Radiation Protection Dosimetry (2014), Vol. 161, No. 1–4, pp. 201 –204 Advance Access publication 15 January 2014
doi:10.1093/rpd/nct355
A RING-SHAPED RECOMBINATION CHAMBER FOR HADRON THERAPY DOSIMETRY E. Jakubowska1, *, M. Zielczyn´ski2, N. Golnik1, M. A. Gryzin´ski2 and Ł. Krzemin´ski1 1 Institute of Metrology and Biomedical Engineering, Warsaw University of Technology, S´w. A. Boboli 8, Warsaw 02-525, Poland 2 National Centre for Nuclear Research, A. Sołtana 7, Otwock 05-400, Poland *Corresponding author: [email protected]
INTRODUCTION Proton and ion therapy beams generate complex radiation fields, with neutrons and scattered protons being the largest contributors to the out-of-field doses in the vicinity of irradiated organs. This includes an extra neutron dose generated by proton therapy machines, proton line with collimator and impossible to avoid secondary radiation produced in the patient’s body(1, 2). Determination of the dose absorbed in the tissues surrounding the irradiated area is important for the estimation of risk associated with the therapy, especially when critical organs are located close to the Bragg peak but outside the beam. For example, in proton eye therapy, the optic nerve is near the irradiated area (0.5 4 3 cm). Due to a set-up error, possible microscopic extension and motion during treatment, the treatment field is extended by a 1.5-mm margin, which may cover the optic nerve field. Information on the absorbed dose should be completed by estimation of parameters characterising radiation quality of the radiation field, either in terms of particle fluence and energy or in terms of parameters dependent on dose versus LET distribution, D(L). In case of eye therapy, information on the absorbed dose and LET-dependent radiation quality parameters in optic nerve is almost as important as knowing the dose inside the radiation fields. The issue of secondary neutrons produced by radiotherapeutic proton beams has recently attracted much research and measurements(3 – 9) using a number of different dosimetric techniques. The measurements are generally difficult, as it is hard to measure high-energy neutron doses in a mixed radiation field, and it is even more difficult to make neutron measurements in realistic anthropomorphic phantoms. Also calculation of dose from stray radiation is computationally
complex, expensive and has only recently become available for proton therapy(10 – 12). The use of tissue-equivalent recombination chambers in dosimetry for radiation therapy(13 – 15) makes it possible to determine not only the absorbed dose but also recombination index of radiation quality(16) Q4, or D(L) distribution, if the recombination microdosimetric method (RMM)(17) is applied. Recombination chambers should operate under conditions of initial recombination of ions, because this kind of recombination does not depend on dose rate and makes the chamber response dependent on LET. Measurements of ionisation current are performed at few different voltages, preferably also at low voltages. At high dose rates, volume recombination may also become considerably high, especially at low voltages. It influences the operational range of the chambers, which depends, in a rather complex way, on the distance between chamber electrodes, filling gas pressure and polarising voltage applied to the chamber. In order to limit the volume recombination of ions, the chambers, which are designed for characterisation of radiation therapy beams, are supposed to have very small distance between electrodes. Usually, this is associated with small gas volume and low sensitivity. Therefore, they cannot be used for the measurements outside the beam. On the other hand, the chambers for radiation protection dosimetry have a sufficiently large gas volume and high sensitivity. However, the large sizes of these chambers prevent them from placing near the beam in order to determine the absorbed dose at small distances from the irradiated organ. A new recombination chamber described here, and denoted as KP-1, has been specially designed for dosimetric measurements at proton eye therapy facility. The chamber is ring shaped, so that the
# The Author 2014. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
Downloaded from http://rpd.oxfordjournals.org/ at Purdue University Libraries ADMN on July 3, 2015
An innovative recombination chamber has been designed for estimation of stray radiation doses and quality factors in hadron therapy. The chamber allows for determination of absorbed dose and recombination index of radiation quality in phantoms at small distances from simulated organs. The chamber body and electrodes are ring shaped, so the beam may be directed through the empty centre of the ring. The ionisation of the filling gas is caused by secondary or scattered radiation and can be related to the dose absorbed in the tissues close to the irradiated target volume.
