A Multichannel Dosimeter Based on Scintillating Fibers for Medical Applications

Transcription

1 A Multichannel Dosimeter Based on Scintillating Fibers for Medical Applications K.-H. Becks 1, J. Drees 1, K. Goldmann 1, I.M. Gregor 1 2, M. Heintz, Arndt Roeser* 1 Fachbereich Physik, Bergische Universität-Gesamthochschule Wuppertal D Wuppertal, Germany 2 Klinische Strahlenphysik, Universitätsklinikum Essen D Essen, Germany *Klinikum Wuppertal GmbH, Klinik f. Strahlentherapie u. Radio-Onkologie D Wuppertal, Germany ABSTRACT Dosimetry with plastic scintillators is an interesting alternative for the measurement of the absorbed dose. A scintillator does barely disturb the radiation field due to its mass absorption coefficients which is water-equivalent in a wide range of energies. Furthermore, plastic scintillation dosimeters provide a fast and direct reading of the measured value combined with a high spatial resolution. In the set-up described so far, the light produced by the scintillators was transported via light-guides to single-channel or multi-channel photomultipliers to be transformed into an electric current read out by pico-amperemeters. The use of photomultipliers becomes expensive and complicated for a dosimeter system with many parallel channels. For such applications, an image intensifier coupled to a CCD is a simpler approach which can read out some 80 fibers in parallel [1] [2]. In this note, the design and the first successful measurements of a prototype set-up are described. Keywords: dosimetry, multichannel, scintillating fibers, brachy therapy, teletherapy 1 Introduction There exist two radiation methods for cancer treatment, brachytherapy and tele therapy [2]. The common method is the teletherapy, a treatment with radiation from the outside (e.g. 60 Co source, linear accelerator). Detailed mapping of the used radiation fields in 2 or 3 dimensions is desirable for the dosimetry. These measurements are usually done with a single ionisation chamber. In this case it is only possible to measure one field position at a time. For a detailed field mapping too much time is needed. With the developed multichannel dosimeter direct and simultaneous reading of the absorbed doses in several field points becomes possible.

2 Another radiation treatment is the brachytherapy. For example, in the eye plaque treatment an applicator with a radioactive source is applied directly to the eye (see figure 1). By changing the form of the applicator the field of the emitted radiation can be influenced. Therefore high field gradients have to be measured without disturbing the physical conditions. A high spatial resolution of the detector is as well necessary. Plastic scintillators can be used as dosimeters because they emit visible light proportional to the absorbed electron and gamma dose rate [3][5]. A scintillator does not disturb the radiation field due to its mass absorption coefficient and its mass stopping power which are water-equivalent in a wide range of energies. Furthermore, plastic scintillation dosimeters provide a fast and direct reading of the measuring values combined with a high spatial resolution. Depending on the spatial resolution needed, the scintillators can be as small as 1 mm 3 or less. The light produced by the scintillators is transported through clear optical plastic fibers. In a set-up discussed by Flühs et al. [3] and by Beddar et al. [5], these fibers are read out by single-channel or multi-channel photomultipliers. For applications where a small number of channels is required, they are the ideal light-to-current converters. For set-ups with more than channels, however, the electronic expenditure to measure the currents is large. The parallel multi-amperemeter systems required for that purpose are commercially not available. An image intensifier coupled to a CCD is an alternative approach. Such a system is easier to handle. Only some supply electronics and a frame grabber card for the data acquisition by a PC are needed. In the following, we describe a system which allows to read out up to 80 detectors in parallel. Using such a multi-channel dosimeter, all necessary basic functionality tests were performed as well as some studies in a clinical environment. 2 The Set-Up of the Multichannel-Dosimeter The described dosimeter will consist altogether of 80 channels, 40 of these channels with a scintillator (1 mm 3 ) 1 and an optical fiber 2. The remaining 40 channels are without scintillator. They will be used for the suppression of the Čerenkov light background produced in the light guiding fibers. The read-out system (see figure 2) is set up in such a way that the incoming light guides are kept at fixed positions by a mask to ensure a good optical contact between fibers and the image intensifier. The image intensifier transforms the light from all 80 fibers into electrons, amplifies them in parallel, transforms them back into photons and re-emits an amplified image of the incoming light signals. The image intensifier keeps both, spatial resolution, i.e. the ability to distinguish between the individual fibers, and the relative intensity of each fiber s light, i.e. the amplification is linear [1] [2]. The image emitted by the image intensifier is transported via an optical fiber taper to a CCD. This fiber taper is a conical optical light guide consisting of a large number of thin glass fibers. The diameter of these fibers all decrease towards one end of the taper, so an image transmitted by the taper will be reduced in size, but its relative spatial resolution will be maintained. The CCD integrates the light signal of each of the 80 fibers in 1 BC 400 from Bicron Technology 2 CWKF E from Cunz Toray 2

