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The Statistical Analysis of Patient Specific Dosimetry Quality Assurance (DQA) of IMRT Plans
I. J. Radiation Oncology d Biology d Physics
S670
Volume 72, Number 1, Supplement, 2008
Results: Preliminary trials of early iterations of this applicator used on cadavers revealed superiority in comfortably inserting the tandem in cervical canal through stent, filling the vaginal cavity with liquid contrast for proper CT/MRI imaging. The unaccounted attenuation from the Stainless Steel tubing in the conventional FSD applicator is no longer an issue when this applicator is implemented. This new applicator, all disposable plastic with no metal parts is CT and MR friendly, allows 3-D treatment planning executable to accurately deliver dose to the target volume as prescribed, while sparing the normal tissue structure at any desirable level. Preliminary dosimetric studies indicate superior volumetric dose coverage to PTV and easy detachment from the conventional Manchester system. Conclusions: The new applicator once applied will overcome most of the disadvantages from the conventional FSD device and offers numerous superior clinical advantages for patients with gynecological cancer. The applicator design, patient comfort studies, GYN dosimetric comparison to FSD will be presented. *This project is supported by Varian Medical System. Author Disclosure: E.I. Parsai, None; Z. Zhang, None; J.J. Feldmeier, None.
3113
Concept of a Clinical Linear Accelerator Optimized for IMRT with Monte Carlo Simulation
M. Yasunaga, M. Yagi, N. Mukumoto, K. Tsujii, K. Uehara, S. Saito, H. Numasaki, T. Teshima Osaka University, Suita, Osaka, Japan Purpose/Objective(s): IMRT (intensity modulated radiation therapy) is the irradiation technique to get the ideal dose distribution by modulating the beam intensity. Therefore, the beam intensity does not need to be flattened and the FF (flattening filter) can be eliminated. Removing the FF could increase dose rate and reduce head scatter significantly. However, it leads to make the beam soft and increase the surface dose. The purpose of this study is to optimize the target of generating the photon beam and the output of the clinical linear accelerator head without the FF. Materials/Methods: The Monte Carlo simulation was performed using EGSnrc/BEAMnrc and EGSnrc/DOSXYZnrc. The properties of shape, composition and combination in target were investigated. The combinations were Al, W, Cu, W - Cu, Cu - W. The control criteria were the properties of the target in a Varian Clinac. The indices such as the photon efficiency, the mean energy of the photon, the contaminating electrons efficiency and the mean energy of the contaminating electrons were used. The selected target was equipped with the machine without the FF. The foil was introduced instead of the FF. The material and thickness of the foil were changed to investigate the output using the P/C which is ratio of photon energy fluence divided by contaminating electrons energy fluence. The best condition of the foil was decided to each target. Considering indices such as beam quality, dose rate and surface dose, the best combination of the target and foil was equipped in the machine without the FF. Results: The dose of column shape W - Cu target was higher than that of only W or Al when the target thickness was equal. In the simulation of the target, 12 W - Cu targets were selected. P/C of the machine introduced the foil increased by a factor of 1.5. The beam quality of the 15 MV Varian machine without the foil was similar to that of the 10 MV Varian machine with the FF. The beam quality of the machine with the best combination of the target and the foil was close to that of 12 MV machine with the FF. The dose rate of the optimized clinical linear accelerator head was 3.6 times higher than that of the machine with the FF. The surface dose was 6% lower than that of the Varian machine without the FF. It was also about 2% lower than that of the machine with the best combination of the Varian target and foil. Conclusions: Removing the FF increased the dose rate significantly. The proper combination of the target and foil leads the beam hardening and decreases the surface dose. To increase the output of a clinical linear accelerator head for IMRT, optimal condition of target properties, thickness and foil was obtained by Monte Carlo simulation. Author Disclosure: M. Yasunaga, None; M. Yagi, None; N. Mukumoto, None; K. Tsujii, None; K. Uehara, None; S. Saito, None; H. Numasaki, None; T. Teshima, None.