E. JAKUBOWSKA ET AL.
proton beam can be directed through the empty centre of the ring, and the outer part of the chamber registers only the signal from secondary particles and radiation background. There is an option to place an eye phantom inside the ring, in order to simulate the realistic conditions of therapeutic irradiation. In that case, the chamber simulates the area surrounding the eye. During the measurements, the chamber is placed on the therapeutic chair with the axis along the direction of the therapeutic beam. The eye phantom should be fixed in therapeutic position.
Recombination chambers designed so far were either parallel-plate or cylindrical(13) with a rod-shaped central electrode. In the ring-shaped chamber discussed in this work, both bias and measuring electrodes as well as outer and inner body are cylindrical and form a coaxial ring with empty volume inside (see Figure 1). The 1.5-mm-thick electrodes are made of polypropylene. The electrode surfaces forming the gas cavity are covered with graphite. The distance between electrodes is 2 mm. The length of the active volume is 82 mm and the internal diameter of the ring is 25 mm, corresponding to small treatment fields used in proton therapy of the eye, which are typically of 10– 15 mm in diameter. The chamber is filled with ethane up to 500 kPa. This high gas pressure is typical for recombination chambers and ensures that the conditions of dominating initial recombination of ions are fulfilled over the whole range of expected dose-rate values.
PRINCIPLE OF OPERATION The chamber is placed at the beam line in such a way that the beam is directed to the inside of the ring, along the chamber axis. A phantom of an irradiated organ can be placed in the ring, at a chosen distance from its edge. It is also possible to place the whole chamber in a large, liquid-filled phantom. External and secondary radiation created in the phantom material, penetrated by the beam, reaches the active volume surrounding the beam and ionises the gas. If the chamber is polarised with sufficiently high voltage and operates in saturation mode, then the collected charge is a measure of the absorbed dose in gas and can be related to absorbed dose in tissue or water. In order to determine the recombination index of radiation quality Q4, the ionisation current (or electrical charge collected in certain time) should be registered at two polarising voltages—the high voltage, the same as for the measurements of the absorbed dose, and a specially chosen lower voltage UR, called recombination voltage(16). It is recommended that the ionisation current should be measured at both positive and negative polarizations of the voltages and the average value should be taken for further considerations. Then, Q4 is calculated as follows: Q4 ¼
1 f ðUR Þ ; 0:04
ð1Þ
where f (UR) is ion collection efficiency measured at the voltage UR. The index Q4 is mostly used in radiation protection, as its values, measured with recombination chamber of appropriate mass and electrode thickness, are similar to the values of radiation quality factor and can be used for the determination of dose equivalent values. Here, it is considered as an indicator of radiation quality averaged over the active volume of the chamber. TEST MEASUREMENTS
Figure 1. 3D cross section of the ring-shaped recombination chamber KP-1.
The chamber was calibrated in a reference radiation field of 137Cs gamma-radiation source in the calibration laboratory of the National Centre for Nuclear Research (NCBJ). It was assumed, on the basis of earlier investigations(13), that the relative neutron sensitivity (kT) (ratio of the chamber sensitivity to neutrons and sensitivity to gamma-rays used in calibration) is 1. Test measurements were performed at the Institute of Nuclear Physics (IFJ PAN) in Cracow. The main
202
Downloaded from http://rpd.oxfordjournals.org/ at Purdue University Libraries ADMN on July 3, 2015
RING-SHAPED RECOMBINATION CHAMBER
Ethane was chosen as filling gas in order to ensure that the chambers would have similar sensitivity to neutron- and gamma-radiation.
RING-SHAPED RECOMBINATION CHAMBER
RESULTS The most important indicator of the correct operation of the chamber was a good correlation of the chamber readings with the beam monitor readings. As mentioned earlier, the first series of measurements were performed without the use of the eye
Figure 2. KP-1 chamber mounted at the ocular proton therapy beam line at IFJ PAN in Cracow.
