- Email: [email protected]
277 poster comparison of photon beam doses determined according to IAEA TRS-398 and IAEA TRS-277 dosimetry protocols
$92
Posters
to be optimal. The relative dose resolution for this composition was better than 3% for doses between 15% and 105% of the normalization dose. The corresponding value for the standard gel was 4%. Conclusion: In the range of doses of clinical interest the new gel composition was found to improve the relative dose resolution from 4% to 3% at a 95% level of confidence. 276
poster
Practical implementation of the new international dosimetry protocol: differences with previous beam calibrations
Results: The directly measured TPR2o,1o values agreed within 0.5% with the calculated TPR2o,10 values. The ratios of absorbed doses determined according to TRS-398 and TRS-277 protocols were between 1.009 and 0.992 for the above photon energies and ionization chambers. Conclusions: The IAEA TRS-398 (Ng,w-based) dosimetry protocol is simpler and easier to use and reduces the possibility of errors in the determination of absorbed dose to water. We found less than 1% difference between the high energy photon beam calibrations for NE 2571, NE 2581, PTW 30001 and IC70 ionization chambers using the new (TRS-398) protocol and the earlier (TRS-277) protocol. 278
M. Westermark, P. Andreo. A. Tilikidis Karolinska Hospital, Hospital Physics, Stockholm, Sweden
poster
Validation of the Siemens virtual wedge for non-standard wedge angles
Sweden and the other Nordic countries have adopted the implementation of the new international Code of Practice IAEA TRS-398 (2000) at national level, which will replace the recommendations given in the previous IAEA TRS-277, formally adopted in the late eighties. No specific recommendations exist at national level for the adoption of the more recent plane-parallel chamber protocol, TRS-381 (1997), which updated TRS-277 in electron dosimetry, and the dissemination of the changes introduced in Swedish hospitals is unknown. The adoption of the new Code of Practice introduces differences in the reference value of the calibration of clinical beams, which have to be determined prior to the clinical implementation of the new Code of Practice. This work presents a systematic dosimetry comparison of the old and the new Codes of Practice in photon and electron beams spanning the range used in clinical radiotherapy. Accelerator beams have been calibrated using the new protocol and results have been compared with dose determinations using the second edition of TRS-277 for high-energy photons and TRS-381 for electron beams. The photons of eight accelerators range from 4 to 50 MV and electrons from 4 to 20 MeV. Different types of Farmer ionization chambers (NE-2571 and PTW-30010) calibrated in terms of NK and ND,w have been used as reference detectors and plane-parallel chambers (NACP and PTW Roos) cross-calibrated for electron beam dosimetry. The results obtained show that TRS-398 yields Dw larger than the previous protocOls by up to 1.2% for photons (TRS-277, cylindrical chambers) and 0.8% for electrons (TRS-381, plane-parallel chambers cross calibrated). About 0.8% is caused by the change to the new type of standards. If ND,w of the plane-parallel chambers are used also for TRS381, the new Dw values for electrons are lower by about -0.5% (influence of kQ values). For photons, the utilization of directly measured TPR20,10 and calculated TPR20,10 from PDD20,10 at SSD=100 cm, using the relation given in the new protocol, has been compared finding negligible differences. The influence of the different recommendations for reference depths for electron dosimetry, both for the cross-calibration of pp-chambers and for dose determination, has been analysed finding negligible discrepancies (influence of pcav values at different depths).
