoated black PMMA may be used for such planar surfaces, which material limits interference from stray light.

The scintillators at the input end of each light guide may be rectangular blocks of PVT. Alternatively, a scintillator unit 220 may be used as shown in Figs. 2 A and 2B. The unit 220 may be formed from PMMA or another suitable water-equivalent material. Dimples 26a, 26b, 26c are formed in a surface of the scintillator unit that in use is positioned adjacent to the light guide unit 210 such that dimples 26a, 26b, 26c line up with light guides 20a, 20b, 20c respectively. The dimples may, for example be milled, or the desired shape may be injection moulded. A paintable scintillator material is located in each dimple to form part of a scintillator at the end of each light guide. A reflector, for example a metallised film, may be provided in the scintillator unit 220 to redirect light that would otherwise escape away from the light guides 20a-c.

Each dosimeter in the array 200 may be individually calibrated. In one approach the array 200 may be irradiated by a beam normal to the longitudinal axis of the light guides 20a-c, such that the same amount of radiation is incident on each scintillator. The corresponding outputs are measured at the photodetectors, enabling calibration of the individual dosimeters in the array 200.

Decoupling the scintillators

If the scintillators are decoupled from the light guides 20a-c it is possible to set a zero level for each of the dosimeters.

Three means for decoupling the scintillators are shown schematically in Fig. 2B. In one arrangement the scintillator unit 220 and light guide unit 210 may be displaced relative to one another such that the scintillators no longer line up with the light guides 20a-c.

In another arrangement a shutter 30 is provided between the scintillator unit 220 and the light guide unit 210. The shutter 30 has at least two positions. In a decoupling position the shutter 30 obstructs the light guides 20a-c such that light emitted by the scintillators does not enter the light guides. In a coupling position, the shutter 30 does not obstruct an optical path between the scintillators and the respective light guides. For example, the shutter 30 may have channels corresponding to each dosimeter such that in the coupling position the channels line up with the scintillators and light guides. An example of such an arrangement is shown in Figure 5B.

In another arrangement a shutter 32 is provided at the output end of the light guide unit 210, ie between the light guides 20a-c and the optical fibres 24a-c. The shutter 32 has at least two positions. In a decoupling position the shutter 32 obstructs the light guides 20a-c such that light emitted by the scintillators does not enter the optical fibres 24a-c. In a coupling position, the shutter 32 does not obstruct an optical path between the optical fibres and the respective light guides. For example, the shutter may have channels corresponding to each dosimeter such that in the coupling position the channels line up with the light guides 20a-c. The optical fibres 24a-c may be held by the shutter 32 so that the optical fibres move with the shutter 32 between the coupling and decoupling positions.

In another arrangement shutters 30 and 32 may both be present in the dosimeter array.

In use, the decoupling means may be operated to decouple the light guide unit 210 from the photodetector 8. The main radiation beam may then be activated and the photodetector array monitored. This may be used to provide a zero reading for each of the dosimeters in the array.

Forming a linear array of dosimeters

A method of manufacturing an array of dosimeters is now described in accordance with the array of dosimeters of Figure 3 A. A sequence of parallel grooves (e.g. 47a) is formed in a sheet of black PMMA 44. In one arrangement the sheet 44 is 6mm thick and 20 cm long and each groove is 2mm wide, extending along the full length of the sheet 44. A strip of PMMA (e.g. 42a) 2mm thick is positioned in each of the grooves, providing a number of channels that have a centre to centre spacing of 4mm. As described above, scintillators (e.g. 41a) may be positioned at an input end of the channels. Another sheet of PMMA (not shown in Figure 3 A) has a complementary sequence of grooves and is positioned so that the strips of PMMA fit into respective grooves of the upper sheet of PMMA. This arrangement forms an array of light guides.

The grooves may be formed by milling or by alternative methods such as injection moulding the sheets 44 with a pattern of grooves therein. An array 500 of seven dosimeters is shown in schematic cross section in Figure 3B.

Upper and lower sheets 54 and 56 each have eight matching grooves. In one arrangement the sheets 54, 56 are each 6mm thick. Strips of PMMA (e.g. 52 a-c) are located in the grooves to form seven light guides, each associated with a scintillator (e.g. 51 a-c). The arrangement of sheets 54, 56 and strips of PMMA form the light guide unit 55. Side grooves 53a and 53b at each end of the light guide unit 55 do not have scintillators.

