WO2013040646A1

WO2013040646A1 - Light guides for scintillation dosimetry

Info

Publication number: WO2013040646A1
Application number: PCT/AU2012/001137
Authority: WO
Inventors: Natalka Suchowerska; David Robert Mckenzie; Pourandokht NASERI
Original assignee: The University Of Sydney
Priority date: 2011-09-23
Filing date: 2012-09-21
Publication date: 2013-03-28

Abstract

There is provided a dosimeter (200) for radiation fields, comprising at least one scintillator (220) and an associated light guide (20a, 20b, 20c) 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. The dosimeter may extend to an array of dosimeters, eg orthogonal rows located in a measurement plane or measurement volume, comprising multiple scintillators and associated light guides in optical communication with each other. Each light guide also comprises a plurality of elongate planar surfaces reflecting light and surrounding a hollow core.

Description

Light guides for scintillation dosimetry

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 arrays of scintillation dosimeters.

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 a fibre optic line. 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 ai 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 and 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. The dosimeter may include a fibre optic line having a first end in optical communication with an output end of the light guide and a detector to detect light output from a second end of the fibre optic line and to provide an output indicative of the intensity of a received light signal. The light guide may have a square cross-sectional shape.

According to a second aspect of the invention there is provided an array of dosimeters for radiation fields, comprising an array of scintillators and an array of light guides having respective input ends in optical communication with associated scintillators, each of the light guides comprising a plurality of elongate planar surfaces surrounding a hollow core, the planar surfaces reflecting light within the hollow core.

The array of dosimeters may include a plurality of fibre optic lines having respective first ends in optical communication with output ends of associated light guides and a detector array to detect light output from respective second ends of the fibre optic lines and to provide an output indicative of the intensity of a received light signal in each dosimeter.

The array of dosimeters may include first decoupling means to decouple the fibre optic lines and the array of light guides such that the fibre optic lines are not in optical communication with the light guides.

The array of dosimeters may include second decoupling means to decouple the array of scintillators and the array of light guides such that the scintillators are not in optical communication with the light guides.

The array of dosimeters may comprise two orthogonal rows of scintillators located on the measurement plane.

The array of dosimeters may comprise three mutually orthogonal rows of scintillators located in the measurement volume. According to a further aspect of the invention there is provided an array of light guides for use in an array of dosimeters, comprising: a first sheet of material with a first plurality of parallel grooves formed in a first surface thereof, wherein the first surface is light reflecting; a second sheet of material with a second plurality of parallel grooves formed in a second surface thereof, wherein the second surface is light reflecting and the second surface is positioned to face the first surface such that the first plurality of grooves aligns with the second plurality of grooves; and a plurality of elongate planar strips of light-reflecting material, each strip located in a groove of the first surface and a groove of the second surface and separating the first sheet and the second sheet to provide a plurality of hollow light-guiding channels.

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 a fibre optic line to a photodetector;

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

Fig. 1 B shows a cross-section of an example of the light-guide of Fig. 1A 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 a fibre optic line;

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. 3A 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. 3A;

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. 5A; 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. 7A 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. 7A

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.

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 Polymethylmethacrylate (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 a fibre optic line 7 such that light travelling in the light guide 2 passes into the fibre optic line. The fibre optic line 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 fibre optic line 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, eg in brachytherapy applications, the fibre optic line 7 should be of sufficient length so that the photodetector 8 is located outside the patient's body.

The fibre optic line 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 fibre optic line 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 fibre optic line 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 fibre optic line;

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 22 to a fibre optic line 24a, 24b, 24c located at an output end of light guides 20a, 20b, 20c respectively. The fibre optic lines 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. 2A 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. J e 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 fibre optic lines 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 fibre optic lines 24a-c. In a coupling position, the shutter 32 does not obstruct an optical path between the fibre optic lines 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 fibre optic lines 24a-c may be held by the shutter 32 so that the fibre optic lines 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 3A. A sequence of parallel grooves (eg 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 (eg 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 (eg 41a) may be positioned at an input end of the channels. Another sheet of PMMA (not shown in Figure 3A) 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 (eg 52 a-c) are located in the grooves to form seven light guides, each associated with a scintillator (eg 51a-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 haying 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 fibre optic lines 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 10cm-100cm, 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 fibre optic lines 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 ^N 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, ie 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. Fibre optic lines are associated with the holes of the shutter 610, and exit the housing 601 via a neck 620. The fibre optic lines 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 (eg 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 fibre optic lines 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 (eg 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 (eg 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 fibre optic lines.

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. Fibre optic lines 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 fibre optic lines. 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 7A, 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 (eg 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 1x1 cm2. Figure 8D shows a depth dose curve measured using a linear array of dosimeters formed by the method illustrated in Figure 3A 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 (eg 940, 944, 946) obtained by the dosimeter array are compared with measurements obtained using an ionization chamber.

