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84 oral An experimental comparison of absorbed dose to water based dosimetry and air kerma based dosimetry in high-energy photon beams with graphite walled cylindrical ionization chambers
Symposia/Proffered papers
Taking pdd7s measured with TLD as the reference it can be seen that measurements with the ionisation chambers agree within the experimental uncertainty inside plastic water once the electronic equilibrium has been reached. Inside lung equivalent material the ionisation chambers over-predict dose in a 5% for the plane parallell chamber and in a 3 % for the cylindrical one. Pdd7s simulated by Montecarlo code Penelope agree within 1% with those measured with TLD inside both materials, for all depths and both energies. However, all correction models implemented in Cadplan over-predict dose in mid-lung in a 3.2%-2.7% (Batho), 9.5%-7% (modified Batho), 13%-5% (Eq. TAR) for 18 MV X-rays and 6 MV X-rays respectively. 5 cm beneath the heterogeneity all algorithms give the same dose, this dose agrees within the experimental uncertainty with the dose measured with all detectors. Conventional correction algorithms implemented in Cadplan over-predict dose inside the lungs and are not accurate at the interfaces. Pdd calculated by Montecarlo code Penelope agrees with that measured with TLD. This work was partially sponsored by the Fondo de Investigacion Sanitaria project number 98/0047-02. 84
oral
An experimental comparison of absorbed dose to water based dosimetry and air kerma based dosimetry in high-energy photon beams with graphite walled cylindrical ionization chambers H. Palmans 1, L, Nafaa 1, J. Dejans2, S. Gilfis3 M.-T. Hoomaerf 4, C. Martens 1, M. Piessens 2, H. Thierens 1, A. Van der Plaetsen 3, S. Vynckier 5, 1Ghent University, Radiation Physics - Medical Physics - div of Radiotherapy, Ghent, Belgium, 2 0 L V Ziekenhuis, Radiotherapy and Oncology, Aalst, Belgium, 3AZ St-Lucas, Radiotherapy and Oncology, Ghent, Belgium, 4Hopital de Jolimont, Radiotherapy and Oncology, Haine-SaintPaul, Belgium, 5UCL Cliniques Univ, St-Luc, Radiotherapy and Oncology, Brussels, Belgium In recent years, a change has been proposed from air kerma based reference dosimetry to absorbed dose based reference dosimetry for all radiotherapy beams of ionizing radiation. A dosimetry study is presented in which both approaches were compared. Measurements were performed with three types of graphite walled ionization chambers: NE2571, PTW30004 and Wellhofer-FC65G. For each type, three ionization chambers were calibrated in terms of absorbed dose to water and in terms of air kerma in 60Co reference beams. Reference dosimetry with all the chambers was performed in eight high-energy clinical photon beams. The data were analysed with IAEA TRS-398, AAPM TG-51, IAEA TRS-277, NCS report 2 (presently used in Belgium) and AAPM TG21. The aim of the work was to study the consistency that can be expected when using the new formalisms and the differences that can be expected compared to air kerma based formalisms. The reproducibility of calibration factors before and after the project was within 0.1% except for the absorbed dose calibrations of the PTW30004'S. One chamber for which the variation was larger than 0.3% was rejected from the study. Polarity corrections were within 0.1% for all chambers in all beams. Recombination corrections were consistent with theoretical predictions, did not vary within a chamber type and only slightly between different chamber types. Maximum chamber to chamber variations of the dose obtained with the different formalisms were 0.2-0.6% for the NE2571, 0.20.6% for the PTW30004 and 0.1-0.3% for the Wellhofer-FC65G. The absorbed dose results for the NE2571's and Wellhofer-FC65G's were in good agreement for all beams and formalisms. The PTW30004's gave a systematically higher result (on the average 0.2% for IAEA TRS-277 and NCS report 2 and 0.4% for IAEA TRS-398, AAPM TG-51 and AAPM TG21). Both absorbed dose to water based formalisms (IAEA TRS-398 and AAPM TG-51) resulted in consistent values within 0.1-0.2%. Within the air kerma based protocols the results obtained with TG-21 were more than 1% higher due to differences in data. The change from old to new formalisms should be discussed together with the traceability of the primary standards in the reference beams. For our particular situation the change amounts to 0.1% based on the routine standards and 0.4% when a previously detected error on the air kerma standard is taken into account. This is similar to the magnitude of the change determined in other countries.