This may provide a mechanism for measuring ambient noise that is not associated with the scintillation. The light guide unit may be positioned in a case 50 made, for example, from 6mm thick PMMA. The case assists in sealing the light guides from stray light. In the depicted example the width of the array module is 34 mm and the width of the case is 48.5 mm. The height of the array module is 14mm. Another method of manufacturing a linear array of dosimeters 300 is described in accordance with the linear dosimeter array illustrated in Figure 4A. The example shows a dosimeter array 300 having three light guides 302, 304, 306, but it will be appreciated that different quantities of light guides may be formed in the array 300.

Three parallel V-shaped grooves 312, 314 and 316 are formed in a first surface of a sheet 308 of suitable material such as perspex or PMMA. Figure 4A shows a cross-section through sheet 308. The grooves may extend along the full length of the sheet 308. The length of the grooves may be in the range 10 cm- 100 cm, or in the range 15 cm - 30 cm. The angle a at the apex of each groove may be 90 degrees.

The grooves are separated by a section of the first surface of sheet 308. For example, grooves 312 and 314 are separated from one another by flat section 303 and grooves 316 and 314 are separated from one another by flat section 307.

The exposed surfaces of the grooves 312, 314, 316 may be milled to provide a smooth reflective surface to reflect light in the light guides 302, 304, 306. Alternatively, the configuration of sheet 308 having grooves 312, 314, 316 may -be formed by press moulding a sheet with the desired structure. Furthermore, the surfaces of the grooves facing the light guides may be coated in a reflective material.

A complementary sheet 310 is formed in a similar fashion to sheet 308. Three V-shaped grooves 320, 322, 324 are formed in the lower sheet 310. Flat surface 305 separates grooves 320 and 322. Flat surface 309 separates grooves 324 and 322. Grooves 320, 322 and 324 have a shape and configuration that corresponds to the shape and configuration of grooves 312, 314, 316. When sheets 308 and 310 are aligned with one another, the matching grooves form light guides 302, 304, 306, which are hollow channels surrounded by four elongate planar surfaces. The matching flat sections between grooves (for example the pair of surfaces 303, 305 or the pair 307, 309) separate the light guides. In one arrangement the centre to centre spacing of the light guides is 2mm. In the illustrated example the light guides have a square cross section. In alternative arrangements the grooves are not V-shaped and the resulting cross section is not square. For example, the angle a may vary and the groove may not be symmetrical in cross section. The grooves may have more than two sides. For example, the grooves may have three sides each such that the resulting light guide has a hexagonal cross-section when the two sheets are assembled.

Sheets 308, 310 may be held together in various ways, for example using adhesives or mechanical fasteners, for example clamps or screws.

Scintillators may be positioned at one end of each light guide 302, 304, 306 and optical fibres may be introduced at the opposite end of each light guide, as illustrated for example in Figure 2A.

A further method of manufacturing a linear array of dosimeters 350 is illustrated in accordance with the linear array shown in Figure 4B. The example shows a dosimeter 350 having three light guides 352, 354, 356, but it will be appreciated that different quantities of light guides may be formed in the array 300.

Three parallel V-shaped grooves 362, 364 and 366 are formed in a first surface of a sheet 358 of suitable material such as perspex or PMMA. The grooves may extend along the full length of the sheet 358. The length of the grooves may be in the range lOcm-lOOcm, or in the range 15 cm - 30 cm. The angle at the apex of each groove may be 90 degrees. The grooves 362, 364, 366 are separated from one another by relatively shallow recesses or grooves 380, 382 that are also formed in the first surface of sheet 358. Grooves 380, 382 have a smaller V-shaped cross section than grooves 362, 364, 366. All the grooves run in parallel along the full length of sheet 358. Grooves 362 and 364 are separated from one another by groove 380 and grooves 366 and 364 are separated from one another by groove 382. The exposed surfaces of the grooves 362, 364, 366 may be milled to provide a smooth reflective surface to reflect light in the light guides 352, 354, 356. Alternatively, the configuration of sheet 358 having grooves 362-366, 380, 382 may be formed by press moulding a sheet with the desired structure. A complementary sheet 360 is formed with three V-shaped grooves 370, 372, 374 that are configured to align with grooves 362, 364 and 366 respectively. However, grooves 370, 372 and 374 are deeper than grooves 362, 364, 366 and are configured such that when sheets 360 and 358 are aligned, an apex between adjacent grooves in sheet 360 (for example apex 390) fits into a corresponding smaller groove in sheet 358 (for example groove 380). Each apex may thus be a detent that cooperates with a corresponding recess to provide a light barrier between adjacent light channels.

