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55 oral First evaluation of the German quality audit system for radiotherapy beam dosimetry
S25
Monday, 15 September 2003
Proffered papers DOSIMETRY 54
oral
EPR analysis of dose to rib bone samples from breast cancer patients following an accident during radiotherapy with 8 MeV electrons A. Wojcik 1, W. Stachowicz 1, J. Sadlo 1, J. Michalik 1, S. Sommer 1, W. Bulski 6, J.-M. Cosset2, K. Clough2, P. Gourmelon 3, J.-F. Bottoliet3, A. Wieczorek 5, J. Sluszniak 5, A. Kulakowski 5, S. Gozdz5, J. Izewska 7 1Institute of Nuclear Chemistry and Technology (INCT), Warsaw, Poland 21nstitut Curie, Paris, France 31nstitut de Radioprotection et de Surete Nucleaire (IRSN), Fontenay-auxRoses, France 5Holycross Oncology Center, Kielce, Poland 6Center for Oncology, Warsaw, Poland 7International Atomic Energy Agency, Vienna, Austria Objective. In February 2001 a radiation accident occurred in a radiotherapy department at one of regional oncological hospitals in Poland. Due to a malfunction of an accelerator, 5 breast cancer patients received a single, high dose of 8 MeV electrons. The patients were at different stages of their radiotherapy treatment course and received different tumour doses of both electron and 60Co radiation prior to the accident. Owing to various circumstances a reconstruction of the accident was not possible. Thus the exact doses received during the accident were not known. However, based on the early and late skin reactions and on a crude estimate by the medical physicist, it was possible to establish that they were heterogeneous and may have reached up to 100 Gy. During the course of surgical reconstruction of the chest wall, pieces of rib bones were excised from 3 patients. This made it possible to measure the absorbed doses with the electron paramagnetic resonance (EPR) methodology. Methods. EPR measurements were performed at the IRSN, France, and INCT, Poland. Irradiation of bone samples measured at the INCT was performed at the Oncology Center in Warsaw, Poland. In order to calibrate the EPR signal the samples were irradiated in the electron beam from the radiotherapy linear accelerator and in the gamma beam from radiotherapy Co-60 machine. Each sample received a series of four dose fractions of 40 Gy. The EPR signal was read between the fractions. The back extrapolation of the EPR signal made it possible to calibrate the initial EPR signal and thus estimate the dose received by the patients. The EPR signal was read with the Bruker spectrometer. Results and conclusions. It was estimated that the doses received by the three patients due to the accidental overexposure ranged from about 60 to 80 Gy. This is in general agreement with the initial dose measurement performed by the medical physicist shortly after the accident. Thus, measurement of EPR signal in bone specimens is a powerful technique of dose estimation in cases of overexposure to ionising radiation. 55
oral
First evaluation of the German quality audit system for radiotherapy beam dosimetry K. Derikum 1, R.-P. Kapsch I, C. Pychlau2 1physikalisch- Technische Bundesanstalt, Braunschweig, Germany 2pTW-Freiburg, Freiburg, Germany Since November 2001 a legal quality audit system for radiotherapy beam dosimetry has been operated in Germany. The aim is to verify local dose measurements in megavoltage photon and electron beams in order to ensure national consistency in radiotherapy dosimetry. The audit measurements are offered on a commercial basis by PTWFreiburg, at present the only measurement service approved for audits in Germany. At the request of local radiotherapy centers the measurement service mails thermoluminiscent dosemeters (TLD) which are to be irradiated to an absorbed dose to water of approximately 1 Gy. The TLD mea-
surements are compared with the dose values stated by the user. Relative deviations exceeding the acceptance limit of 3% lead to an immediate investigation of the causes which is jointly conducted by the local physicist and the measurement service. The anonymized data of the dosimetry intercomparisons are evaluated by the Physikalisch-Technische Bundesanstalt aiming at further improving the audit system. Every dosemeter to be irradiated consists of 6 TLD100 discs 4.5 mm in diameter, placed concentrically in a waterproof housing which has approximately the size and shape of a typical plane-parallel ionization chamber. The local clinic receives a set of 9 dosemeters, one of which pro-irradiated, eight of which can be used for irradiation. Irradiations are performed at the highest and the lowest photon and electron beam energies clinically used. Every ionization chamber and every electrometer used as part of a reference dosemeter is checked at least at one radiation quality. The radiotherapy centers measure the absorbed dose to water with ionization chambers following the German code of practice DIN 6800-2 based on NDw calibration factors at Co-60. All irradiations are performed under reference conditions in a water phantom. The audit measurements check both the dosimetry equipment and the correct application of the code of practice. Relative differences between measured and stated dose values of more than 3% occurred in about t0% of the measurements. The largest discrepancies, some greater than 10%, were found in electron beams. Resolution of the discrepancies has led to improved practice. The distributions of the relative differences show a standard deviation of 1.5% for both electron and photon beams. This is slightly smaller than the corresponding values reported by the ESTRO-EQUAL quality assurance network (2%) but consistent with their data. 56