Tuesday, 18 September 2001
$35
MARGINS AND GEOMETRIC A C C U R A C Y I 85
invited
Incorporation of geometric uncertainties in treatment plan evaluation M. van Herk. P. Remeijer, J. V. Lebesque The Netherlands Cancer Institute, Radiotherapy Department, Amsterdam, The Netherlands With IMRT, margin reduction is often applied to spare organs at risk close to the tumor. It is, however, unclear whether the potential clinical benefit of these changes outweighs the risk of missing the tumor. The objective of this work is to explicitly incorporate geometrical uncertainties in the evaluation of treatment plans in terms of TCP for a population of patients, TCPpop. First, the delivered cumulative dose distribution is estimated by blurring it for random errors. Next, systematic errors are included by a Monte Carlo approach, i.e., the CTV is randomly displaced and rotated many times with respect to its delineated position, while computing the TCP for the CTV. By combining the results with the probability of each particular systematic error, TCPpop is computed. We tested a spherical case (spherical CTV and spherical symmetric dose distribution) and several realistic threefield prostate plans. We varied both the (uniform) margin and systematic and random error SD for the spherical case. There is a threshold value for the errors above which the TCPpop starts to decrease. The decrease in TCPpop for increasing SD of geometric errors indicates the presence of geometrical misses: failure to control the tumor for some cases due to a geometrical error. The margin at which the TCPpop decreases by 1% follows a straight line as function of the error SD's for small values of the random errors. A 1% TCPpop loss occurs for a PTV margin of 2.5 SIGMA + 0.7 sigma - 3 mm, where SIGMA is the SD of systematic errors and sigma is the SD of random errors. Next, we computed dose-TCPpop curves for various margin choices for prostate plans, with realistic error levels of SIGMA= 4.5 mm and sigma = 2.6 mm and rotations about the left-right axis with a SD of 4 degrees. Above 10mm uniform margin the TCP curves overlap, while the curves decrease sharply for smaller margins. These plots allow analysis of the possibility of compensation TCP loss due to geometrical misses caused by (too small) margins by an increase in dose. For instance, at a prescribed dose of 80 Gy and a margin of 10mm, a TCP level of 86% is achieved. To maintain the same TCP level when using a margin of 6mm, a dose level of 87 Gy is required. At a margin of 4 mm, the required dose level is 98 Gy. In conclusion, too small margins complete cancel the effect of dose escalation ! 86
invited
Respiratory gating in conformal radiotherapy G.S. Maeeras Memorial Sloan-Kettering Cancer Ctr, Dept. of Medical Physics, New York, USA This presentation focusses on experience at MSKCC with the application of respiratory gating (RG) to radiation treatment and imaging. RG is technically challenging in that not only treatment delivery, but also acquisition of different image modalities, must be synchronized with respiration. We have been studying a commercial system, consisting of a position sensitive device placed on the patient to detect respiration, that can be interfaced with CT, fluoroscopy, PET and EPID systems. There are two means of synchronization: for CT and EPI, acquisition is triggered by the RG system; whereas for PET and fluoroscopy, image data are tagged or 'time stamped' with phase from the respiration waveform, then selectively viewed according to phase. The system is also capable of gated IMRT delivered with dynamic or step-and-shoot multileaf collimation. Fluoroscopy studies show external monitor movement to correlate well with that of the diaphragm in most patients, although phase lags have been observed in patients with impaired lung function. RG-triggered CT, repeated at the same respiration phase, show that motion artifacts are reduced and organ position reproducibility is improved relative to standard CT. For example, the mean difference (6 patients) in liver centroid position along the superior-inferior direction, between repeat scans at end expiration, is 2.5mm (standard deviation, SD 1.8mm), compared to a mean expiration-to-inspiration excursion of 11.5mm (SD 4.0mm). Further, 'gated' FDG-PET reduces motion-induced blurring, resulting in reductions of -20-30% in apparent lung tumor volume compared to the standard mode, thus yielding a more accurate definition of the GTV. Amorphous silicon EPIDs can acquire an image within a single