When sheets 358 and 360 are aligned with one another, the matching grooves 362-366, 370-374 form light guides 352, 354, 356, which are hollow channels surrounded by four elongate planar surfaces. In one arrangement the centre to centre spacing of the light guides is 2mm and the light guides have a square cross section. The matching peaks 390 and grooves 380, 382 separate the light guides and may assist in limiting any leakage of light between light guides.

Sheets 358, 360 may be held together in various ways, for example using adhesives or mechanical fasteners, for example clamps or screws. Scintillators may be positioned at one end of each light guide 352, 354, 356 and optical fibres may be introduced at the opposite end of each light guide, as illustrated for example in Figure 2A.

Two-dimensional array of dosimeters

Figures 5A to 5C show an example of a two-dimensional array of dosimeters 600, in which two rows of scintillators are arranged in an x-y plane.

Figure 5A shows a first row of scintillators 602, each of the scintillators 602 associated with a light guide 604 defined by a plurality of elongate planar surfaces around a hollow core. A second row of scintillators 606 is arranged in the same plane as the first row 602, each of the scintillators 606 associated with a light guide 608 defined by a plurality of elongate planar surfaces around a hollow core. In the array 600 the two rows of scintillators 602, 604 are configured in a symmetric cross shape. Other configurations may also be used, and there may be more than two rows of scintillators arranged in the same x-y plane.

In one arrangement the array is built with a 2mm centre to centre spacing for each dosimeter, in a probe 5 cm in diameter. Figure 5B shows a shutter 610 that may be used in conjunction with the two-dimensional array 600. The shutter 610 is a disc having two rows of holes 612, 614. The configuration of the holes 612, 614 in the plane of the shutter corresponds to the configuration of the rows of light guides 604, 608, i.e. in this example a symmetric cross shape. The shape of each hole matches the cross-sectional shape of the light guides 604, 608.

The shutter 610 may be rotated relative to a housing 601 holding the array 600 in order to move the array of dosimeters from a coupled position in which the two rows of holes 612, 614 of the shutter 610 are aligned with the two rows of scintillators 602, 604 to an uncoupled position, where the shutter 610 obstructs the scintillators. Thus a "dark signal" may be measured whenever necessary.

Figure 5C shows a view of the housing 601, which is illustrated as transparent to show the internal configuration of light guides 604, 608. The housing may also be opaque. Optical fibres are associated with the holes of the shutter 610, and exit the housing 601 via a neck 620. The optical fibres may lead to a photomultiplier array. Three dimensional array of dosimeters

In another arrangement, scintillators may be arranged in a three dimensional configuration. An example of a three-dimensional array 700 is shown in Figure 6A. Three mutually orthogonal rows of scintillators 702, 704, 708 are located at an input end of housing 701. Row 708 runs parallel to the longitudinal axis of the housing 701. Rows 702 and 704 lie in an x-y plane orthogonal to the z-axis represented by row 708.

Figure 6B shows the light guides 712 associated with row 702, each light guide defined by a plurality of elongate planar surfaces around a hollow core.

Figure 6C shows in addition the light guides 714 associated with row 704, each light guide defined by a plurality of elongate planar surfaces around a hollow core. Figure 6D adds the light guides (e.g. 718a and 718b) associated with row 708. Each light guide is defined by a plurality of elongate planar surfaces around a hollow core. As illustrated in Figure 6E, the light guides associated with the scintillators in row 708 are L-shaped. The guide 718a leaves scintillator 720 in a plane parallel to the x-y plane defined by rows 702, 704. After a distance 730 there is a bend in the light guide. After the bend the light guide 718a lies generally parallel to row 708. Thus, after the L-shaped bend, all of the light guides 712, 714, 718 run generally parallel to one another, leading to an output end of the housing 701 from which optical fibres may lead the light to a photodetector array.

At the bend in light guides 718a a reflective surface 722 is provided to reflect light originating from the scintillator 720. As seen in Figure 6D, the light guides (e.g. 718a) between the input end of the housing 701 and the plane of rows 702, 704 run away from the scintillators 708 in a first direction and the light guides (e.g. 718b) that lie below the plane of rows 702, 704 run away from the scintillators of row 708 in a second direction. The length 730 between the scintillator and the L-bend is different for adjacent light guides to allow for a regular spacing of the light guides at the end closest the optical fibres.

A shutter may be provided for use with the three-dimensional array of Figures 6A-6E.