phantom. The measured ionisation current is proportional to the monitor with accuracy of+1.5 %, except for the beginning of the measurements, where some deviation from the proportionality was caused by the expected initial instability of the measuring system (see, for example, Figure 3). The main source of small deviations from the proportionality is the integration time constant of the chamber and measuring system. The values of the ionisation current averaged over the time of at least 30 s are proportional to the indications of the monitor with accuracy of .0.5 %. Time of measurements was set to 214 s due to limitations of the irradiation system. The next series of measurements were performed with the eye phantom placed inside the chamber. The phantom was fixed in the therapeutic position, but the chamber was moved along the beam axis. Three measuring positions were used—with the phantom at the front of the chamber, at the centre and finally at the rear end of the chamber. Ionisation current measured with the phantom in the centre of the chamber was 1.5 times higher than the current measured without phantom (see Table 1). The highest value of the ionisation current was recorded for the position of the chamber being shifted towards the elements of the beam line (eye phantom at the rear of the chamber) and the lowest when the chamber was moved back from the line. Changes of the chamber readings with the distance follow rather well the inverse squares rule. These results confirm earlier observations(18) that the majority of secondary neutrons is produced on the beam line. The value of the absorbed dose in the vicinity of the eye phantom, estimated on the basis of actual preliminary calibration of the chamber, was 2 mGy per treatment course, i.e. 0.0033 % of the 60 Gy dose delivered to the tumour in the eye. The value concerns the situation when the phantom was placed in the centre of the chamber.
Figure 3. Comparison of the chamber readings (diamonds, left axis) with the readings of the beam monitor (squares, arbitrary units). The ratio of the measured values to the monitor (triangles) has nearly constant value as indicated on the right vertical axis.
203
Downloaded from http://rpd.oxfordjournals.org/ at Purdue University Libraries ADMN on July 3, 2015
goal of the tests was a qualitative check of the chamber performance. The radiotherapy facility at IFJ PAN, equipped with cyclotron AIC-144, generates a proton beam of energy 60 MeV passively scattered on a single scattering foil made of Tantalum(9). The set-up mounted along the beam line includes beam-forming and monitoring elements, range shifter and range modulator (Figure 2). During the measurements, the chamber was attached to the treatment chair. The distance between the end of the proton line (collimator) and the centre of the phantom was 10 cm. The beam was directed to the centre of the chamber ring. Irradiation time was 214 s. The measurements started 20 s before and ended 20 s after the beam closing. The beam size was 10 mm in diameter, and the Bragg peak was broadened to 5 mm. Tests of the chamber were performed first without and then with the use of the eye phantom inside the ring. The phantom was in the position of eye in a real treatment. It was placed on the treatment chair with precise pinpointing, using a laser positioning system, the same that is used for radiation therapy. In three series of the measurements with the phantom, the chamber was moved along the beam, i.e. the position of the Bragg peak was shifted relatively to the centre of the chamber. Such measurements allowed for checking the change of the chamber’s signal depending on its position towards the end of the beam line. The tests could indicate the proportion of the signals caused by neutrons generated in the eye phantom and in the optical line and were considered as the most useful for the first check of the chamber at the therapeutic beam.
E. JAKUBOWSKA ET AL. Table 1. Ionisation currents of the KP-1 chamber measured for different phantom positions at proton radiotherapy facility in IFJ PAN. Position of the eye phantom
Ionisation current [pA]
Without phantom Front of the chamber Centre of the chamber Rear of the chamber
60.8 57.6 89.9 122.8
CONCLUSIONS A recently designed and constructed ring-shaped recombination chamber successfully passed first operational tests. The measurements performed on a proton eye therapy beam line showed potential usefulness of the chamber for determination of the scattered radiation dose in the vicinity of the beam. In the future, the chamber can be applied both for patient dosimetry and for monitoring changes in the scattered radiation field, associated with different configurations of the elements forming the proton beam line. FUNDING The work was partly supported by National Science Centre, for project ‘Recombination dose meter of new generation for exposure assessment on workplaces in radiation fields of reactors and accelerators’ [grant number 1350/B/P01/2010/39] and by Warsaw University of Technology, the Faculty of Physics as a part of Human Capital Operational Programme nr 4.1.1 ( project title: Preparing and realization of medical physics faculty). REFERENCES 1. Brenner, D. J. and Hall, E. J. Secondary neutrons in clinical proton radiotherapy: a charged issue. Radiother. Oncol. 86, 165 –170 (2008). 2. Xu, X. G., Bednarz, B. and Paganetti, H. A review of dosimetry studies on external-beam radiation treatment with respect to second cancer induction. Phys. Med. Biol. 53, 193 –241 (2008).
