WO2016075717A1

WO2016075717A1 - A ionizing radiation beam detector

Info

Publication number: WO2016075717A1
Application number: PCT/IT2015/000272
Authority: WO
Inventors: Alessandro Montanari; Mauro IORI; Giuseppe FELICI
Original assignee: Istituto Nazionale Di Fisica Nucleare; AZIENDA OSPEDALIERA Dl REGGIO EMILIA ARCISPEDALE S. MARIA; S.I.T.-Sordina Iort Technologies S.P.A.
Priority date: 2014-11-12
Filing date: 2015-11-09
Publication date: 2016-05-19

Abstract

A ionizing radiation detector comprising a plurality of scintillators (12) of plastic material, wherein said scintillators (12) are in form of co-planar plates disposed according to a two-dimensional ordered arrangement, each plate having a respective optical fiber (13) joined to a face or to a perimetral edge portion of the plate for conveying scintillation light pulses to a photodetector (20).

Description

A IONIZING RADIATION BEAM DETECTOR

The present invention generally relates to systems for detecting ionizing radiations.

In particular, the present invention relates to detection systems for monitoring and control the radiation dose delivered to the patient during a treatment of intraoperative radiotherapy with electrons ("Intraoperative Electron Radiation Therapy", IOERT) . Said technique involves irradiation with electron beams which is carried out immediately after the removal of the cancer, when the surgical operation is still in place, in order to completely eliminate any remaining cancer cells. This technique is of particular interest in the treatment of mammary carcinomas as well as in the treatment of prostatic carcinomas or of the liver's metastases .

The electron beam, which has an energy of between 2 and 12 MeV, is produced by a compact linear accelerator, which can be transported in the operating room. Said type of accelerator provides a pulsed beam of great intensity, with pulses having a typical duration of about one microsecond and a frequency of a few Hertz . The dose of each pulse is equal to a few cGy. The treatment involves a single exposure having a total dose of the order of about ten Gy, for a duration of a few minutes. The electron beam is collimated in the area to be treated, through a tube made of polymethyl- methacrylate (PMMA) , a so-called "applicator", which has a diameter typically between four and ten centimeters. During the clinical practice a punctual detector is used, generally a micromosfet, to monitor the dose at the input of the radiation field, which is placed at the base of the applicator. Moreover a suitable screen shaped as a plate is inserted immediately below the organic tissue to be treated, so that the electronic radiation is attenuated and does not affect the internal organs. The plate has also a thickness of plastic material to remove the electrons which are back-scattered towards the base of the irradiated tissue.

To ensure that the protection is effective, it is obviously essential that the applicator is directed towards the treatment area and at the same time said applicator is aligned with the shielding plate. The alignment procedure is not easy because the plate is not visible to the surgeon after it has been inserted into the human tissue. The protective plate generally has a radius greater than the radius of the applicator and the surgeon is thus able to check the centering of said plate only with his/her fingers. The plate or disk is finally removed after the irradiation.

Since the total dose is delivered, during a single treatment, in a very short time, the real-time control of the dose and the correct alignment of the shielding plate are particularly important. In order to accurately monitor the value of the input dose, almost all the radiotherapy centers apply clinical protocols according to which the input dose delivered towards the area to be treated is measured at the output of the applicator by means of a punctual solid state detector. A proper alignment can be verified only after the treatment through an analysis of a radiocromic film which is applied over the protective metal plate. The percentage of correct alignments is far from 100% and greatly depends on the position of the area to be treated, that is to say of the area from which the cancer was removed. When the position of the cancer is angled, there is a geat probability that the plate moves by gravity.

The document US 2009/0114823 discloses different embodiments of ionizing radiation detectors which are constituted by scintillators and which can be used for measuring the radiation dose during a radiological exam of a patient . One of the embodiments of said detector comprises a single plate of plastic material having a respective optical fiber combined with a portion of the peripheral edge to carry the light pulses of the scintillator to a remote photo-detector. Another embodiment described in US 2009/0114823 comprises an array of scintillators shaped as thread-like rods, which have practically the diameter of an optical fiber. Said document does not describe any application of the detector for checking the centering of a radiation beam, nor in the IOERT field. In any case, by using a detector configured according to the first embodiment described in US 2009/0114823 it would not be possible to make measurements that allow to carry out the alignment of the beam with respect to the detector. A detector according to the second embodiment and constituted by a matrix of scintillators with high multiplicity would be able to measure with great spatial granularity the incident beam and therefore it could be configured to perform measurements useful to the centering of the beam; in this case, however, the device would require a large multiplicity of reading channels with consequent problems of complexity of the system, slow reading and bulk of the system.

Therefore, there is a need to suggest a system for measuring the ionizing radiations, which is able to obviate at least partially the above mentioned drawbacks .

In this regard, an object of the present invention is to suggest a ionizing radiation detector comprising a plurality of scintillators of plastic material, wherein said scintillators are in form of co-planar plates disposed according to a two-dimensional ordered arrangement, each plate having a respective optical fiber joined to a face or to a perimetral edge portion of the plate for conveying scintillation light pulses to a photodetector .