The dosimeter arrays described herein may be used in planning and programming radiation doses that are to be delivered to a patient. During the planning procedure, the dosimeter array is positioned in a 'phantom' that provides a volume representing the portion of the patient's anatomy that will subsequently be treated by radiation therapy. An example is shown in Figure 7A, which illustrates a generally cylindrical head phantom 800 that is used to represent a patient's head. The array 600 of scintillators is mounted in housing 601 inside the head phantom 800 such that the array 600 may be moved to any specified location within the phantom. Optical fibres capture the light emitted by the scintillators and exit the housing 601 via neck 620. Inside the housing 601 an array of light guides separates the scintillators from the optical fibres.

The array 600 may be moved along the longitudinal axis 806 of the phantom 800. In addition, the housing 601 is supported by an eccentric rotation assembly 820 at an end of the phantom 800, as illustrated further in the end view of Figure 7B and the perspective view of Figure 7C. The housing 601 is positioned away from the centre of disc 802, which in turn is positioned away from the centre of disc 804. By adjusting the relative positions of the housing 601 and the components 802, 804 of the rotation assembly 820, the scintillator array 600 may be positioned at any desired radial location within the cylindrical phantom 800. In this way it is possible to measure the dose at a greater number of points in the volume than the number of scintillators in the array. By driving the array to a set of predetermined positions, the dose distribution can be finely sampled throughout the entire volume of interest. The example of Figures 7 A, 7B and 7C shows the two-dimensional array 600. However, the phantom 800 may also be used with a single dosimeter 100, a linear array of dosimeters 500 or three-dimensional array 700.

Results Figures 8A-8D show experimental results obtained using an array of dosimeters formed by the method illustrated in Figure 3A and 3B.

Figure 8A is a graph that compares the performance of light guides having a circular cross section with light guides as described with reference to Figure 3A, ie having a square cross section. The y-axis is a normalised transmission of light along the light guide and the x-axis is wavelength in mm. Data points 902 show the efficiency of light transmission along the square light guides and data points 904 show the efficiency of light transmission along the circular light guides. It may be seen that across the illustrated frequency range the square light guides provide a higher transmission of light than the circular light guides. It is conjectured that the improved efficiency offered by the square light guides arises from the planar surfaces that surround the hollow core. Planar surfaces may be provided that are smoother than the curved surfaces of circular fibre optic light guides. The smoother surfaces reduce losses when light is reflected from the sides of the light guides.

Figure 8B shows an example of the accuracy and resolution that may be obtained using an array of dosimeters formed by the method illustrated in Figure 3A and 3B. The array is used to measure the intensity of a 5mm beam of radiation produced by a Varian linear accelerator. The beam centre was located 1mm away from the centre of the array. This slight asymmetry allows for more dose readings from the centre of the beam assuming the beam is symmetric about its centre. The beam 950 is orthogonally incident on the axis defined by the row of scintillators in array 500 as illustrated in Figure 9A, and thus the readings show the profile of the beam. The readings obtained from the dosimeter array are shown as a series of points (e.g. 912, 914) in the graph, in which the y-axis is a normalised dose and the x-axis is distance. The dosimeter results may be captured without correction for perturbation effects, angular dependencies density or dose rates. For comparison, measurements obtained using EBT2 film are shown as the continuous line 910. It may be seen that there is a close match over the entire range of distances illustrated in Figure 8B. Figure 8C shows a set of results 920-930 for different beam field sizes measured using a linear array of dosimeters formed by the method illustrated in Figure 3A and 3B. The y-axis is normalised dose and the x-axis is distance in mm. The beam is orthogonally incident on the axis defined by the row of scintillators, and thus the readings show the profile of the beam. Results 920 were obtained with a field beam size of 8x8 cm². Results 922 were obtained with a field beam size of 6x6 cm². Results 924 were obtained with a field beam size of 4x4 cm². Results 926 were obtained with a field beam size of 3x3 cm². Results 928 were obtained with a field beam size of 2x2 cm². Results 930 were obtained with a field beam size of lxl cm².

Figure 8D shows a depth dose curve measured using a linear array of dosimeters formed by the method illustrated in Figure 3 A and 3B. In this case the beam 952 is incident on the plane defined by the row of scintillators in the array 500, as illustrated in Figure 9B so that one scintillator is closest to the beam source (giving reading 944) and another scintillator is furthest from the beam source (giving reading 946). The results (e.g. 940, 944, 946) obtained by the dosimeter array are compared with measurements obtained using an ionization chamber. Arrangement with twisted fibre optic pair