3114
The Statistical Analysis of Patient Specific Dosimetry Quality Assurance (DQA) of IMRT Plans
J. Kim1, J. Chung1, I. Kim1, I. Kim2, S. Ye2, T. Suh3 1 Seoul National University Bundang Hospital, Seongnam City, Republic of Korea, 2Seoul National University Hospital, Seoul, Republic of Korea, 3The Catholic University of Korea, Seoul, Republic of Korea
Purpose/Objective(s): By analyzing measured results of patient specific DQA (dosimetry quality assurance) of IMRT plans, we identified several factors that affected the discrepancy between calculated and measured doses. Materials/Methods: From July 2006 to February 2008, patient specific DQA for 153 IMRT plans of head and neck (H&N) cancer, prostate cancer, and brain tumor was performed by using a home-made cylindrical phantom. A verification point was selected in the regions of uniform and high dose. A diode array was used to compare individual fluence maps in a g-index method. Results: The average difference between calculated and measured doses was -1.54 ± 1.35% for H&N cancer, 0.13 ± 1.05% for prostate cancer, and -0.70 ± 1.34% for brain tumor. The maximum difference was -3.4% for H&N cancer, -3.4% for brain tumor, and 2.5% for prostate cancer. Planed and measured fluence maps were in agreement of 95.8 ± 7.4% with the g-index criteria of ±3 mm and ±3%. The number of PTVs, the number of split fields, and the degree of modulation showed strong correlations with the discrepancy between them. Most IMRT plans (96%) agreed with measured doses within the tolerance level of ±3% on patient specific DQA. However, we found systematic underdosage of highly modulated H&N plans while dose differences of other cancer patients followed the Gaussian distribution. Conclusions: We recommend that highly modulated plans over the tolerance level should be avoided even though they can be generated by RTP. We suggest that dosimetric corrections for IMRT plans over the tolerance level could be achievable only with an established IMRT QA protocol. Author Disclosure: J. Kim, None; J. Chung, None; I. Kim, None; I. Kim, None; S. Ye, None; T. Suh, None.
S670
Volume 72, Number 1, Supplement, 2008
Results: Preliminary trials of early iterations of this applicator used on cadavers revealed superiority in comfortably inserting the tandem in cervical canal through stent, filling the vaginal cavity with liquid contrast for proper CT/MRI imaging. The unaccounted attenuation from the Stainless Steel tubing in the conventional FSD applicator is no longer an issue when this applicator is implemented. This new applicator, all disposable plastic with no metal parts is CT and MR friendly, allows 3-D treatment planning executable to accurately deliver dose to the target volume as prescribed, while sparing the normal tissue structure at any desirable level. Preliminary dosimetric studies indicate superior volumetric dose coverage to PTV and easy detachment from the conventional Manchester system. Conclusions: The new applicator once applied will overcome most of the disadvantages from the conventional FSD device and offers numerous superior clinical advantages for patients with gynecological cancer. The applicator design, patient comfort studies, GYN dosimetric comparison to FSD will be presented. *This project is supported by Varian Medical System. Author Disclosure: E.I. Parsai, None; Z. Zhang, None; J.J. Feldmeier, None.
