ESTRO 38 Abstract book

S1013 ESTRO 38

Results All esophageal cancer plans generated with the auto- planning module met the clinical dose requirements. In comparison group of upper thoracic esophageal cancer plan, homogeneity index has no significant differences between two groups(P=0.115), the conformity Index of targets was superior in the automated plans than in manual plans (P=0.027). In addition, the automated plans had significantly reduced the dose of lung received, lung’s V 5 , V 10 , V 20 , V 30 was reduced by 2.1%, 1.6%, 3%, 1.2% respectively (P=0.001, 0, 0, 0.01). The dose of cord received was not significant differences between two groups(P=0.892). In comparison group of thoracic esophageal cancer plan, homogeneity index was not significant differences between two groups(P=0.428), the conformity Index of targets was superior in the manual plans than in automated plans (P=0.048). Lung’s V 5 had no significant differences(P=0.053), lung’s V 10 , V 20 , V 30 was reduced by 2%, 3.1%, 1.3% （ P=0.032, 0, 0.008 ） . The dose of cord and heart received has no significant differences between two groups(P=0.204, 0.894). Conclusion Auto-Planning module can improve the overall quality of treatment plans, reduced plans quality differences due to inexperienced planners. Compared with the manual planning, the Auto-planning have substantially shorter manual planning time and improved planning efficiency. It is feasible to generate automated IMRT plans with automated planning module for esophageal cancer patients. The relatively complex esophageal cancer plans still needs to be manually optimized after an automatic planning of optimization. EP-1865 A feasibility study: 6 VMAT arcs full collimator opening for hippocampal sparing M. Maffei 1 , S. Bou Selman 1 , H. Stefan 2 , M. Haller 2 , P. Ferrari 2 1 Azienda Sanitaria dell'Alto Adige, Servizio di radioterapia oncologica, Bozen, Italy ; 2 Azienda Sanitaria dell'Alto Adige, Servizio di fisica sanitaria, Bozen, Italy Purpose or Objective to assess the feasibility of 6 arcs with full collimator opening for an efficient VMAT planning approach for hippocampal-avoidance whole-brain radiation therapy Material and Methods 11 patient were considered, prescribed dose was 30 Gy in 10 fractions. Contouring was in accordance to RTOG 0933. VMAT Treatment Planning Techniques was applied using 6 coplanar arcs. Full collimator opening was used with the following parameters: arc1 181°-179° clockwise and 95° collimator, arc2 179°-181° counterclockwise and 265° collimator, arc3 181°-300° clockwise and 350° collimator, arc4 60°-179° clockwise and 350° collimator, arc5 179°- 60° counterclockwise and 10° collimator and arc6 300°- 181° counterclockwise and 10° collimator. Specific physical objective were used for PTV coverage and hippocampal sparing as RTOG protocol suggested. Conformity and homogeneity index were calculated. Statistical analysis was performed. Results The coverage of PTV was warranted D90% = 28.8±0.3 Gy, D2% = 31.6±0.4 Gy and for Hippocampal D100% =8.5±0.6 Gy, max dose = 14.9±0.3 Gy. Conformity and homogeneity index were equal to 1.11±0.01 and 0.86±0.03 respectively. 600±10 segments and a total of 1170±150 MU were used. Total delivery time 516±16 sec. Conclusion 6 VMAT arcs full collimator opening for hippocampal sparing was feasible and showed a easy coverage in PTV saving hippocampal in accordance of RTOG 0933 constrains. The specific rotation of collimator in the 6 arcs, warranted a easy calculation and deliverability of the plan, with 1000 MU and 500 sec of delivery time.

EP-1866 An interplay effect study comparing two different VMAT techniques for free-breathing moving targets M. Gil Conde 1 , Á. Seguro 1 , F. Campos 1 , M.J. Albertos 1 , M.A. Iborra 1 , L. Díaz 2 1 Hospital de Jerez, Radiation Physics and Radiological Protection, Jerez de la Frontera, Spain ; 2 Hospital de Jerez, Radiation Oncology, Jerez de la Frontera, Spain Purpose or Objective Dosimetric impact due to interplay effect is studied in VMAT plans for lung tumors in free-breathing patients. The magnitude of this effect is compared in two treatment planning techniques: pure VMAT and hybrid VMAT. Material and Methods A programmable respiratory motion phantom was used, simulating a typical breathing pattern with a maximum longitudinal amplitude of 20 mm and a variable period around 5 s. A Pinpoint chamber (0.016 cc) was introduced inside the phantom in a methacrylate holder that moved in the longitudinal direction following the mentioned pattern. 5 VMAT plans that had been previously planned for real patients were used as irradiation technique in a Varian Truebeam. They used 6 MV, variable dose rate of maximum 600 MU/min and 2 half-arcs of 180.1º-0º/0º- 180.0º, depending on the laterality of the tumor. Alternatively, a hybrid plan (80% 3DCRT – 20% VMAT) was designed for each patient, consisting of 3 conformal 6 MV static fields plus a 6 MV half-arc to homogenize the plan. The phantom was positioned aligning the chamber with the Linac isocenter so during the breathing cycle the chamber moved around this point. Absolute dose was measured under these conditions for each plan in 5 treatment fractions. Results Measured dose per fraction and after the 5 fractions of each plan was obtained separately. For all the patients studied, the mean deviation of the measured dose ( D meas ) from the expected one ( D exp ) after 5 fractions for VMAT and hybrid plans was 0.43% and -0.64%, respectively. Mean standard deviation of D meas in all fractions was of 4.85 cGy for VMAT versus 2.96 cGy for the hybrid plans, however. Maximum deviation found of D meas in one fraction between all the VMAT and hybrid plans was of -4.00% and -3.58%, respectively. The maximum mean dose deviation along 5 fractions in a single beam was of 16.7% with VMAT and - 8.50% when using the hybrid technique. This last deviation was observed in an arc. Maximum mean dose deviation found between all the 3DCRT beams was of 3.18%, being the average standard deviation for the 5 patients 0.14 cGy.

Conclusion Despite the standard deviation of absolute dose values between fractions for the same patient is higher with VMAT than with the hybrid technique and even higher when compared to 3DCRT, differences in the mean dose after 5 treatment fractions between both cases are not significant. Interplay effect when combining VMAT with respiratory movement is observed in single arcs. Nevertheless, the random character of this effect makes deviation of different arcs in the same plan or inter- fraction dose deviation cancel out, not being clinically relevant after adding all the fractions.