It is possible to compensate for the contribution of Cerenkov radiation induced in the optical fibre (i.e. the signal optical fibre) by determining the Cerenkov light in the optical fibre, for example by using a partner optical fibre. The signal and partner optical fibres may be used in a twisted fibre optic pair technique. In the arrangement of Figure 1 a signal optical fibre 7 couples the end of the light guide 2 to the photodetector 8. In some environments Cerenkov radiation resulting in Cerenkov light may be generated in the optical fibre 7, introducing noise into the measured signal. For example, there may be scattered radiation in a treatment room that strikes the optical fibre 7. Figure 10 shows a system in which the Cerenkov radiation may be taken into account. As before, the scintillator 1 creates a light output dependent on radiation dose. The light propagates along hollow-core light guide 2 to a point outside the main radiation field, from which one optical fibre of a twisted fibre optic pair 10 conveys the signal to a measurement system. In the twisted fibre optic pair 10 the output of the hollow-core light guide is coupled to the end 12 of optical fibre 7, here termed the signal fibre, which transmits the light signal to a measurement device. A second optical fibre, termed the partner fibre 18, is provided to measure the Cerenkov radiation. The partner fibre 18 (also called the background fibre) is similar to the signal fibre 7 thereby to ensure that the Cerenkov radiation generated in the background fibre accurately reproduces the radiation generated in the signal fibre. The dimensions and optical characteristics of the signal fibre 7 and the partner fibre 18 are thus kept substantially the same. The end 14 of the partner fibre 18 is prepared in the same way as the end 12 of the signal fibre 7 so that there is the same degree of internal reflection of Cerenkov radiation in the fibres.

The ends 12, 14 of the fibres 7, 18 are terminated immediately adjacent to one another, but the end 14 of the partner fibre 18 is blind and does not receive light from the hollow core light guide 2. In a preferred embodiment, the signal fibre 7 and the partner fibre 18 are twisted around one another so as to form a double helix.

In one arrangement the periodicity of the double helix is finest at the proximal end of the twisted pair 10 (i.e. the end closest to the radiation field). The centre-to-centre distance of the fibres 7, 18 may be kept small by reducing the thickness of the protective sleeve around each optical fibre.

In order to reduce the total Cerenkov background radiation in both fibres 7, 18, the twisted pair 10 is encased in an additional protective sleeve (not shown) that shields both fibres from scattered low energy radiation. The operating principle of the twisted pair is to equalise the exposure to radiation of each fibre. If the spatial periodicity of the helix is high enough, each fibre is equally exposed to radiation from any direction.

Figure 12 illustrates schematically how the twisted pair 10 may be used in conjunction - with the dosimeter array 200 shown in Figure 2A. The light guide unit 210 is shown in cross section. Elongate planar members 22a, 22b, 22c and 22d are positioned in parallel to define three light guides 20a, 20b and 20c. The planar members 22a-22d are formed from a material having a refractive index greater than that of the medium filling the light guides 20a-c (in this case air) such that in use light is guided along the channels from a scintillator in scintillator unit 220 at an input end of each guide 20a, 20b, 20c to an output end of light guides 20a, 20b, 20c respectively. A twisted fibre pair 10 is provided at the output end of the light guides 20a, 20b, 20c to convey the light to an array of photodetectors to measure the light in each dosimeter. For clarity of illustration only the twisted pair associated with light guide 20c is shown. The signal line 7 is coupled to the end of the light guide 20c and receives the light signal that propagates along light guide 20c. The partner fibre 18 may be terminated in a blind hole drilled into the planar member 22d such that the partner fibre 18 does not receive light propagating along the light guide unit 210. Measurement and data processing

As seen in Figure 11 A, each fibre in the twisted pair 10 may be associated with a photodetector of the measurement system. For example, the signal fibre 7 is coupled to photodetector 101a and the partner fibre 18 is coupled to photodetector 101b. The photodetectors 101a, b may be any suitable device, for example a photomultiplier or photodiode device. Alternatively, as seen in Figure 1 IB, the signal fibre 7 and the partner fibre 18 both terminate in a switch 103 that allows the output of the signal fibre 7 and the partner fibre 18 to be transmitted to one shared detector, photodetector 101c. The switch alternates rapidly between the fibres 7, 18 and thus the photodetector 101c measures the signal in both fibres. The frequency of switching should be higher than the frequency of variations in the radiation field striking the twisted pair 10.

