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Dosimetric Evaluation of Tungsten Eye-Shield in 9, 12, and 16 MeV Electron Beams Using EBT2 Gafchromic Films
Poster Viewing Abstracts S873
Volume 84 Number 3S Supplement 2012 Purpose/Objective(s): A planning study was performed to evaluate the performance of volumetric modulated arc therapy (VMAT) and fixed-field conventional IMRT with (FF) or without flattening filter (FFF) for headand neck cancers and to investigate the possible dosimetric differences on target and normal tissues. Materials/Methods: We selected 7 head and neck patients who were treated with IMRT in our department for the study. For each patient, VMAT and 9-field IMRT were generated both with 6X-FF (600 MU/min) and 6X-FFF (1400 MU/min) for linac. Four plans have been generated for each patient using planning system. The gantry arc-spans, collimator and couch angles were the same in all plans for each patient. To provide an unbiased comparison, optimization objectives and priorities were kept the same as given by standard guidelines in all planning process. Dose prescription was set to PTV with simultaneous integrated boost; 70 Gy to high-risk PTV, 59.4 Gy to intermediate-risk PTV and 50.4 Gy to low-risk PTV in 33 fractions. The planning objectives for PTV were Dmin>95% at Vmin>99% of the PTV. The maximum dose criteria was defined as no more than 1% of the PTV receives maximal 106%. Dmax for spinal cord and brain stem was limited to 45 Gy and 54 Gy, respectively. Dmean<26 Gy and V30Gy<50% were given as the objectives for parotids. The plan quality was evaluated by calculating homogeneity (HI) and conformity indices (CI), mean dose to parotid, the maximum dose to the spinal cord and the brain stem, and the volume of normal tissue receiving 10 Gy. For plan efficiency evaluation, the cumulative MUs were compared. Results: The coverage of 95% of the PTV70 was >99% of the prescribed dose for both IMRT and VMAT plans in both energy modes. For IMRT-6X, IMRT6XFFF, VMAT-6X and VMAT-6XFFF mean CI-PTV70 were 1.38, 1.38, 1.45 and 1.44, mean CI-PTV59.4 was 1.39, 1.37, 1.49 and 1.47, mean CI-PTV50.4 was 1.68, 1.62, 1.94 and 1.92, respectively. HI and V10 normal tissue were similar in all groups. Mean doses to the left parotid were 27.7 Gy, 27.3 Gy, 24.7 Gy, 23.7 Gy and, mean doses to the right parotid were 28.3 Gy, 28.3 Gy, 25.3 Gy, 23.8 Gy. Dmax for the spinal cord were 40.4 Gy, 40.8 Gy, 43.5 Gy, 43.4 Gy. The brain stem maximum doses were 44.2 Gy, 44.2 Gy, 45.7 Gy, 46.2 Gy. The mean MUs were 1792312, 3048518, 496101, 545105. Conclusions: For each energy mode, clinically acceptable plans were achieved for both IMRT and VMAT plans, although IMRT plans typically required higher monitor units and FFF mode significantly decreased the beam-on time. Minor dosimetric differences were observed in some organs, however clinical significance needs further evaluation. Author Disclosure: M. Abacioglu: None. S. Unal: None. Z. Ozen: None. B. Gunhan: None. S. Gurdalli: None. A. Arifoglu: None.
