SU-E-T-541: A Clinical Approach to Account for the Attenuation of Varian Exact IGRT Couch Top for Spine Stereotatic Radiosurgery Patients

Abstract

Purpose: The purpose of this work is to develop an approach to account for the attenuation of Varian Exact IGRT couch top for spine cancer patients in spine stereotatic radiosurgery. Methods: Varian Exact IGRT couch which has carbon fiber plates sandwiched with a plastic foam core was used for this study. Transmission of 6MV x‐rays through the couch was measured at various oblique beam angles. Based on the measured values, we modeled the couch top as a stored organ in Pinnacle treatment planning system (TPS) for spine stereotatic radiosurgery (SSRS) patients. A planner can load the virtual couch top and override the density when designing a treatment plan to account for the couch attenuation explicitly. The dosimetric impact of the couch top on DVHs was investigated in 3 representative patients. Results: At normal incidence, percentage of the attenuation of the Exact IGRT couch for a 6MV beam was measured to be 2.40% and 2.68% for field size of and , respectively. The attenuation was found to be gantry angle dependent, with maximum attenuation 3.90% at 120° for field size of and 4.25% at 130° for , respectively. By using a fixed density of 1.05 within the thin carbon fiber layers, the calculated attenuation for the virtual couch top were 2.38% and 2.94% for field, 4.15%, and 4.57% for field, respectively, which agreed well with the measurement. Without considering the couch attenuation, the average loss of target coverage was 7.1% in three representative cases we analyzed. Conclusions: The attenuation of Varian Exact IGRT Couch Top should not be ignored when using the couch top for SSRS patients. A clinical approach was developed to account for the attenuation. A calibrated virtual couch model can be used to account for the attenuation in the treatment planning process.