The photodetectors convert the received light signal into an electronic signal, which may be processed by an electronic processing system 106, for example as illustrated schematically in Figure 1 1C. The photodetectors, e.g. 101a, 101b, are coupled to a data communication network 104, which is also coupled to the elements of the electronic processing system 106, e.g. 105, 107, 109. The electronic processing system 106 includes at least one data processing system or computer system, shown by processor 105, which may be implemented as any conventional personal computer or server. However, those skilled in the art will appreciate that implementations of various technologies described herein may be practiced in other computer system configurations, including hypertext transfer protocol (HTTP) servers, hand-held devices, multiprocessor systems, microprocessor-based or programmable consumer electronics, network PCs, minicomputers, Linux computers, mainframe computers, and the like.

The processor 105 may be in communication with at least one disk storage device or at least one memory device, which may be external hard disk storage devices. The processor 105 may retrieve data from the disk storage device to process the data according to program instructions that correspond to implementations of various technologies described herein. The program instructions may be written in a computer programming language, such as FORTRAN, C, C++, Java and the like. The program instructions may be stored in a computer-readable medium, such as a program disk storage device. Such computer- readable media may include computer storage media and communication media. Computer storage media may include volatile and non-volatile, and removable and non-removable media implemented in any method or technology for storage of information, such as computer-readable instructions, data structures, program modules or other data. Computer storage media may further include RAM, ROM, erasable programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), flash memory or other solid state memory technology, CD-ROM, digital versatile disks (DVD), or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium hich can be used to store the desired information and which can be accessed by the computing system 105.

Software instructions running on the electronic processing system 106 may be used to implement computational steps in the workflow 1 10 of Figure 11D. Software running on the electronic processing system 106 may also determine the switching of the switch 103 and the storage of data measured by photodetectors 101 a, b, c. In one implementation, the electronic processing system 106 may include at least one graphical user interface (GUI) components such as a graphics display 107 and a user input device 109 which can include a pointing device (e.g., a mouse, trackball, or the like, not shown) to enable interactive operation. The GUI components may be used both to display data and processed data products and to allow the user to select among options for implementing aspects of the method. The electronic processing system 106 may store the results of the methods described above on disk storage for later use and further analysis.

Figure 1 ID shows a method 1 10 for compensating for induced Cerenkov radiation in the dosimeter of Figures 10 and 11. In an initial process 1 12 described in more detail below, the relative sensitivity of the fibres 7, 18 in a twisted pair 10 is determined. Then, during a measurement process, the light output from the signal fibre 7 is measured (step 1 14) and the light output from the partner fibre 18 is measured (step 1 16). Then, in step 1 18, software running on the electronic processing system 106 removes the effect of the background from the measured signal by subtracting an inferred amount of Cerenkov light from the output of the signal fibre 7. This step is described in more detail in the following section.

Determination of relative sensitivity of the two fibres in the twisted pair 10 Three methods for determining the relative sensitivity of the fibres 7, 18 are described below.

First method

A standard dosimeter, such as an ionisation chamber, is co-located with a dosimeter including the twisted pair 10 in the beam of a linear accelerator. Two exposures are required. In the first exposure, the reading of the light intensity in the signal fibre 7 is measured using a constant field intensity of dose rate ^D as measured by the standard dosimeter. The reading #a of the light intensity in the partner fibre 18 is also recorded. The relation between # i , # J and D is:

A second exposure is carried out which results in readings Λ Ί , R'₃ and & R = k_t D^' + k_tR (2)

In general, the exposures should be carried out so that R ± * R^'i and ^ff a * ^Rt . For example, the scintillation dosimeter and ionisation chamber may be left in place while the two exposures are carried out at different field sizes. Alternatively, the field can be moved relative to the dosimeters. Equations 1 and 2 are solved simultaneously to obtain ^ki and ^fcs . Once ^fei and ^fca have been determined, any other dose ^{D >} can be measured using:

where ^R'i and ^R'i are the respective readings of the two channels, ie fibres 7 and 18. Equation (3) is used in step 1 18 of method 1 10 to remove the effect of the background from the measured signal.

Second method

An absolute calibration is performed for a radiation beam using a secondary standard dosimeter, such as an ionisation chamber. The relative sensitivity of the two readout channels, ie fibres 7, 18 is determined using two exposures to the calibrated beam. In the first exposure, the reading of the light intensity in the signal fibre 7 is measured using a constant field intensity of dose rate ^D as determined by the standard dosimeter. The reading ^fl? of the light intensity in the partner fibre 18 is also recorded. The relation between ^R \ , ^R z and 9 is:

A second exposure to a dose rate ^D is carried out which results in readings ^Λι and ^R n

where the dose rate D' is determined using known beam treatment planning parameters.