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The measurements of the recombination index of radiation quality Q4, with the phantom in the centre of the chamber, resulted in a value of 2.5. This, relatively low, value of the index suggests that there was a considerable contribution of low-LET radiation produced when protons stopped in the collimator, from scattered primary protons and prompt gammaradiation.
3. Agosteo, S., Birattari, C., Caravaggio, M., Silari, M. and Tosi, G. Secondary neutron and photon dose in proton therapy. Radiother. Oncol. 48, 293–305 (1998). 4. Newhauser, W. D., Burns, J. and Smith, A. R. Dosimetry for ocular proton beam therapy at the Harvard Cyclotron Laboratory based on the ICRU Report 59. Med. Phys. 29, 1953–1961 (2002). 5. Yan, X., Titt, U., Koehler, A. M. and Newhauser, W. D. Measurement of neutron dose equivalent to proton therapy patients outside of the proton radiation field. Nucl. Instrum. Meth. Phys. Res. A. 476, 429 – 434 (2002). 6. Tayama, R., Fujita, Y., Tadokoro, M., Fujimaki, H., Sakae, T. and Terunuma, T. Measurement of neutron dose distribution for a passive scattering nozzle at the Proton Medical Research Center (PMRC). Nucl. Instrum. Meth. Phys. Res. A. 564, 532–536 (2006). 7. Wroe, A., Rosenfeld, A. and Schulte, R. Out-of-field dose equivalents delivered by proton therapy of prostate cancer. Med. Phys. 34, 3449–3456 (2007). 8. Moyers, M. F., Benton, E. R., Ghebremedhin, A. and Coutrakon, G. Leakage and scatter radiation from a double scattering based proton beamline. Med. Phys. 35, 128– 144 (2008). 9. Kim, D. W., Chung, W. K., Shin, J., Lim, Y. K., Shin, D., Lee, S. B., Yoon, M., Park, S-Y., Shin, D. O. and Cho, J. K. Secondary neutron dose measurements for proton eye treatment using an eye snout with a borated neutron absorber. Radiat. Oncol. 8, 182 (2013). 10. Jiang, H., Wang, B., Xu, X. G., Suit, H. D. and Paganetti, H. Simulation of organ-specific patient effective dose due to secondary neutrons in proton radiation treatment. Phys. Med. Biol. 50, 4337– 4153 (2005). 11. Polf, J. C. and Newhauser, W. D. Calculations of neutron dose equivalent exposures from range-modulated proton therapy beams. Phys. Med. Biol. 50, 3859–3873 (2005). 12. Zheng, Y., Newhauser, W., Fontenot, J., Taddei, P. and Mohan, R. Monte Carlo study of neutron dose equivalent during passive scattering. Phys. Med. Biol. 52(15), 4481–4496 (2007). 13. Golnik, N. Recombination methods in the dosimetry of mixed radiation. Poland, IAE-20/A, ISSN 1232–5317. Report of the Institute of Atomic Energy (1996). 14. Golnik, N. Review of recent achievements of recombination methods. Radiat. Prot. Dosim. 70, 211– 214 (1997). 15. Zielczyn´ski, M., Golnik, N., Gryzin´ski, M. A. and Tulik, P. The use of recombination chambers at radiation therapy facilities. Radiat. Meas. 45, 1472– 1475 (2010). 16. Zielczynski, M. and Golnik, N. Recombination index of radiation quality—measuring and applications. Radiat. Prot. Dosim. 52, 419–422 (1994). 17. Golnik, N. and Zielczyn´ski, M. Determination of restricted LET distribution for mixed (n,g) radiation fields by high pressure ionization chamber. Radiat. Prot. Dosim. 52, 35– 38 (1994). 18. Cywicka-Jakiel, T., Stolarczyk, L., Swakon´, J., Olko, P. and Waligo´rski, M. P. R. Individual patient shielding for a proton eye therapy facility. Radiat. Meas. 45, 1127–1129 (2010).