A Siemens Primus H linear accelerator incorporating a 'Virtual' wedge has recently been installed at the Northern Centre for Cancer Treatment based at Newcastle General Hospital. Rather than the look up table approach used by Varian, Siemens utilise an algorithm to compute jaw speeds and running dose rates just prior to beam delivery. This design approach allows the production of any virtual wedge angle in the range 15 to 60 degrees. Previously reported virtual wedge validation has described only the validation of the 15, 30 45 and 60 degrees virtual wedges; angles that are intended to mimic Siemens physical wedge filters. MDS-Nordion have implemented the Siemens virtual wedge in their TMS planning system. Practical utilisation of all intermediate wedge angles between the 15 and 60 degree limits offers considerable flexibility in treatment plan optimisation at the potential expense of considerable commissioning effort. This paper describes how this effort was minimised and the steps taken to verify agreement between calculated and TMS generated dose distributions for the full range of available wedge angles for both 6 and 15 MV photon energies. A minimal set of wedged collimator scatter data was also collected for use in an independent software algorithm for TMS monitor unit confirmation. Agreement between predicted and measured dose distributions for all virtual wedge angles, field sizes and energies was found to be comparable to the agreement obtained with equivalent open beams. The monitor units generated by TMS for absolute dose delivery were also found to be in good agreement with those predicted by the independent software model. It was concluded that Siemens virtual wedge may be employed to produce any desired wedge angle within the stated limits and that TMS models the resultant dose distribution with sufficient accuracy for its clinical implementation.
277
279
poster
C.P. Walker, R.H. Kermode, J.P. Byrne Regional Medical Physics Department, Newcastle General Hospital, Newcastle Upon Tyne, UK
poster
Comparison of photon beam doses determined according to IAEA TRS-398 and IAEA TRS-277 dosimetry protocols
Dosimetry characteristics of degraded electron beams investigated by Monte Carlo calculations
G. Kontra, G. Nemeth National Institute of Oncology, Radiotherapy Department, Budapest, Hungary
P. Bi6rk, P. Nilsson, T. Kn5Os Department of Radiation Physics, Lund University Hospital, Lund, Sweden
Introduction: The International Atomic Energy Agency published a new, ND,w-based dosimetry protocol (TRS-398) to determine absorbed dose to water in clinical radiation beams. In this new protocol the basic concept is that the ionization chamber has a calibration factor in terms of absorbed dose to water. In the previous, NK -based protocol (TRS-277) the ionization chambers were calibrated against a primary standards of air kerma. Objective: Our purpose was to determine the difference in high energy photon beam calibrations if the new TRS-398 dosimetry protocol is followed instead of the earlier TRS-277 code of practice. We investigated this difference for different type of commercially available ionization chambers. Materials and methods: Co-60, 6 MV, 18 MV and 23 MV photon beams were calibrated in water phantom according to TRS-398 protocol with NE 2571, NE 2581, PTW 30001 and Wellh0fer IC70 ionization chambers using the ND,w calibration factors of these chambers. (Zpe, - Zp = 0.6-rw=) was applied.) The ND,Wand Nk factors were determined by the same calibrating dosimetry laboratory. The beam quality index (TPR2o,lo) values were directly measured in PTW MP3 water phantom for each photon energy and were calculated from measured PDD (10cm) and PDD2o,lo values according to TRS-398 protocol too.