References:

Beddar A S, Mackie 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

Clift 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 1 21-33

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 J 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 3000-6

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 dosimeter for radiation fields, comprising a scintillator; and 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.

2. The dosimeter of claim 1 comprising: a fibre optic line having a first end in optical communication with an output end of the light guide; and a detector to detect light output from a second end of the fibre optic line and to provide an output indicative of the intensity of a received light signal.

3. The dosimeter of claim 1 or 2 wherein the light guide has a square cross- sectional shape.

4. The dosimeter of claim 1 or 2 wherein the light guide has a rectangular cross- sectional shape.

5. The dosimeter of any one of the preceding claims comprising a light-reflective layer located between the elongate planar surfaces and the hollow core of the light guide.

6. The dosimeter of any one of the preceding claims wherein the elongate planar surfaces comprise polymethyl methacrylate (PMMA).

7. The dosimeter of any one of the preceding claims wherein a length of the light guide is in the range of 5- 00 cm.

8. The dosimeter of claim 7 wherein the length is in the range of 5-20 cm.

9. An array of dosimeters for radiation fields, comprising an array of scintillators; and an array of light guides having respective input ends in optical communication with associated scintillators, each of the light guides comprising a plurality of elongate planar surfaces surrounding a hollow core, the planar surfaces reflecting light within the hollow core.

10. The array of dosimeters of claim 9 comprising: a plurality of fibre optic lines having respective first ends in optical communication with output ends of associated light guides; and a detector array to detect light output from respective second ends of the fibre optic lines and to provide an output indicative of the intensity of a received light signal in each dosimeter. .

11. The array of dosimeters of claim 10 comprising: first decoupling means to decouple the fibre optic lines and the array of light guides such that the fibre optic lines are not in optical communication with the light guides.

12. The array of dosimeters of claims 10 or 11 comprising: second decoupling means to decouple the array of scintillators and the array of light guides such that the scintillators are not in optical communication with the light guides.

13. The array of dosimeters of claim 11 or 12 wherein the decoupling means comprises a shutter movable between a coupling position enabling optical communication in the array of dosimeters and one or more decoupling positions in which optical communication is disabled in the array of dosimeters.

14. The array of dosimeters of claim 9 wherein the array of scintillators is movable relative to the array of light guides to enable or disrupt optical communication between the scintillators and the light guides.

15. The array of dosimeters of any one of claims 9-14 wherein the array of scintillators comprises a two-dimensional plurality of scintillators located on a measurement plane.

16. The array of dosimeters of claim 15 comprising two orthogonal rows of scintillators located on the measurement plane.

17. The array of dosimeters of any one of claims 9-14 wherein the array of scintillators comprises a three-dimensional plurality of scintillators located in a measurement volume.

18. The array of dosimeters of claim 17 comprising three orthogonal rows of scintillators located in the measurement volume.

19. An array of light guides for use in an array of dosimeters, comprising: a first sheet of material with a first plurality of parallel grooves formed in a first surface thereof, wherein the first surface is light reflecting; a second sheet of material with a second plurality of parallel grooves formed in a second surface thereof, wherein the second surface is light reflecting and the second surface is positioned to face the first surface such that the first plurality of grooves aligns with the second plurality of grooves; and a plurality of elongate planar strips of light-reflecting material, each strip located in a groove of the first surface and a groove of the second surface and separating the first sheet and the second sheet to provide a plurality of hollow light-guiding channels.

20. The array of light guides of claim 19 comprising a light-reflecting layer applied to the interior surfaces of the hollow light-guiding channels.

21. The array of light guides of claim 19 or 20 made of polymethyl methacrylate (PMMA).

22. The array of light guides of any one of claims 19-21 wherein a length of the light guides is in the range of 5-100 cm.

23. The array of light guides of claim 22 wherein the length is in the range of 5-20 cm.

24. An array of light guides for use in an array of dosimeters, comprising: a first sheet of material with a first plurality of parallel grooves formed in a first surface thereof, wherein the grooves comprise a plurality of planar light reflecting surfaces; and „ a second sheet of material with a second plurality of parallel grooves formed in a second surface thereof, wherein the grooves of the second plurality of grooves each comprise a plurality of planar light reflecting surfaces; wherein the second surface is positioned to face the first surface such that the first plurality of grooves aligns with the second plurality of grooves such that the planar surfaces of the first and second grooves define a plurality of hollow light-guiding channels.

25. The array of light guides of claim 24 wherein: the first sheet comprises a plurality of parallel recesses running parallel to the first plurality of parallel grooves, wherein the recesses are located between adjacent grooves of the first plurality of grooves; and the second sheet comprises a plurality of elongate detents running parallel to the second plurality of parallel grooves, wherein the detents are located between adjacent grooves of the second plurality of grooves; wherein when the second surface is positioned to face the first surface the detents are received within the recesses to form a plurality of light barriers between adjacent light guides.