Thanks to a two-dimensional well-ordered arrangement of the plates, the measurement of the beam radiation which hits the detector is automatically spatially integrated on the single plates and conveyed on a limited number of fibers which can be read by using a few reading channels. Moreover, the reading electronic circuits can be sufficiently refined to provide a measure in real time with a great temporal resolution (for example of the order of a few tens of nanoseconds) .

This configuration is relevant when a spatial reconstruction with high resolution is not necessary and when the simplicity and the speed of the system are preferred. Many advantages are also provided during surgical operations . The detector can be very thin and compact, it requires no electric power supply and requires only few fibers to transmit outside the optical signal. The scintillator is also an equivalent tissue and does not alter the profile of the beam, has low cost and therefore can be disposable .

Another important point relates to the ease of use during a surgery. The device can be realized so as to be of simple constitution and so as to be easily inserted, together with the shielding plate, by the surgeon .

Although the present invention has been conceived in the context of the techniques of intraoperative radiotherapy with electrons, it is clear that it may find application in other therapeutic, surgical, diagnostic or laboratory fields, as well as in other fields where it is required to make observations on ionizing radiation beams by using simple and quick- reading devices.

Other preferred embodiments of the invention are disclosed in the dependent claims, which are intended as an integral part of the present description.

Further characteristics and advantages of the detector according to the invention will become more clear from the following detailed description of an embodiment of the invention, and with reference to the enclosed drawings, in which

- figure 1 is a schematic plan view of a detector according to the invention;

- figure 2 is a schematic view of a radiation beam having a circular geometry and directing perpendicularly to the detector of figure 1;

- figures 3 and 4 show two graphs representing the results of tests carried out on a plastic scintillator; and

- figures 5 to 7 show schematic plan views of possible other embodiments of the detector according to the invention.

With reference to figure 1, a preferred embodiment of the detector according to the invention is disclosed. The detector 10 is configured to be positioned along the trajectory of a ionizing radiation beam, in particular an electron beam, so as to intercept the beam, and is connected to a transduction element 20. The detector 10 comprises a support 11 and a plurality of scintillators 12 made of plastic material. The scintillators 12 are shaped in form of coplanar plates arranged in a two-dimensional ordered arrangement. According to an alternative embodiment (not shown) , the support is not present and the scintillators constitute a self-supporting structure.

When used in a treatment of intraoperative radiotherapy with electrons, the detector is associated with a shielding element. According to an embodiment of the invention, the shielding element can be constituted by the support 11 itself.

Each plate of the scintillator 12 has a respective optical fiber 13, in particular a Wavelength Shifter (WLS) fiber, which is coupled (for example glued) to a portion of the peripheral edge of the plate 12 for optically connecting the single plate to the transduction element 10. The transduction element 10 comprises a plurality of photodetectors , and is connected to an electronic processing and control unit to process the electrical signals provided by the photodetectors. According to another embodiment, the optical fiber may be joined to a face of the plate.

Each fiber 13 then collects the light pulses of the scintillation which are produced by the respective plate of the scintillator 12 as a result of the interaction of said plate with the ionizing radiation, and conveys said light pulses to the respective photodetector which then provides to their transduction into electrical signals.

In the embodiment of figure 1, the plates of the scintillator 12 are arranged according to a roundabout symmetry with respect to a center point x of the detector. In particular, the plates of the scintillator 12 are arranged in a roundabout symmetry of order 4 with respect to the central point x of the detector. More specifically, the plates of the scintillator 12 are shaped as circular sectors. In the above embodiment, said plates 12 are four.

As shown in figure 1, the optical fibers 13 which are respectively joined to the plates of the scintillator 12 are conducted along the same center line of the detector. This arrangement is advantageous because it allows to collect the fibers in a single bundle.

As part of the IOERT treatments, the detector 10 can be inserted in the human tissues together with the shielding plate and is able to provide a real-time measurement of the correct alignment of the beam.

The passage of an ionizing particle produces a fluorescence process in the organic molecules contained in the single scintillator plate, wherein the light emitted is proportional to the energy lost by the particle. The light collected by the fiber which is coupled with the plate of the scintillator is converted into an electrical signal by means of the respective photodetector. According to said application the plastic scintillator has the double advantage of not requiring any power supply and of having a density equal to the density of the water. The above mentioned properties make it suitable to be used for surgical operations without problems of electrical interference and without altering the beam dosimetric profile.

A detector suitable for IOERT applications may have a thickness equal to about one millimeter. The four plates, in the shape of circular sectors of ninety degrees, may have a radius typically from two to five centimeters, so as to form a disk or plate having a diameter from four to ten centimeters. The four fibers coupled with the four plates can be read independently by four separate photodetectors and placed in a remote position with respect to the scintillator. The electrical signal produced by each photodetector can be sampled by an analog-digital converter at a frequency of some tens of MHz. The signal amplitude is proportional to the energy lost by the particles that pass through the scintillator, and therefore is linked to their flow. The duration of a beam pulse of IOERT radiation therapy is typically of the order of one microsecond, and thanks to the speed of the reading electronic circuits it is possible to reconstruct the time pattern of said beam pulse. The integral of the signal corresponding to a beam pulse is proportional to the total energy lost in the scintillator, and therefore is proportional both to the fluence and to the absorbed dose. The acquisition system is thus able to check in real time each pulse of the accelerator and to store the entire sequence of delivered pulses for carrying out a possible later analysis.