doi:10.1093/rpd/nct355
A RING-SHAPED RECOMBINATION CHAMBER FOR HADRON THERAPY DOSIMETRY E. Jakubowska1, *, M. Zielczyn´ski2, N. Golnik1, M. A. Gryzin´ski2 and Ł. Krzemin´ski1 1 Institute of Metrology and Biomedical Engineering, Warsaw University of Technology, S´w. A. Boboli 8, Warsaw 02-525, Poland 2 National Centre for Nuclear Research, A. Sołtana 7, Otwock 05-400, Poland *Corresponding author: [email protected]
INTRODUCTION Proton and ion therapy beams generate complex radiation fields, with neutrons and scattered protons being the largest contributors to the out-of-field doses in the vicinity of irradiated organs. This includes an extra neutron dose generated by proton therapy machines, proton line with collimator and impossible to avoid secondary radiation produced in the patient’s body(1, 2). Determination of the dose absorbed in the tissues surrounding the irradiated area is important for the estimation of risk associated with the therapy, especially when critical organs are located close to the Bragg peak but outside the beam. For example, in proton eye therapy, the optic nerve is near the irradiated area (0.5 4 3 cm). Due to a set-up error, possible microscopic extension and motion during treatment, the treatment field is extended by a 1.5-mm margin, which may cover the optic nerve field. Information on the absorbed dose should be completed by estimation of parameters characterising radiation quality of the radiation field, either in terms of particle fluence and energy or in terms of parameters dependent on dose versus LET distribution, D(L). In case of eye therapy, information on the absorbed dose and LET-dependent radiation quality parameters in optic nerve is almost as important as knowing the dose inside the radiation fields. The issue of secondary neutrons produced by radiotherapeutic proton beams has recently attracted much research and measurements(3 – 9) using a number of different dosimetric techniques. The measurements are generally difficult, as it is hard to measure high-energy neutron doses in a mixed radiation field, and it is even more difficult to make neutron measurements in realistic anthropomorphic phantoms. Also calculation of dose from stray radiation is computationally
complex, expensive and has only recently become available for proton therapy(10 – 12). The use of tissue-equivalent recombination chambers in dosimetry for radiation therapy(13 – 15) makes it possible to determine not only the absorbed dose but also recombination index of radiation quality(16) Q4, or D(L) distribution, if the recombination microdosimetric method (RMM)(17) is applied. Recombination chambers should operate under conditions of initial recombination of ions, because this kind of recombination does not depend on dose rate and makes the chamber response dependent on LET. Measurements of ionisation current are performed at few different voltages, preferably also at low voltages. At high dose rates, volume recombination may also become considerably high, especially at low voltages. It influences the operational range of the chambers, which depends, in a rather complex way, on the distance between chamber electrodes, filling gas pressure and polarising voltage applied to the chamber. In order to limit the volume recombination of ions, the chambers, which are designed for characterisation of radiation therapy beams, are supposed to have very small distance between electrodes. Usually, this is associated with small gas volume and low sensitivity. Therefore, they cannot be used for the measurements outside the beam. On the other hand, the chambers for radiation protection dosimetry have a sufficiently large gas volume and high sensitivity. However, the large sizes of these chambers prevent them from placing near the beam in order to determine the absorbed dose at small distances from the irradiated organ. A new recombination chamber described here, and denoted as KP-1, has been specially designed for dosimetric measurements at proton eye therapy facility. The chamber is ring shaped, so that the
# The Author 2014. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
Downloaded from http://rpd.oxfordjournals.org/ at Purdue University Libraries ADMN on July 3, 2015
An innovative recombination chamber has been designed for estimation of stray radiation doses and quality factors in hadron therapy. The chamber allows for determination of absorbed dose and recombination index of radiation quality in phantoms at small distances from simulated organs. The chamber body and electrodes are ring shaped, so the beam may be directed through the empty centre of the ring. The ionisation of the filling gas is caused by secondary or scattered radiation and can be related to the dose absorbed in the tissues close to the irradiated target volume.
E. JAKUBOWSKA ET AL.
proton beam can be directed through the empty centre of the ring, and the outer part of the chamber registers only the signal from secondary particles and radiation background. There is an option to place an eye phantom inside the ring, in order to simulate the realistic conditions of therapeutic irradiation. In that case, the chamber simulates the area surrounding the eye. During the measurements, the chamber is placed on the therapeutic chair with the axis along the direction of the therapeutic beam. The eye phantom should be fixed in therapeutic position.