In general, the exposures should be carried out so that ^≠ K'i and ≠ J?i . For example, the scintillation dosimeter can be left in place while the two exposures are carried out at different field sizes. Alternatively, the field can be moved relative to the dosimeters. Equations 4 and 5 are solved simultaneously to obtain ^t and ^fe* . Once ^i and have been determined, the dose D' can be measured using:

where A . and ^R 'j are the respective readings of the two channels (fibres 7 and 18).

Third method

The relative sensitivity of the two readout channels, ie fibres 7, 18 may be determined with a temporary removal of the scintillator 1 so that #i and are measurements of only Cerenkov radiation. This can also be achieved if the scintillator is sufficiently shielded or sufficiently far from the primary beam so that the dose rate to the scintillator D can be assumed to be 0. In either case, the relative sensitivity can be obtained with a single irradiation where:

= k,R₃ (7) Once has been determined, the residual Cerenkov radiation can be subtracted from the reading R Ί and the dose ^D' can be measured using:

D^' = R'_x - k_tR^' _a (8)

Method of forming the twisted pair Figure 13 shows schematically a method of forming a twisted pair 10 with a defined periodicity between twists.

A casing 130 is provided to hold the fibres 7, 18. The casing has a sequence of partitions at defined intervals 133a, 133b, for example partitions 131a, 131 b, 131c. The intervals 133a, 133b may be equal resulting in a uniform helix. The intervals 133a, 133b may be variably spaced resulting in a helix with a variable spatial periodicity.

Each partition 131 has a notch or groove sized to accommodate and hold two fibres 7, 18. The casing 130 is initially open. A first twist in the fibres 7, 18 is effected and positioned in notch 132a. A second twist is effected and positioned in notch 132b. This is repeated for the required length of the twisted pair. The partitions thus define a series of nodes at which the fibre twists occur. In one example, the nodes are about 1 cm apart. The casing 130 is then closed up to protect the twisted pair from scattered radiation.

The fibres 7, 18 may be lengths of optical fibre cut from the same original piece of optical fibre to have the same dimensions and optical characteristics.

Experimental results The twisting of the fibres in twisted pair 10 is thought to provide an averaging of the effects of radiation striking the individual fibres from different directions. This may be compared with other physical configurations, for example one in which the signal fibre 7 and the partner fibre 18 are positioned side by side, but without twisting.

Figure 14A shows the result of an experiment in which a twisted pair of fibres is placed 5cm from the edge of a radiation field. The gantry of the linear accelerator providing the radiation field is rotated around the twisted pair in an axial plane and the Cerenkov radiation generated in each fibre is measured. Ideally, the ratio of the signal in the signal fibre to the signal in the partner fibre should be unity. The further this ratio departs from unity, the higher the error in the measured dose may be when Cerenkov subtraction is performed in step 118. The axial plane relative to the twisted pair is thought to be where the fibre pair is most vulnerable to changes in this ratio.

The x-axis of Figure 14A is the beam angle in the axial plane, and the y-axis is the ratio of signals in the signal fibre and the partner fibre. The measured ratios 142 of the twisted pair show a maximum deviation of 4.5%.

The same procedure is followed where the signal fibre 7 and partner fibre 18 lie adjacent to one another but are not twisted. In this case the measured ratios 141 show large deviations of up to 30%. These large deviations are thought to arise when one fibre shields the other from the incident radiation. This effect is mitigated when the fibres are twisted into a double helix.

Figure 14B shows an angular plot 143 of the ratios in the twisted pair at all beam angles in the axial plane. Figure 14C shows the ratio of signals in the twisted signal and partner fibres when the gantry of the linear accelerator is moved through different azimuthal angles, i.e. through various angles in the plane parallel to the length of the twisted pair. The x-axis is the gantry angle and the y-axis is the ratio of signals induced in the two fibres. With a twisted pair the maximum deviation is less than 3% in this experiment. Figure 14D compares a beam output measured using the twisted pair 10 with the beam output measured using an ionisation chamber as a reference. It may be seen that the twisted pair measurements (e.g. 147) closely match the ionisation chamber measurements 145.

Figure 14E shows further how the relative dose measured by each fibre of the twisted pair varies dependent on the angle of the gantry in the axial plane. Variations

It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.

The twisted pair 10 may be used independently of the light guide 2. For example, the twisted pair configuration may be used in conjunction with the light pipe described in co- assigned patent application WO 2007/085060 "Fibre Optic Dosimeter" filed on 30 Januaiy 2007, which is incorporated herein by cross reference. The twisted pair configuration may also be used in conjunction with a dosimeter or array of dosimeters as described in patent application PCT/AU2012/001 137 filed 21 September 2012, which is incorporated herein by cross reference.