3 parallel over 20 millisecond. The image is transformed into standard TV format and read out by a personal computer equipped with a frame grabber 3, an extension card for a PC converting a video signal into digital information. To increase the precision, the digital signal can be summed up in the PC over several integration periods of the CCD - typically between a few hundred and a few thousand corresponding to total measurement times of one to five minutes. The dynamic range of the multi-channel dosimeter is determined by two factors. For an individual reading, it is given by the maximum signal the CCD can handle without saturation, and the minimum signal that can be distinguished from the background noise. These limitations give a dynamic range of Furthermore, from one measurement to another, the amplification of the multi-channel dosimeter s image intensifier can be adjusted over a range of The smallest detectable signals are in the order of a few hundred photons per second emerging from a fiber of 1 mm diameter. The amount of radiation equivalent to this signal depends on the size of the scintillator coupled to the fiber, the transmission characteristics of the fiber, and the background noise. 3 Measurements To test the system, depth dose measurements performed with a ionisation chamber were compared to measurements performed with a 10-channel prototype of the described multichannel dosimeter. The used source of radiation was a Siemens linear accelerator at the Clinic for Radiation Therapy, Radio Oncology and Nuclear Medicine in Wuppertal. The acceleration bias was adjustable from 6 MV to 18 MV. Measurements at different acceleration biases were performed. The depth dose distribution is the dose distribution in a body or in a phantom along the central beam. Mostly relative or proportional depth doses are indicated, which refer to the absorbed dose maximum or an indicated reference depth. The so called build up effect describes an increase of the absorbed dose, the passing through a maximum and the following dose decline. For photons this build up effect is due to secondary electrons, which are generated by photoelectric effect, Compton-effect, or pair production depending upon the photon energy. Starting from a photon energy of 0,6 MeV the Comptoneffect outweighs in water. The secondary electrons move in the same direction as the primary beam and increase the absorbed dose up to a depth, which corresponds to the range of the secondary electrons. The dose decline is caused by the attenuation of the photon beams. From a certain depth the curve follows approximately an exponential function. The slope of the dose decline depends on the quality of the radiation, the field size and the divergence of the jet bundle [4]. This progression is shown in figure 3 and figure 4. In the described measurements a field size of 5 cm 5 cm was used. In a plate made of rigid water (RW3 4 ) two out of x10 fibers were placed in a drilling for this purpose. The fibers were positioned in such way, that the tips with the scintillating material were 3 National Instruments PCI a material, which has the same absorption coefficient as water 3