3737 Planning Quality and Delivery Accuracy Assessment of VMAT Plans for Whole Brain Treatment With Hippocampus Avoidance L. Huang, A. Rizzo, S. Chao, P. Qi, and P. Xia; Cleveland Clinic Foundation, Cleveland, OH Purpose/Objective(s): To investigate planning quality and delivery accuracy of volumetric modulated arc (VMAT) therapy plans for whole brain treatment with hippocampus avoidance following RTOG 0933. Materials/Methods: Five patients eligible for whole brain treatment with hippocampus avoidance were modeled for this study. For each case, three VMAT plans were created and delivered using two different linear accelerators from two different manufactories with three different dose rates, 400 MU/min, 600 MU/min, and 1000 MU/min. Each plan were created using the planning system for a prescribed daily dose of 3Gy. To meet the dose constraints from RTOG 0933 for the PTV and hippocampus, each plan included one full arc of 360 and one partial arc of 200 . Subsequently, each plan was delivered with a matrix device, measuring planar dose agreement between the planned and delivered dose, using the gamma index. Results: The average of total monitor units (MU) for each plan with the low, intermediate, and high dose rates were 848.787.5, 1201.3151.1 and 1155.4189.1, respectively. Accordingly, the dose received by 100% of the hippocampus (D100) were 8.03 0.57 Gy, 8.22 0.40 Gy and 7.400.50 Gy. The maximum doses of the hippocampus were 14.7 1.1 Gy, 15.11 1.17 Gy and 16.12 0.74 Gy. The average volume of the PTV receiving 37.50 Gy (V37.5) were 0.59 0.50%, 1.8 1.1%, and 1.00.58%. The
average volumes of the PTV received the prescription dose of 30 Gy were 91.6 1.35%, 91.0 0.85% and 91.0 1.25%. The volume of the PTV receiving 25 Gy were 97.01.2%, 96.20.8% and 97.50.9%. The maximum point doses for the whole brain were 41.13 2.47 Gy, 42.55 2.18 Gy and 44.97 3.19 Gy. All these plans were considered acceptable according to the RTOG protocol 0933. The accuracy of the plan delivery decreased as the dose rate increase, with the gamma index (3% 3mm) passing rates of 95.3 0.96%, 92.33.65% and 91.7 2.40%, respectively. Conclusion: The volumetric modulated arc therapy provides a good treatment option for the whole brain treatment with hippocampus sparing. The delivery accuracy is slightly decreased with the increased of the dose rate. Author Disclosure: L. Huang: None. A. Rizzo: None. S. Chao: None. P. Qi: None. P. Xia: None.
3738 Dosimetric Evaluation of Tungsten Eye-Shield in 9, 12, and 16 MeV Electron Beams Using EBT2 Gafchromic Films A. Anand, S. Tung, R. Tailor, W.H. Morrison, and R.X. Zhu; U.T. M.D.Anderson Cancer Center, Houston, TX Purpose/Objective(s): Commercially available tungsten eye-shield has been clinically tested in the past for protection to the ocular structures when treated with 9 MeV electron beam. Dosimetry underneath these eye shields is being reported for special situations requiring higher electron energies (12, and 16 MeV) for treatment of deep seated adenocarcinoma of eye. Materials/Methods: Large sized tungsten eye shield (wall thickness 5.8 mm, width 25mm) coated with 2mm thick dental acrylic was filled up with agarose gel (98 Wt.%) in order to mimic clinical setup conditions for these shields. Calibrated films (EBT2 Gafchromic) were employed for dosimetry measurements immediately under the shield, at dmax, and at deeper depths for 9, 12, and 16 MeV electron beams using a 10x10 electron-cone applicator. At each beam-energy, eye-shield attenuation was determined as ratio of the dose measured at depth of interest to the dose in an open field at dmax. Additionally, surface-dose enhancement was measured close to the eye-shield (which is primarily caused by scatter from eye-shield itself). Results: Immediately under the eye-shield, dose transmission from 9 and 12 MeV beams was found to be 2 % and 5.5 % respectively. However, transmission for the16 MeV beam was measured to be significantly larger (20%), implying higher doses to the surface of the phantom (cornea). At deeper depths, transmitted doses are relatively higher than the doses immediately under shield. This is primarily due to contribution from the lateral scatter generated within the phantom which increases with beam energy due to increase in range of scattered electrons. On the surface close to the shield, doses to the structures are enhanced due to lateral scatter from the shield itself. Dose enhancement, determined as the ratio of surface dose close to the shield versus dose 1.5 cm away from the shield on the surface, was found to be practically same 16-17% for all beam energies. Dose enhancement region extends over w10mm region around the shield. Conclusions: Even though this eye shield was designed for use with 9 MeV electron beams, results suggest that shielding of the ocular structures is adequate also for 12 MeV beam. Decision for use of this shield with higher energy (16 MeV) remains the subject of separate clinical discussion. It is important to note that electron-scattering from the shield itself, causes significant enhancement to surface doses to the structures around the shield Author Disclosure: A. Anand: None. S. Tung: None. R. Tailor: None. W.H. Morrison: None. R.X. Zhu: None.