oral
IMAT treatment verification with monomer/polymer gel dosimetry K. Verqote 1, Y. De Deene 1, 14/+Duthoy 1, 14,( De Gersem 1, !4( De Neve I, E. Achten 2, C. De Wagter 1 1Ghent University Hospital, Radiotherapy and Nuclear Medicine, Gent, Belgium 2Ghent University Hospital, Radiology, Gent, Belgium Introduction: Gel dosimetry was used to verify the computer treatment planning of an intensity-modulated arc therapy (IMAT) treatment. The IMAT treatment was designed to irradiate the whole abdominopelvic region for patients with relapsed ovarian cancer. Materials and methods: A Barex cast, vacuum molded on the abdominal region of the Rando phantom, was used for the experiment. Several issues need to be addressed when applying gel dosimetry in large phantoms: (1) temperature drift during MRI of the gel has to be minimized, (2) inhomogeneities in the MRI-derived dose maps due to variations in the transmitted radiofrequency (RF) field have to be compensated and (3) spatial variations in the dose response due to inhomogeneous cooling of the gel after fabrication should be minimized. For this purpose, a homogeneity study was performed on the phantom filled with a blank gel. The IMAT treatment consisted of 6 arcs and 1 sliding window, and was planned on CT scans of the gel-filled phantom with anatomical structures derived from patient data. After applying this treatment on the gel phantom, the resulting dose distribution in the phantom was visualized by MRI, and compared with computer planning results from Pinnacle. As a way to validate the use of gel dosimetry in the voluminous phantom (10 I), the gel-filled phantom was irradiated with two rectangular beams (in a separate experiment) and the resulting dose distribution was compared with Helax-TMS computations. Results: The homogeneity study revealed that, even after compensating for temperature effects and RF inhomogeneities, dose deviations up to 5% (compared to maximum dose) can still be expected in parts of the phantom. Gel-derived dose maps from the rectangular beam experiment showed good agreement with Helax-TMS computations: absolute dose differences that exceed 3% were only observed at high dose gradient locations. For the IMAT experiment itself, gel-derived dose-volume histograms (DVHs) were compared with DVHs computed by the Pinnacle planning software: median
Monday, 15 September 2003
Proffered papers DOSIMETRY 54
oral
EPR analysis of dose to rib bone samples from breast cancer patients following an accident during radiotherapy with 8 MeV electrons A. Wojcik 1, W. Stachowicz 1, J. Sadlo 1, J. Michalik 1, S. Sommer 1, W. Bulski 6, J.-M. Cosset2, K. Clough2, P. Gourmelon 3, J.-F. Bottoliet3, A. Wieczorek 5, J. Sluszniak 5, A. Kulakowski 5, S. Gozdz5, J. Izewska 7 1Institute of Nuclear Chemistry and Technology (INCT), Warsaw, Poland 21nstitut Curie, Paris, France 31nstitut de Radioprotection et de Surete Nucleaire (IRSN), Fontenay-auxRoses, France 5Holycross Oncology Center, Kielce, Poland 6Center for Oncology, Warsaw, Poland 7International Atomic Energy Agency, Vienna, Austria Objective. In February 2001 a radiation accident occurred in a radiotherapy department at one of regional oncological hospitals in Poland. Due to a malfunction of an accelerator, 5 breast cancer patients received a single, high dose of 8 MeV electrons. The patients were at different stages of their radiotherapy treatment course and received different tumour doses of both electron and 60Co radiation prior to the accident. Owing to various circumstances a reconstruction of the accident was not possible. Thus the exact doses received during the accident were not known. However, based on the early and late skin reactions and on a crude estimate by the medical physicist, it was possible to establish that they were heterogeneous and may have reached up to 100 Gy. During the course of surgical reconstruction of the chest wall, pieces of rib bones were excised from 3 patients. This made it possible to measure the absorbed doses with the electron paramagnetic resonance (EPR) methodology. Methods. EPR measurements were performed at the IRSN, France, and INCT, Poland. Irradiation of bone samples measured at the INCT was performed at the Oncology Center in Warsaw, Poland. In order to calibrate the EPR signal the samples were irradiated in the electron beam from the radiotherapy linear accelerator and in the gamma beam from radiotherapy Co-60 machine. Each sample received a series of four dose fractions of 40 Gy. The EPR signal was read between the fractions. The back extrapolation of the EPR signal made it possible to calibrate the initial EPR signal and thus estimate the dose received by the patients. The EPR signal was read with the Bruker spectrometer. Results and conclusions. It was estimated that the doses received by the three patients due to the accidental overexposure ranged from about 60 to 80 Gy. This is in general agreement with the initial dose measurement performed by the medical physicist shortly after the accident. Thus, measurement of EPR signal in bone specimens is a powerful technique of dose estimation in cases of overexposure to ionising radiation. 55