Taking pdd7s measured with TLD as the reference it can be seen that measurements with the ionisation chambers agree within the experimental uncertainty inside plastic water once the electronic equilibrium has been reached. Inside lung equivalent material the ionisation chambers over-predict dose in a 5% for the plane parallell chamber and in a 3 % for the cylindrical one. Pdd7s simulated by Montecarlo code Penelope agree within 1% with those measured with TLD inside both materials, for all depths and both energies. However, all correction models implemented in Cadplan over-predict dose in mid-lung in a 3.2%-2.7% (Batho), 9.5%-7% (modified Batho), 13%-5% (Eq. TAR) for 18 MV X-rays and 6 MV X-rays respectively. 5 cm beneath the heterogeneity all algorithms give the same dose, this dose agrees within the experimental uncertainty with the dose measured with all detectors. Conventional correction algorithms implemented in Cadplan over-predict dose inside the lungs and are not accurate at the interfaces. Pdd calculated by Montecarlo code Penelope agrees with that measured with TLD. This work was partially sponsored by the Fondo de Investigacion Sanitaria project number 98/0047-02. 84
oral
An experimental comparison of absorbed dose to water based dosimetry and air kerma based dosimetry in high-energy photon beams with graphite walled cylindrical ionization chambers H. Palmans 1, L, Nafaa 1, J. Dejans2, S. Gilfis3 M.-T. Hoomaerf 4, C. Martens 1, M. Piessens 2, H. Thierens 1, A. Van der Plaetsen 3, S. Vynckier 5, 1Ghent University, Radiation Physics - Medical Physics - div of Radiotherapy, Ghent, Belgium, 2 0 L V Ziekenhuis, Radiotherapy and Oncology, Aalst, Belgium, 3AZ St-Lucas, Radiotherapy and Oncology, Ghent, Belgium, 4Hopital de Jolimont, Radiotherapy and Oncology, Haine-SaintPaul, Belgium, 5UCL Cliniques Univ, St-Luc, Radiotherapy and Oncology, Brussels, Belgium In recent years, a change has been proposed from air kerma based reference dosimetry to absorbed dose based reference dosimetry for all radiotherapy beams of ionizing radiation. A dosimetry study is presented in which both approaches were compared. Measurements were performed with three types of graphite walled ionization chambers: NE2571, PTW30004 and Wellhofer-FC65G. For each type, three ionization chambers were calibrated in terms of absorbed dose to water and in terms of air kerma in 60Co reference beams. Reference dosimetry with all the chambers was performed in eight high-energy clinical photon beams. The data were analysed with IAEA TRS-398, AAPM TG-51, IAEA TRS-277, NCS report 2 (presently used in Belgium) and AAPM TG21. The aim of the work was to study the consistency that can be expected when using the new formalisms and the differences that can be expected compared to air kerma based formalisms. The reproducibility of calibration factors before and after the project was within 0.1% except for the absorbed dose calibrations of the PTW30004'S. One chamber for which the variation was larger than 0.3% was rejected from the study. Polarity corrections were within 0.1% for all chambers in all beams. Recombination corrections were consistent with theoretical predictions, did not vary within a chamber type and only slightly between different chamber types. Maximum chamber to chamber variations of the dose obtained with the different formalisms were 0.2-0.6% for the NE2571, 0.20.6% for the PTW30004 and 0.1-0.3% for the Wellhofer-FC65G. The absorbed dose results for the NE2571's and Wellhofer-FC65G's were in good agreement for all beams and formalisms. The PTW30004's gave a systematically higher result (on the average 0.2% for IAEA TRS-277 and NCS report 2 and 0.4% for IAEA TRS-398, AAPM TG-51 and AAPM TG21). Both absorbed dose to water based formalisms (IAEA TRS-398 and AAPM TG-51) resulted in consistent values within 0.1-0.2%. Within the air kerma based protocols the results obtained with TG-21 were more than 1% higher due to differences in data. The change from old to new formalisms should be discussed together with the traceability of the primary standards in the reference beams. For our particular situation the change amounts to 0.1% based on the routine standards and 0.4% when a previously detected error on the air kerma standard is taken into account. This is similar to the magnitude of the change determined in other countries.