3739 Dosimetric Comparison of Treatment Techniques for Skin Cancer Above the Clavicle: High-dose-rate 3-dimensional Topographic Applicator Brachytherapy Versus Electron Therapy R. Weinberg, S. Vyas, N. Thawani, D. Rangaraj, and S. Mutyala; Scott & White Healthcare, Temple, TX Purpose/Objective(s): Conventional radiation therapy treatment for skin cancers above the clavicle is electron beam therapy. An alternative treatment modality is high dose rate (HDR) brachytherapy with the use of surface
Volume 84 Number 3S Supplement 2012 Purpose/Objective(s): A planning study was performed to evaluate the performance of volumetric modulated arc therapy (VMAT) and fixed-field conventional IMRT with (FF) or without flattening filter (FFF) for headand neck cancers and to investigate the possible dosimetric differences on target and normal tissues. Materials/Methods: We selected 7 head and neck patients who were treated with IMRT in our department for the study. For each patient, VMAT and 9-field IMRT were generated both with 6X-FF (600 MU/min) and 6X-FFF (1400 MU/min) for linac. Four plans have been generated for each patient using planning system. The gantry arc-spans, collimator and couch angles were the same in all plans for each patient. To provide an unbiased comparison, optimization objectives and priorities were kept the same as given by standard guidelines in all planning process. Dose prescription was set to PTV with simultaneous integrated boost; 70 Gy to high-risk PTV, 59.4 Gy to intermediate-risk PTV and 50.4 Gy to low-risk PTV in 33 fractions. The planning objectives for PTV were Dmin>95% at Vmin>99% of the PTV. The maximum dose criteria was defined as no more than 1% of the PTV receives maximal 106%. Dmax for spinal cord and brain stem was limited to 45 Gy and 54 Gy, respectively. Dmean<26 Gy and V30Gy<50% were given as the objectives for parotids. The plan quality was evaluated by calculating homogeneity (HI) and conformity indices (CI), mean dose to parotid, the maximum dose to the spinal cord and the brain stem, and the volume of normal tissue receiving 10 Gy. For plan efficiency evaluation, the cumulative MUs were compared. Results: The coverage of 95% of the PTV70 was >99% of the prescribed dose for both IMRT and VMAT plans in both energy modes. For IMRT-6X, IMRT6XFFF, VMAT-6X and VMAT-6XFFF mean CI-PTV70 were 1.38, 1.38, 1.45 and 1.44, mean CI-PTV59.4 was 1.39, 1.37, 1.49 and 1.47, mean CI-PTV50.4 was 1.68, 1.62, 1.94 and 1.92, respectively. HI and V10 normal tissue were similar in all groups. Mean doses to the left parotid were 27.7 Gy, 27.3 Gy, 24.7 Gy, 23.7 Gy and, mean doses to the right parotid were 28.3 Gy, 28.3 Gy, 25.3 Gy, 23.8 Gy. Dmax for the spinal cord were 40.4 Gy, 40.8 Gy, 43.5 Gy, 43.4 Gy. The brain stem maximum doses were 44.2 Gy, 44.2 Gy, 45.7 Gy, 46.2 Gy. The mean MUs were 1792312, 3048518, 496101, 545105. Conclusions: For each energy mode, clinically acceptable plans were achieved for both IMRT and VMAT plans, although IMRT plans typically required higher monitor units and FFF mode significantly decreased the beam-on time. Minor dosimetric differences were observed in some organs, however clinical significance needs further evaluation. Author Disclosure: M. Abacioglu: None. S. Unal: None. Z. Ozen: None. B. Gunhan: None. S. Gurdalli: None. A. Arifoglu: None.