oral
First evaluation of the German quality audit system for radiotherapy beam dosimetry K. Derikum 1, R.-P. Kapsch I, C. Pychlau2 1physikalisch- Technische Bundesanstalt, Braunschweig, Germany 2pTW-Freiburg, Freiburg, Germany Since November 2001 a legal quality audit system for radiotherapy beam dosimetry has been operated in Germany. The aim is to verify local dose measurements in megavoltage photon and electron beams in order to ensure national consistency in radiotherapy dosimetry. The audit measurements are offered on a commercial basis by PTWFreiburg, at present the only measurement service approved for audits in Germany. At the request of local radiotherapy centers the measurement service mails thermoluminiscent dosemeters (TLD) which are to be irradiated to an absorbed dose to water of approximately 1 Gy. The TLD mea-
surements are compared with the dose values stated by the user. Relative deviations exceeding the acceptance limit of 3% lead to an immediate investigation of the causes which is jointly conducted by the local physicist and the measurement service. The anonymized data of the dosimetry intercomparisons are evaluated by the Physikalisch-Technische Bundesanstalt aiming at further improving the audit system. Every dosemeter to be irradiated consists of 6 TLD100 discs 4.5 mm in diameter, placed concentrically in a waterproof housing which has approximately the size and shape of a typical plane-parallel ionization chamber. The local clinic receives a set of 9 dosemeters, one of which pro-irradiated, eight of which can be used for irradiation. Irradiations are performed at the highest and the lowest photon and electron beam energies clinically used. Every ionization chamber and every electrometer used as part of a reference dosemeter is checked at least at one radiation quality. The radiotherapy centers measure the absorbed dose to water with ionization chambers following the German code of practice DIN 6800-2 based on NDw calibration factors at Co-60. All irradiations are performed under reference conditions in a water phantom. The audit measurements check both the dosimetry equipment and the correct application of the code of practice. Relative differences between measured and stated dose values of more than 3% occurred in about t0% of the measurements. The largest discrepancies, some greater than 10%, were found in electron beams. Resolution of the discrepancies has led to improved practice. The distributions of the relative differences show a standard deviation of 1.5% for both electron and photon beams. This is slightly smaller than the corresponding values reported by the ESTRO-EQUAL quality assurance network (2%) but consistent with their data. 56
oral
IMAT treatment verification with monomer/polymer gel dosimetry K. Verqote 1, Y. De Deene 1, 14/+Duthoy 1, 14,( De Gersem 1, !4( De Neve I, E. Achten 2, C. De Wagter 1 1Ghent University Hospital, Radiotherapy and Nuclear Medicine, Gent, Belgium 2Ghent University Hospital, Radiology, Gent, Belgium Introduction: Gel dosimetry was used to verify the computer treatment planning of an intensity-modulated arc therapy (IMAT) treatment. The IMAT treatment was designed to irradiate the whole abdominopelvic region for patients with relapsed ovarian cancer. Materials and methods: A Barex cast, vacuum molded on the abdominal region of the Rando phantom, was used for the experiment. Several issues need to be addressed when applying gel dosimetry in large phantoms: (1) temperature drift during MRI of the gel has to be minimized, (2) inhomogeneities in the MRI-derived dose maps due to variations in the transmitted radiofrequency (RF) field have to be compensated and (3) spatial variations in the dose response due to inhomogeneous cooling of the gel after fabrication should be minimized. For this purpose, a homogeneity study was performed on the phantom filled with a blank gel. The IMAT treatment consisted of 6 arcs and 1 sliding window, and was planned on CT scans of the gel-filled phantom with anatomical structures derived from patient data. After applying this treatment on the gel phantom, the resulting dose distribution in the phantom was visualized by MRI, and compared with computer planning results from Pinnacle. As a way to validate the use of gel dosimetry in the voluminous phantom (10 I), the gel-filled phantom was irradiated with two rectangular beams (in a separate experiment) and the resulting dose distribution was compared with Helax-TMS computations. Results: The homogeneity study revealed that, even after compensating for temperature effects and RF inhomogeneities, dose deviations up to 5% (compared to maximum dose) can still be expected in parts of the phantom. Gel-derived dose maps from the rectangular beam experiment showed good agreement with Helax-TMS computations: absolute dose differences that exceed 3% were only observed at high dose gradient locations. For the IMAT experiment itself, gel-derived dose-volume histograms (DVHs) were compared with DVHs computed by the Pinnacle planning software: median