At the beginning the detector is calibrated with the beam of the accelerator, by centering the applicator with respect to said detector: it is thus possible to establish the amount of the signal supplied by each circular sector in correspondence of a beam which is perfectly centered.

In clinical practice, the detector has the same dimensions of the shielding plate and is inserted above said plate before irradiation. By comparing the response during treatment with the calibration data, it is possible to know if the applicator is correctly centered with respect to the shielding plate.

In fact, since in the considered example the profile of the beam has a circular geometry, when the beam B is not centered as shown in figure 2, the responses of the four circular sectors 1, 2, 3, 4 are different. An analysis of the related responses also allows to know how the applicator should be moved in order to correct the centering. Note also that it is enough to have only one single beam pulse for doing the measurement, thus preventing release of high doses in the human tissues. Please also note that the sum of the signals of the four circular sectors is proportional to the flow and the sum of the integrals of the pulses is proportional to the fluence and therefore to the dose. When placed in the human tissues, the detector according to the invention is also able to provide a real-time measure of the absorbed dose . When placed at the output of the applicator, the detector according to the invention is able to give also a real-time measure of the delivered dose .

A prototype consisting of a single scintillator was inserted into a puppet of polymethylmethacrylate (PMMA) and tested on a IOERT beam accelerator at the Santa Maria- IRCCS Hospital based at Reggio Emilia. It has verified the possibility of temporally reconstructing the single pulse of the accelerator. In figure 3 the signal (in ADC counts) corresponding to one pulse beam sampled every 20 nsec is shown.

Since the integral of the pulse is proportional to the total energy released in the scintillator, by adding the integrals of all pulses it is possible to evaluate the total dose. Figure 4 shows the response of a calibrated ionization chamber, added on two thousand pulses, for different beam energies (to the left) , and the corresponding measurement of the scintillator (to the right) . It is clear from said figure the linearity of response of the detector and the analogy of the response with the response of the ionisation chamber. The detector, suitably calibrated, provides a measure of the dose in a water-equivalent material.

Figure 5 shows another possible geometric configuration of the scintillator plates. The elements which correspond to the elements of the embodiment which has been previously described have the same reference numbers. In the embodiment of figure 5 each single scintillator plate is square shaped.

Figure 6 shows a further embodiment, in which the plates of the scintillator 12 have the shape of circular crown sectors.

Figure 7 shows another embodiment, in which the plates of the scintillator 12 are arranged according to an arrangement of concentric circles. An inner area formed by two scintillators 12 having the shape of circular sectors of 180° is surrounded by an outer area formed by two scintillators 12 having the shape of circular crown sectors of 180°.

Claims

1. A ionizing radiation detector comprising a plurality of scintillators (12) of plastic material, characterised in that said scintillators (12) are in form of co-planar plates disposed according to a two- dimensional ordered arrangement, each plate having a respective optical fiber (13) joined to a face or to a perimetral edge portion of the plate for conveying scintillation light pulses to a photodetector (20) .

2. A detector according to claim 1, wherein the scintillator plates (12) are disposed according to an arrangement which is rotationally symmetric with respect to a central point (x) of the detector.

3. A detector according to claim 2, wherein the scintillator plates (12) are disposed according to an arrangement which is 4^th-order rotationally symmetric with respect to the central point (x) of the detector.

4. A detector according to claim 2 or 3 , wherein the scintillator plates (12) are in form of circular sectors or circular crown sectors.

5. A detector according to any of the preceding claims, wherein the scintillator plates (12) are four.

6. A detector according to any of claims 2 to 5, wherein the optical fibers (13) respectively joined to the scintillator plates (12) are guided along a same center line of the detector.

7. A detector according to any of the preceding claims, said detector being associated to a shielding element (11) to be used in a treatment of intraoperative electron radiotherapy.

8. A detector according to claim 7, wherein the detector is joined to the shielding element (11) .

9. A method for centering a ionizing radiation beam with a ionizing radiation detector (10) comprising a plurality of scintillators (12) of plastic material in form of co-planar plates disposed according to an arrangement which is rotationally symmetric with respect to a central point (x) of the detector, each scintillator plate (12) having a respective optical fiber (13) joined to a face or to a perimetral edge portion of the plate for conveying scintillation light pulses to a photodetector (20) , said method comprising the following steps:

- disposing the detector (10) so that a desired beam orientation axis intersects with known angle the detector (10) at the central point (x) thereof;

receiving from each scintillator plate (12) a respective signal indicative of the energy deposited by the ionizing radiation beam in that plate;

- determining a signal difference between the signal received from the individual scintillator plate (12) and a reference signal relating to a condition wherein the ionizing radiation beam is centered with respect to the central point (x) of the detector and incident with said known angle, and

- correcting the path of the beam on the basis of said signal difference.