3737 Planning Quality and Delivery Accuracy Assessment of VMAT Plans for Whole Brain Treatment With Hippocampus Avoidance L. Huang, A. Rizzo, S. Chao, P. Qi, and P. Xia; Cleveland Clinic Foundation, Cleveland, OH Purpose/Objective(s): To investigate planning quality and delivery accuracy of volumetric modulated arc (VMAT) therapy plans for whole brain treatment with hippocampus avoidance following RTOG 0933. Materials/Methods: Five patients eligible for whole brain treatment with hippocampus avoidance were modeled for this study. For each case, three VMAT plans were created and delivered using two different linear accelerators from two different manufactories with three different dose rates, 400 MU/min, 600 MU/min, and 1000 MU/min. Each plan were created using the planning system for a prescribed daily dose of 3Gy. To meet the dose constraints from RTOG 0933 for the PTV and hippocampus, each plan included one full arc of 360 and one partial arc of 200 . Subsequently, each plan was delivered with a matrix device, measuring planar dose agreement between the planned and delivered dose, using the gamma index. Results: The average of total monitor units (MU) for each plan with the low, intermediate, and high dose rates were 848.787.5, 1201.3151.1 and 1155.4189.1, respectively. Accordingly, the dose received by 100% of the hippocampus (D100) were 8.03 0.57 Gy, 8.22 0.40 Gy and 7.400.50 Gy. The maximum doses of the hippocampus were 14.7 1.1 Gy, 15.11 1.17 Gy and 16.12 0.74 Gy. The average volume of the PTV receiving 37.50 Gy (V37.5) were 0.59 0.50%, 1.8 1.1%, and 1.00.58%. The
average volumes of the PTV received the prescription dose of 30 Gy were 91.6 1.35%, 91.0 0.85% and 91.0 1.25%. The volume of the PTV receiving 25 Gy were 97.01.2%, 96.20.8% and 97.50.9%. The maximum point doses for the whole brain were 41.13 2.47 Gy, 42.55 2.18 Gy and 44.97 3.19 Gy. All these plans were considered acceptable according to the RTOG protocol 0933. The accuracy of the plan delivery decreased as the dose rate increase, with the gamma index (3% 3mm) passing rates of 95.3 0.96%, 92.33.65% and 91.7 2.40%, respectively. Conclusion: The volumetric modulated arc therapy provides a good treatment option for the whole brain treatment with hippocampus sparing. The delivery accuracy is slightly decreased with the increased of the dose rate. Author Disclosure: L. Huang: None. A. Rizzo: None. S. Chao: None. P. Qi: None. P. Xia: None.
3738 Dosimetric Evaluation of Tungsten Eye-Shield in 9, 12, and 16 MeV Electron Beams Using EBT2 Gafchromic Films A. Anand, S. Tung, R. Tailor, W.H. Morrison, and R.X. Zhu; U.T. M.D.Anderson Cancer Center, Houston, TX Purpose/Objective(s): Commercially available tungsten eye-shield has been clinically tested in the past for protection to the ocular structures when treated with 9 MeV electron beam. Dosimetry underneath these eye shields is being reported for special situations requiring higher electron energies (12, and 16 MeV) for treatment of deep seated adenocarcinoma of eye. Materials/Methods: Large sized tungsten eye shield (wall thickness 5.8 mm, width 25mm) coated with 2mm thick dental acrylic was filled up with agarose gel (98 Wt.%) in order to mimic clinical setup conditions for these shields. Calibrated films (EBT2 Gafchromic) were employed for dosimetry measurements immediately under the shield, at dmax, and at deeper depths for 9, 12, and 16 MeV electron beams using a 10x10 electron-cone applicator. At each beam-energy, eye-shield attenuation was determined as ratio of the dose measured at depth of interest to the dose in an open field at dmax. Additionally, surface-dose enhancement was measured close to the eye-shield (which is primarily caused by scatter from eye-shield itself). Results: Immediately under the eye-shield, dose transmission from 9 and 12 MeV beams was found to be 2 % and 5.5 % respectively. However, transmission for the16 MeV beam was measured to be significantly larger (20%), implying higher doses to the surface of the phantom (cornea). At deeper depths, transmitted doses are relatively higher than the doses immediately under shield. This is primarily due to contribution from the lateral scatter generated within the phantom which increases with beam energy due to increase in range of scattered electrons. On the surface close to the shield, doses to the structures are enhanced due to lateral scatter from the shield itself. Dose enhancement, determined as the ratio of surface dose close to the shield versus dose 1.5 cm away from the shield on the surface, was found to be practically same 16-17% for all beam energies. Dose enhancement region extends over w10mm region around the shield. Conclusions: Even though this eye shield was designed for use with 9 MeV electron beams, results suggest that shielding of the ocular structures is adequate also for 12 MeV beam. Decision for use of this shield with higher energy (16 MeV) remains the subject of separate clinical discussion. It is important to note that electron-scattering from the shield itself, causes significant enhancement to surface doses to the structures around the shield Author Disclosure: A. Anand: None. S. Tung: None. R. Tailor: None. W.H. Morrison: None. R.X. Zhu: None.
3739 Dosimetric Comparison of Treatment Techniques for Skin Cancer Above the Clavicle: High-dose-rate 3-dimensional Topographic Applicator Brachytherapy Versus Electron Therapy R. Weinberg, S. Vyas, N. Thawani, D. Rangaraj, and S. Mutyala; Scott & White Healthcare, Temple, TX Purpose/Objective(s): Conventional radiation therapy treatment for skin cancers above the clavicle is electron beam therapy. An alternative treatment modality is high dose rate (HDR) brachytherapy with the use of surface