Authors: Lauren Wilson, MSS, Rob Rohe, BS, MS, RT(T), Nishele Lenards, PhD, CMD, RT(R)(T), FAAMD, Ashley Hunzeker, MS, CMD, Matt Tobler, CMD, RT(T), Sabrina Zeiler, MS, CMD, RT(T), Ashley Fellows, MS, CMD, RT(T)

Medical Dosimetry Program at the University of Wisconsin-La Crosse, La Crosse, WI 54601 USA

Abstract
The prone position is frequently used for breast irradiation in an effort to minimize dose to normal tissue and reduce skin toxicities. Immobilization required for prone breast irradiation can cause collision issues with the linear accelerator, disrupting treatment and negatively affecting the patient experience. The purpose of this retrospective study was to determine if an isocenter location guideline could be developed to prevent collisions with the prone breast immobilization device and gantry head, while still creating a clinically acceptable treatment plan. Clearance isocenter guidelines were established by measuring clearance between the Civco Horizon breast board and Varian linear accelerator. Fourteen patients with known clearance issues at a single institution were selected for this study and re-planned using clearance isocenter guidelines. Collision plans were compared to clearance plans created within the established clearance threshold through the institutions breast treatment guidelines based on arm II of the Radiation Therapy Oncology Group (RTOG) 1005 recommendations. Researchers in this study demonstrated clinical relevance by establishing that a clearance isocenter location guideline can be developed to prevent collisions with the prone breast immobilization and gantry head, while still creating a clinically acceptable treatment plan.