Purpose: the aim is to investigate the accuracy of different methods to determine the absorbed dose in degraded electron beams. Material and methods: standard electron beams and degraded beams, which are present in e.g. intraoperative radiation therapy (IORT), have been modelled using the EGS4 based Monte Carlo code BEAM. By comparing characteristics of degraded beams with conventional electron fields, where the procedures to determine the absorbed dose are well documented, limitations and uncertainties of commonly used dosimetric techniques in IORT electron fields have been quantified. Results and discussion: the simulated IORT beams exhibit broader energy spectra as well as wider angular distributions at the phantom surface compared to conventional beams. It is mainly a result of the increased number of scattered electrons from the intraoperative cone. The total scatter contribution to the absorbed dose at dmax can be up to 40% depending on energy and cone size. The difference in beam characteristics has an influence on the response of the detector. The water-to-air stopping-power ratios vs. depth in IORT beams differ about 1-2% compared to those from monoenergetic parallel beams. This limits the direct use of ionization chamber dosimetry in degraded beams. Conclusion: we recommend that the relative absorbed dose distributions
Posters
to be optimal. The relative dose resolution for this composition was better than 3% for doses between 15% and 105% of the normalization dose. The corresponding value for the standard gel was 4%. Conclusion: In the range of doses of clinical interest the new gel composition was found to improve the relative dose resolution from 4% to 3% at a 95% level of confidence. 276
poster
Practical implementation of the new international dosimetry protocol: differences with previous beam calibrations
Results: The directly measured TPR2o,1o values agreed within 0.5% with the calculated TPR2o,10 values. The ratios of absorbed doses determined according to TRS-398 and TRS-277 protocols were between 1.009 and 0.992 for the above photon energies and ionization chambers. Conclusions: The IAEA TRS-398 (Ng,w-based) dosimetry protocol is simpler and easier to use and reduces the possibility of errors in the determination of absorbed dose to water. We found less than 1% difference between the high energy photon beam calibrations for NE 2571, NE 2581, PTW 30001 and IC70 ionization chambers using the new (TRS-398) protocol and the earlier (TRS-277) protocol. 278
M. Westermark, P. Andreo. A. Tilikidis Karolinska Hospital, Hospital Physics, Stockholm, Sweden
poster
Validation of the Siemens virtual wedge for non-standard wedge angles
Sweden and the other Nordic countries have adopted the implementation of the new international Code of Practice IAEA TRS-398 (2000) at national level, which will replace the recommendations given in the previous IAEA TRS-277, formally adopted in the late eighties. No specific recommendations exist at national level for the adoption of the more recent plane-parallel chamber protocol, TRS-381 (1997), which updated TRS-277 in electron dosimetry, and the dissemination of the changes introduced in Swedish hospitals is unknown. The adoption of the new Code of Practice introduces differences in the reference value of the calibration of clinical beams, which have to be determined prior to the clinical implementation of the new Code of Practice. This work presents a systematic dosimetry comparison of the old and the new Codes of Practice in photon and electron beams spanning the range used in clinical radiotherapy. Accelerator beams have been calibrated using the new protocol and results have been compared with dose determinations using the second edition of TRS-277 for high-energy photons and TRS-381 for electron beams. The photons of eight accelerators range from 4 to 50 MV and electrons from 4 to 20 MeV. Different types of Farmer ionization chambers (NE-2571 and PTW-30010) calibrated in terms of NK and ND,w have been used as reference detectors and plane-parallel chambers (NACP and PTW Roos) cross-calibrated for electron beam dosimetry. The results obtained show that TRS-398 yields Dw larger than the previous protocOls by up to 1.2% for photons (TRS-277, cylindrical chambers) and 0.8% for electrons (TRS-381, plane-parallel chambers cross calibrated). About 0.8% is caused by the change to the new type of standards. If ND,w of the plane-parallel chambers are used also for TRS381, the new Dw values for electrons are lower by about -0.5% (influence of kQ values). For photons, the utilization of directly measured TPR20,10 and calculated TPR20,10 from PDD20,10 at SSD=100 cm, using the relation given in the new protocol, has been compared finding negligible differences. The influence of the different recommendations for reference depths for electron dosimetry, both for the cross-calibration of pp-chambers and for dose determination, has been analysed finding negligible discrepancies (influence of pcav values at different depths).
