TU-D-ValA-08: Geometric Considerations for Optimizing Beam Directions for IMAT Treatment of Lung Cancer with Mediastinum Nodal Irradiation

Abstract

Purpose: Based on geometric considerations, we optimized beam directions and arc ranges for intensity‐modulated arc therapy (IMAT) to maximize the sparing of lung and spinal cord while treating the mediastinum nodes and gross disease. Methods and Materials: A phantom with multiple planning target volumes (PTV) and lung with varying geometric shapes was created on Pinnacle 7.6c (Philips Medical Systems). A spinal cord was placed posteriorly. Mean lung and maximum cord dose were obtained for the various geometries for AP/PA and IMAT fields that were designed to spare the cord. We combined AP/PA and IMAT fields in order to achieve an optimal plan for each geometry, and seek correlations between patient geometry and their optimal relative weights. This combination of AP/PA and the IMAT fields were used to treat a patient in 2004 diagnosed with non‐small cell lungcancer with concurrent chemotherapy.Results: For a circular PTV in the phantom, the mean lungdose varied for AP/PA between 50% and 29% of the mean PTV dose, depending on the lung geometry, and for IMAT, between 50% and 46%. The maximum cord dose was 107% for AP/PA versus 33% for IMAT. Optimized composite plan of AP/PA and IMAT were created for each phantom geometry, balancing PTV dose,lung and cord tolerance. For the patient plan, the prescription dose was 60Gy, and the composite AP/PA‐IMAT plan gave a maximum cord dose of 45Gy with mean lungdose =16.1Gy and V20Gy =23%. Patient completed treatment and had no acute or late lung or spinal cord toxicities. Conclusions: IMAT was found to be useful in sparing the cord but not the lung, while AP/PA maximizes lung sparing but not the cord. A combination of AP/PA and IMAT provides an optimal class solution for treatment of lungcancer where mediastinum nodal irradiation is indicated.