4 situated in the iso center 5. One of the three fibers was a reference fiber without scintillator. Therefore it was possible to measure the background and the signal simultaneously. The background has been subtracted from the measured values afterwards. In order to suppress the Čerenkov light background. Further RW3 plates were subsequently placed onto the previous and measurements were repeated until a depth of 20 cm was achieved. As comparison, the measurements were repeated under same conditions with an ionization chamber, calibrated for the same energy region. The determined values are shown in figure 3. Only two out of ten fibres are shown exemplarily. The build up effect is more strongly pronounced with the fibers than with the ionization chamber, which is to due to the characteristics of the fibers. Figure 3 depicts the depth dose distribution measured at 6MV with an ionisation chamber and the prototype multichannel dosimeter. In larger depths than the maximum of the build up effect, the gradient of the fiber data corresponds to the reference data. In smaller depths the curves of the fibers are clearly steeper than those of the reference data. Due to their physical characteristics the scintillators do not influence the physical environment. However the photons are more influenced by the gas volume of the ionization chamber, the gas and the material of the chamber cause a modification of the environment. The depth dose distribution at an accelerating voltage of 18 MV is shown in figure 4. Here a steeper rise of the measured values for smaller depths is being detected for the same reasons as described above. However the gradient of the fiber values does not correspond to those of the ionization chamber. The depth dose distribution of the fibers are substantially steeper, than those of the ionization chamber. The assumption is that this is due to the coupling of the optical fibers to the opto-electronic read-out. The losses may be explained by a different quality of the optical coupling between fiber ends and the image intensifier. Because of the different manufacturing quality of the fiber ends, the losses during the transition diverse slightly and therefore different Čerenkov light background is registered in the reference fiber as actually occur in the other fibers. In higher energy regions this effect becomes more apparent. Therefore one should take more care when preparing the fibers for the coupling to the image intesifier. A reference fiber for each scintillating fiber would improve the background reduction. Measuring the Čerenkov light background of each fiber before attaching the scintillator would indicate the fluctuations within the manufacturing of the fiber ends. It is possible to reduce the Čerenkov light background further more by using longer light guides. The attenuation of the guides is wavelength depending. The Čerenkov light will be more attenuated in some wavelenth ranges than the information signal. All this will be done for a further prototype before taking it into clinical environment. 4 Conclusion and Future Work In conclusion, we have developed a new parallel read-out system for dosimetry with plastic scintillators, plastic light guides, an image intensifier coupled to a CCD and read out by a PC. This system combines the advantages of the plastic scintillator dosimetry, such as water-equivalence, direct reading and high spatial resolution, with a parallel, computer-controlled read-out of the measured signals. Further advantages over com- 5 intersection of the central beam with the rotation axis of the irradiation device 4

5 monly used systems are its intrinsic simplicity, the high number of channels that can be read out in parallel with negligible cross-talk and in its low cost per channel. Although no long term test of the entire system has been performed under clinical conditions. Yet, the first experiences are very encouraging. Depth dose measurements with a prototye of this dosimeter were performed at different particle energies and compared to measurments with an ionisation chamber. For lower energies the results agree with each other. The uncertainty due to Čerenkov light can be reduced by using reference fibers. At the prototype dosimeter problems with the coupling of the light guides to the read out system appeared. For a further prototype this coupling has to be optimised by cutting the fibers more carful and polishing the ends finer. Also it is possible to reduce the Čerenkov light by using about 10 m long light guides. Due to the wavelength depending attenuation of the light guides the Čerenkov light will be more attenuated in some wavelength ranges than the information signal. When applying the dosimeter in the teletherapy it is also necessary to operate the read out electronic in a sufficient large distance to the radiation source. This can automatically be ensured by the proposed long light guides. Further studies will include long time measurements in the teletherapy with a new prototype, an individual calibration of each channel, and user friendly software tools for the operating of this system. References [1] M. Garg, Konzeption, Aufbau und Test eines Vielkanal-Dosimeters mit szintillierenden Fasern, WUB DIS 96 17, Universität Wuppertal, 1996 [2] I.M. Gregor, Ein Vielkanaldosimeter mit szintillierenden Fasern Klinische Messungen bei Energien oberhalb 1 MeV, WU D 98 14, Universität Wuppertal, 1998 [3] D. Flühs, M. Heintz, F. Indenkämpen, C. Wieczorek, H. Kolanoski and U. Quast, Direct reading measurement of absorbed dose with plastic scintillators - The general concept and applications to ophthalmic plaque dosimetry, Med. Phys. 23 (1996), pages [4] H. Reich, Dosimetrie ionisierender Strahlung, B.G. Teubner, Stuttgart, 1990 [5] A.S. Beddar, T.R. Mackie, and F.H. Attix, Water-equivalent plastic scintillation detectors for high-energy beam dosimetry: 1. Physical characteristics and theoretical considerations, Phys. Med. Biol. 37 (1992), pages