A Siemens Primus H linear accelerator incorporating a 'Virtual' wedge has recently been installed at the Northern Centre for Cancer Treatment based at Newcastle General Hospital. Rather than the look up table approach used by Varian, Siemens utilise an algorithm to compute jaw speeds and running dose rates just prior to beam delivery. This design approach allows the production of any virtual wedge angle in the range 15 to 60 degrees. Previously reported virtual wedge validation has described only the validation of the 15, 30 45 and 60 degrees virtual wedges; angles that are intended to mimic Siemens physical wedge filters. MDS-Nordion have implemented the Siemens virtual wedge in their TMS planning system. Practical utilisation of all intermediate wedge angles between the 15 and 60 degree limits offers considerable flexibility in treatment plan optimisation at the potential expense of considerable commissioning effort. This paper describes how this effort was minimised and the steps taken to verify agreement between calculated and TMS generated dose distributions for the full range of available wedge angles for both 6 and 15 MV photon energies. A minimal set of wedged collimator scatter data was also collected for use in an independent software algorithm for TMS monitor unit confirmation. Agreement between predicted and measured dose distributions for all virtual wedge angles, field sizes and energies was found to be comparable to the agreement obtained with equivalent open beams. The monitor units generated by TMS for absolute dose delivery were also found to be in good agreement with those predicted by the independent software model. It was concluded that Siemens virtual wedge may be employed to produce any desired wedge angle within the stated limits and that TMS models the resultant dose distribution with sufficient accuracy for its clinical implementation.
277
279
poster
C.P. Walker, R.H. Kermode, J.P. Byrne Regional Medical Physics Department, Newcastle General Hospital, Newcastle Upon Tyne, UK
poster
Comparison of photon beam doses determined according to IAEA TRS-398 and IAEA TRS-277 dosimetry protocols
Dosimetry characteristics of degraded electron beams investigated by Monte Carlo calculations
G. Kontra, G. Nemeth National Institute of Oncology, Radiotherapy Department, Budapest, Hungary
P. Bi6rk, P. Nilsson, T. Kn5Os Department of Radiation Physics, Lund University Hospital, Lund, Sweden
Introduction: The International Atomic Energy Agency published a new, ND,w-based dosimetry protocol (TRS-398) to determine absorbed dose to water in clinical radiation beams. In this new protocol the basic concept is that the ionization chamber has a calibration factor in terms of absorbed dose to water. In the previous, NK -based protocol (TRS-277) the ionization chambers were calibrated against a primary standards of air kerma. Objective: Our purpose was to determine the difference in high energy photon beam calibrations if the new TRS-398 dosimetry protocol is followed instead of the earlier TRS-277 code of practice. We investigated this difference for different type of commercially available ionization chambers. Materials and methods: Co-60, 6 MV, 18 MV and 23 MV photon beams were calibrated in water phantom according to TRS-398 protocol with NE 2571, NE 2581, PTW 30001 and Wellh0fer IC70 ionization chambers using the ND,w calibration factors of these chambers. (Zpe, - Zp = 0.6-rw=) was applied.) The ND,Wand Nk factors were determined by the same calibrating dosimetry laboratory. The beam quality index (TPR2o,lo) values were directly measured in PTW MP3 water phantom for each photon energy and were calculated from measured PDD (10cm) and PDD2o,lo values according to TRS-398 protocol too.
Purpose: the aim is to investigate the accuracy of different methods to determine the absorbed dose in degraded electron beams. Material and methods: standard electron beams and degraded beams, which are present in e.g. intraoperative radiation therapy (IORT), have been modelled using the EGS4 based Monte Carlo code BEAM. By comparing characteristics of degraded beams with conventional electron fields, where the procedures to determine the absorbed dose are well documented, limitations and uncertainties of commonly used dosimetric techniques in IORT electron fields have been quantified. Results and discussion: the simulated IORT beams exhibit broader energy spectra as well as wider angular distributions at the phantom surface compared to conventional beams. It is mainly a result of the increased number of scattered electrons from the intraoperative cone. The total scatter contribution to the absorbed dose at dmax can be up to 40% depending on energy and cone size. The difference in beam characteristics has an influence on the response of the detector. The water-to-air stopping-power ratios vs. depth in IORT beams differ about 1-2% compared to those from monoenergetic parallel beams. This limits the direct use of ionization chamber dosimetry in degraded beams. Conclusion: we recommend that the relative absorbed dose distributions