Recombination chambers designed so far were either parallel-plate or cylindrical(13) with a rod-shaped central electrode. In the ring-shaped chamber discussed in this work, both bias and measuring electrodes as well as outer and inner body are cylindrical and form a coaxial ring with empty volume inside (see Figure 1). The 1.5-mm-thick electrodes are made of polypropylene. The electrode surfaces forming the gas cavity are covered with graphite. The distance between electrodes is 2 mm. The length of the active volume is 82 mm and the internal diameter of the ring is 25 mm, corresponding to small treatment fields used in proton therapy of the eye, which are typically of 10– 15 mm in diameter. The chamber is filled with ethane up to 500 kPa. This high gas pressure is typical for recombination chambers and ensures that the conditions of dominating initial recombination of ions are fulfilled over the whole range of expected dose-rate values.
PRINCIPLE OF OPERATION The chamber is placed at the beam line in such a way that the beam is directed to the inside of the ring, along the chamber axis. A phantom of an irradiated organ can be placed in the ring, at a chosen distance from its edge. It is also possible to place the whole chamber in a large, liquid-filled phantom. External and secondary radiation created in the phantom material, penetrated by the beam, reaches the active volume surrounding the beam and ionises the gas. If the chamber is polarised with sufficiently high voltage and operates in saturation mode, then the collected charge is a measure of the absorbed dose in gas and can be related to absorbed dose in tissue or water. In order to determine the recombination index of radiation quality Q4, the ionisation current (or electrical charge collected in certain time) should be registered at two polarising voltages—the high voltage, the same as for the measurements of the absorbed dose, and a specially chosen lower voltage UR, called recombination voltage(16). It is recommended that the ionisation current should be measured at both positive and negative polarizations of the voltages and the average value should be taken for further considerations. Then, Q4 is calculated as follows: Q4 ¼
1 f ðUR Þ ; 0:04
ð1Þ
where f (UR) is ion collection efficiency measured at the voltage UR. The index Q4 is mostly used in radiation protection, as its values, measured with recombination chamber of appropriate mass and electrode thickness, are similar to the values of radiation quality factor and can be used for the determination of dose equivalent values. Here, it is considered as an indicator of radiation quality averaged over the active volume of the chamber. TEST MEASUREMENTS
Figure 1. 3D cross section of the ring-shaped recombination chamber KP-1.
The chamber was calibrated in a reference radiation field of 137Cs gamma-radiation source in the calibration laboratory of the National Centre for Nuclear Research (NCBJ). It was assumed, on the basis of earlier investigations(13), that the relative neutron sensitivity (kT) (ratio of the chamber sensitivity to neutrons and sensitivity to gamma-rays used in calibration) is 1. Test measurements were performed at the Institute of Nuclear Physics (IFJ PAN) in Cracow. The main
202
Downloaded from http://rpd.oxfordjournals.org/ at Purdue University Libraries ADMN on July 3, 2015
RING-SHAPED RECOMBINATION CHAMBER
Ethane was chosen as filling gas in order to ensure that the chambers would have similar sensitivity to neutron- and gamma-radiation.
RING-SHAPED RECOMBINATION CHAMBER
RESULTS The most important indicator of the correct operation of the chamber was a good correlation of the chamber readings with the beam monitor readings. As mentioned earlier, the first series of measurements were performed without the use of the eye
Figure 2. KP-1 chamber mounted at the ocular proton therapy beam line at IFJ PAN in Cracow.
phantom. The measured ionisation current is proportional to the monitor with accuracy of+1.5 %, except for the beginning of the measurements, where some deviation from the proportionality was caused by the expected initial instability of the measuring system (see, for example, Figure 3). The main source of small deviations from the proportionality is the integration time constant of the chamber and measuring system. The values of the ionisation current averaged over the time of at least 30 s are proportional to the indications of the monitor with accuracy of .0.5 %. Time of measurements was set to 214 s due to limitations of the irradiation system. The next series of measurements were performed with the eye phantom placed inside the chamber. The phantom was fixed in the therapeutic position, but the chamber was moved along the beam axis. Three measuring positions were used—with the phantom at the front of the chamber, at the centre and finally at the rear end of the chamber. Ionisation current measured with the phantom in the centre of the chamber was 1.5 times higher than the current measured without phantom (see Table 1). The highest value of the ionisation current was recorded for the position of the chamber being shifted towards the elements of the beam line (eye phantom at the rear of the chamber) and the lowest when the chamber was moved back from the line. Changes of the chamber readings with the distance follow rather well the inverse squares rule. These results confirm earlier observations(18) that the majority of secondary neutrons is produced on the beam line. The value of the absorbed dose in the vicinity of the eye phantom, estimated on the basis of actual preliminary calibration of the chamber, was 2 mGy per treatment course, i.e. 0.0033 % of the 60 Gy dose delivered to the tumour in the eye. The value concerns the situation when the phantom was placed in the centre of the chamber.