3113
Concept of a Clinical Linear Accelerator Optimized for IMRT with Monte Carlo Simulation
M. Yasunaga, M. Yagi, N. Mukumoto, K. Tsujii, K. Uehara, S. Saito, H. Numasaki, T. Teshima Osaka University, Suita, Osaka, Japan Purpose/Objective(s): IMRT (intensity modulated radiation therapy) is the irradiation technique to get the ideal dose distribution by modulating the beam intensity. Therefore, the beam intensity does not need to be flattened and the FF (flattening filter) can be eliminated. Removing the FF could increase dose rate and reduce head scatter significantly. However, it leads to make the beam soft and increase the surface dose. The purpose of this study is to optimize the target of generating the photon beam and the output of the clinical linear accelerator head without the FF. Materials/Methods: The Monte Carlo simulation was performed using EGSnrc/BEAMnrc and EGSnrc/DOSXYZnrc. The properties of shape, composition and combination in target were investigated. The combinations were Al, W, Cu, W - Cu, Cu - W. The control criteria were the properties of the target in a Varian Clinac. The indices such as the photon efficiency, the mean energy of the photon, the contaminating electrons efficiency and the mean energy of the contaminating electrons were used. The selected target was equipped with the machine without the FF. The foil was introduced instead of the FF. The material and thickness of the foil were changed to investigate the output using the P/C which is ratio of photon energy fluence divided by contaminating electrons energy fluence. The best condition of the foil was decided to each target. Considering indices such as beam quality, dose rate and surface dose, the best combination of the target and foil was equipped in the machine without the FF. Results: The dose of column shape W - Cu target was higher than that of only W or Al when the target thickness was equal. In the simulation of the target, 12 W - Cu targets were selected. P/C of the machine introduced the foil increased by a factor of 1.5. The beam quality of the 15 MV Varian machine without the foil was similar to that of the 10 MV Varian machine with the FF. The beam quality of the machine with the best combination of the target and the foil was close to that of 12 MV machine with the FF. The dose rate of the optimized clinical linear accelerator head was 3.6 times higher than that of the machine with the FF. The surface dose was 6% lower than that of the Varian machine without the FF. It was also about 2% lower than that of the machine with the best combination of the Varian target and foil. Conclusions: Removing the FF increased the dose rate significantly. The proper combination of the target and foil leads the beam hardening and decreases the surface dose. To increase the output of a clinical linear accelerator head for IMRT, optimal condition of target properties, thickness and foil was obtained by Monte Carlo simulation. Author Disclosure: M. Yasunaga, None; M. Yagi, None; N. Mukumoto, None; K. Tsujii, None; K. Uehara, None; S. Saito, None; H. Numasaki, None; T. Teshima, None.
3114
The Statistical Analysis of Patient Specific Dosimetry Quality Assurance (DQA) of IMRT Plans
J. Kim1, J. Chung1, I. Kim1, I. Kim2, S. Ye2, T. Suh3 1 Seoul National University Bundang Hospital, Seongnam City, Republic of Korea, 2Seoul National University Hospital, Seoul, Republic of Korea, 3The Catholic University of Korea, Seoul, Republic of Korea
Purpose/Objective(s): By analyzing measured results of patient specific DQA (dosimetry quality assurance) of IMRT plans, we identified several factors that affected the discrepancy between calculated and measured doses. Materials/Methods: From July 2006 to February 2008, patient specific DQA for 153 IMRT plans of head and neck (H&N) cancer, prostate cancer, and brain tumor was performed by using a home-made cylindrical phantom. A verification point was selected in the regions of uniform and high dose. A diode array was used to compare individual fluence maps in a g-index method. Results: The average difference between calculated and measured doses was -1.54 ± 1.35% for H&N cancer, 0.13 ± 1.05% for prostate cancer, and -0.70 ± 1.34% for brain tumor. The maximum difference was -3.4% for H&N cancer, -3.4% for brain tumor, and 2.5% for prostate cancer. Planed and measured fluence maps were in agreement of 95.8 ± 7.4% with the g-index criteria of ±3 mm and ±3%. The number of PTVs, the number of split fields, and the degree of modulation showed strong correlations with the discrepancy between them. Most IMRT plans (96%) agreed with measured doses within the tolerance level of ±3% on patient specific DQA. However, we found systematic underdosage of highly modulated H&N plans while dose differences of other cancer patients followed the Gaussian distribution. Conclusions: We recommend that highly modulated plans over the tolerance level should be avoided even though they can be generated by RTP. We suggest that dosimetric corrections for IMRT plans over the tolerance level could be achievable only with an established IMRT QA protocol. Author Disclosure: J. Kim, None; J. Chung, None; I. Kim, None; I. Kim, None; S. Ye, None; T. Suh, None.