Tuesday, 18 September 2001
$35
MARGINS AND GEOMETRIC A C C U R A C Y I 85
invited
Incorporation of geometric uncertainties in treatment plan evaluation M. van Herk. P. Remeijer, J. V. Lebesque The Netherlands Cancer Institute, Radiotherapy Department, Amsterdam, The Netherlands With IMRT, margin reduction is often applied to spare organs at risk close to the tumor. It is, however, unclear whether the potential clinical benefit of these changes outweighs the risk of missing the tumor. The objective of this work is to explicitly incorporate geometrical uncertainties in the evaluation of treatment plans in terms of TCP for a population of patients, TCPpop. First, the delivered cumulative dose distribution is estimated by blurring it for random errors. Next, systematic errors are included by a Monte Carlo approach, i.e., the CTV is randomly displaced and rotated many times with respect to its delineated position, while computing the TCP for the CTV. By combining the results with the probability of each particular systematic error, TCPpop is computed. We tested a spherical case (spherical CTV and spherical symmetric dose distribution) and several realistic threefield prostate plans. We varied both the (uniform) margin and systematic and random error SD for the spherical case. There is a threshold value for the errors above which the TCPpop starts to decrease. The decrease in TCPpop for increasing SD of geometric errors indicates the presence of geometrical misses: failure to control the tumor for some cases due to a geometrical error. The margin at which the TCPpop decreases by 1% follows a straight line as function of the error SD's for small values of the random errors. A 1% TCPpop loss occurs for a PTV margin of 2.5 SIGMA + 0.7 sigma - 3 mm, where SIGMA is the SD of systematic errors and sigma is the SD of random errors. Next, we computed dose-TCPpop curves for various margin choices for prostate plans, with realistic error levels of SIGMA= 4.5 mm and sigma = 2.6 mm and rotations about the left-right axis with a SD of 4 degrees. Above 10mm uniform margin the TCP curves overlap, while the curves decrease sharply for smaller margins. These plots allow analysis of the possibility of compensation TCP loss due to geometrical misses caused by (too small) margins by an increase in dose. For instance, at a prescribed dose of 80 Gy and a margin of 10mm, a TCP level of 86% is achieved. To maintain the same TCP level when using a margin of 6mm, a dose level of 87 Gy is required. At a margin of 4 mm, the required dose level is 98 Gy. In conclusion, too small margins complete cancel the effect of dose escalation ! 86
invited
Respiratory gating in conformal radiotherapy G.S. Maeeras Memorial Sloan-Kettering Cancer Ctr, Dept. of Medical Physics, New York, USA This presentation focusses on experience at MSKCC with the application of respiratory gating (RG) to radiation treatment and imaging. RG is technically challenging in that not only treatment delivery, but also acquisition of different image modalities, must be synchronized with respiration. We have been studying a commercial system, consisting of a position sensitive device placed on the patient to detect respiration, that can be interfaced with CT, fluoroscopy, PET and EPID systems. There are two means of synchronization: for CT and EPI, acquisition is triggered by the RG system; whereas for PET and fluoroscopy, image data are tagged or 'time stamped' with phase from the respiration waveform, then selectively viewed according to phase. The system is also capable of gated IMRT delivered with dynamic or step-and-shoot multileaf collimation. Fluoroscopy studies show external monitor movement to correlate well with that of the diaphragm in most patients, although phase lags have been observed in patients with impaired lung function. RG-triggered CT, repeated at the same respiration phase, show that motion artifacts are reduced and organ position reproducibility is improved relative to standard CT. For example, the mean difference (6 patients) in liver centroid position along the superior-inferior direction, between repeat scans at end expiration, is 2.5mm (standard deviation, SD 1.8mm), compared to a mean expiration-to-inspiration excursion of 11.5mm (SD 4.0mm). Further, 'gated' FDG-PET reduces motion-induced blurring, resulting in reductions of -20-30% in apparent lung tumor volume compared to the standard mode, thus yielding a more accurate definition of the GTV. Amorphous silicon EPIDs can acquire an image within a single