Figure 3. Comparison of the chamber readings (diamonds, left axis) with the readings of the beam monitor (squares, arbitrary units). The ratio of the measured values to the monitor (triangles) has nearly constant value as indicated on the right vertical axis.
203
Downloaded from http://rpd.oxfordjournals.org/ at Purdue University Libraries ADMN on July 3, 2015
goal of the tests was a qualitative check of the chamber performance. The radiotherapy facility at IFJ PAN, equipped with cyclotron AIC-144, generates a proton beam of energy 60 MeV passively scattered on a single scattering foil made of Tantalum(9). The set-up mounted along the beam line includes beam-forming and monitoring elements, range shifter and range modulator (Figure 2). During the measurements, the chamber was attached to the treatment chair. The distance between the end of the proton line (collimator) and the centre of the phantom was 10 cm. The beam was directed to the centre of the chamber ring. Irradiation time was 214 s. The measurements started 20 s before and ended 20 s after the beam closing. The beam size was 10 mm in diameter, and the Bragg peak was broadened to 5 mm. Tests of the chamber were performed first without and then with the use of the eye phantom inside the ring. The phantom was in the position of eye in a real treatment. It was placed on the treatment chair with precise pinpointing, using a laser positioning system, the same that is used for radiation therapy. In three series of the measurements with the phantom, the chamber was moved along the beam, i.e. the position of the Bragg peak was shifted relatively to the centre of the chamber. Such measurements allowed for checking the change of the chamber’s signal depending on its position towards the end of the beam line. The tests could indicate the proportion of the signals caused by neutrons generated in the eye phantom and in the optical line and were considered as the most useful for the first check of the chamber at the therapeutic beam.
E. JAKUBOWSKA ET AL. Table 1. Ionisation currents of the KP-1 chamber measured for different phantom positions at proton radiotherapy facility in IFJ PAN. Position of the eye phantom
Ionisation current [pA]
Without phantom Front of the chamber Centre of the chamber Rear of the chamber
60.8 57.6 89.9 122.8
CONCLUSIONS A recently designed and constructed ring-shaped recombination chamber successfully passed first operational tests. The measurements performed on a proton eye therapy beam line showed potential usefulness of the chamber for determination of the scattered radiation dose in the vicinity of the beam. In the future, the chamber can be applied both for patient dosimetry and for monitoring changes in the scattered radiation field, associated with different configurations of the elements forming the proton beam line. FUNDING The work was partly supported by National Science Centre, for project ‘Recombination dose meter of new generation for exposure assessment on workplaces in radiation fields of reactors and accelerators’ [grant number 1350/B/P01/2010/39] and by Warsaw University of Technology, the Faculty of Physics as a part of Human Capital Operational Programme nr 4.1.1 ( project title: Preparing and realization of medical physics faculty). REFERENCES 1. Brenner, D. J. and Hall, E. J. Secondary neutrons in clinical proton radiotherapy: a charged issue. Radiother. Oncol. 86, 165 –170 (2008). 2. Xu, X. G., Bednarz, B. and Paganetti, H. A review of dosimetry studies on external-beam radiation treatment with respect to second cancer induction. Phys. Med. Biol. 53, 193 –241 (2008).
204
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The measurements of the recombination index of radiation quality Q4, with the phantom in the centre of the chamber, resulted in a value of 2.5. This, relatively low, value of the index suggests that there was a considerable contribution of low-LET radiation produced when protons stopped in the collimator, from scattered primary protons and prompt gammaradiation.
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