203. Patient specific pre-treatment Quality Assurance in multiple lesions lung SBRT VMAT plans

Purpose SBRT of lung multiple lesions is a frequent clinical request. VMAT is preferentially used because it is possible to treat multiple lesions with single arc and single isocenter in a relatively short time. This treatment technique frequently make use of MLC interdigitation and the peripheral part of the treatment field, challenging conditions for both TPS calculation and LINAC delivery. The aim of this study is to evaluate the reliability of this irradiation technique in our department through different methods of patient specific pre-treatment Quality Assurance (QA). Methods VMAT plans have been calculated with the Monaco TPS for an Elekta Synergy Linac mounting the Agility MLC (80 leaf pair, 5 mm width, interdigitation). PTW Octavius 4D system and Octavius detector have been used to measure 3D dose distributions. Each plan has also been recalculated on the Tomotherapy Cheese phantom in order to measure absorbed dose at the center of each lesion with an Exradin A1SL ionization chamber. EBT3 gafchromic films have also been exposed in the Cheese phantom in coronal, sagittal or transversal planes to measure absorbed dose distributions. Results Even with good gamma passing rate (>90%, 3%, 3 mm, local), measurements with PTW Octavius 4D are sometime affected by poor spatial resolution and/or limited acquisition volume, and occasional failures in the high dose region have also been found. On the contrary, very good agreement ( Conclusions By using different independent methods of pre-treatment QA we came to the conclusion that lung SBRT VMAT plans of multiple lesion with single isocenter can be accurately delivered with the technology in use in our department.