References:

Beddar A S, ackie T R and Attix F H 1992 Water-equivalent plastic scintillation detectors for high- energy beam dosimetry: 2. Properties and measurements Phys. Med. Biol. 37 1901-13

Gift M A, Sutton R A and Webb D V 2002 A temporal method of avoiding the Cerenkov radiation in organic scintillator dosimeters by pulsed mega-voltage electron and photon beams Phys. Med. Biol. 47 1421-33

Deboer S F, Beddar A S and Rawlinson J 1993 Optical filtering and spectral measurements of radiation- induced light in plastic scintillation dosimetry Phys. Med. Biol. 38 945-58

Fontbonne } M et al 2002 Scintillating fiber dosimeter for radiation therapy accelerator IEEE Trans. Nucl. Sci.49 2223-7

Frelin A M, Fontbonne J M, Ban G, Colin J, Labalme M, Batalla A, Isambert A, Vela A and Leroux T 2005 Spectral discrimination of Cerenkov radiation in scintillating dosimeters Med. Phys. 32 3000-0

Lacroix F, Archambault L, Gingras^L, Guillot M, Beddar A S and Beaulieu L 2008 Clinical prototype of a plastic water-equivalent scintillating fiber dosimeter array for QA applications Med. Phys. 35 3682-90

Scott A J D, Kumar S, Nahum AE, Fenwick JD 2012 Characterizing the influence of detector density on dosimeter response in non-equilibrium small photon fields Phys. Med. Biol. 57 (2012) 4461-4476

Claims

1. A system for compensating for induced Cerenkov light, the system comprising: a first optical fibre that conveys a signal from a source end of the first optical fibre to a measurement end of the first optical fibre; a second optical fibre having a blind end adjacent the source end of the first optical fibre, wherein the first optical fibre and the second optical fibre are twisted around one another to form a double helix; a measurement system that measures a signal output from the first optical fibre and a partner output from the second optical fibre, the partner output being indicative of Cerenkov light induced in the second optical fibre; and a data processor for subtracting an inferred amount of Cerenkov light from the signal output, wherein the inferred amount is dependent on the measured partner output.

2. The system of claim 1 wherein a periodicity of the double helix varies between the source end and the measurement end of the first optical fibre. -

3. A method of determining a relative sensitivity of the first optical fibre and the second optical fibre in the system of any one of claims 1 to 2, the method comprising: measuring the signal output and the partner output during a first exposure of the system to a first field intensity with a known measured dose rate; measuring the signal and partner output during a second exposure of the system to a second field intensity with a second known dose rate, wherein the first and second field intensities allow for different dose rates; determining at least one parameter characterising a relationship between the signal _λ output, the partner output and the measured dose rates; and storing the at least one parameter for use in compensating for induced Cerenkov light.

4. The method of claim 3 wherein the second known dose rate is determined by using known beam treatment planning parameters.

5. The method of claim 2 further comprising measuring the signal output and the partner output during an exposure of the system where the measurement system is sufficiently shielded or removed from the signal output to assume that the first known dose rate is zero.

6. A dosimeter for radiation fields, comprising a scintillator; a light guide having an input end in optical communication with the scintillator, the light guide comprising a plurality of elongate planar surfaces surrounding a hollow core, the planar surfaces reflecting light within the hollow core; a signal optical fibre having a first end in optical communication with an output end of the light guide; a signal detector to detect light output from a second end of the signal optical fibre and to provide a signal output indicative of the intensity of a first received light signal; a partner optical fibre having a blind end located adjacent the first end of the signal optical fibre; and a partner detector to detect light output from an output end of the partner optical fibre and to provide a partner output indicative of the intensity of a second received light signal.

7. The dosimeter of claim 6 wherein the partner optical fibre and the signal optical fibre are twisted together to form a double helix.

8. The dosimeter of claim 7 wherein a periodicity of the double helix varies between a first end and a second end of the signal optical fibre.

9. The dosimeter of any one of claims 6 to 8 wherein: the signal detector and the partner detector are a shared detector; and the dosimeter further comprises a switch for switching an input to the shared detector between the second end of the signal optical fibre and the output end of the partner optical fibre so that, in use, the shared detector detects light from the signal optical fibre and the partner optical fibre.

10. The dosimeter of any one of claims 6 to 9 comprising: a data processor to adjust the signal output associated with the signal optical fibre based on the partner output associated with the partner optical fibre.

1 1. A dosimeter array comprising at least two dosimeters according to any one of claims 6 to 10.