TH‐A‐116‐03: Photon and Proton Radiotherapy of the Lung Would Benefit From 4D Dose Calculation Techniques

Abstract

Purpose: To compare the effects of respiratory motion by calculating four‐dimensional (4D) dose accumulation on proton and photon radiotherapy plans for a stage III lung cancer cohort.Methods and Materials: This study used passively scattered proton therapy (PSPT) and photon intensity modulated radiotherapy (IMRT) plans designed to meet the same criteria for an ongoing clinical trial. For 20 patients, 4D dose evaluation was calculated on both the PSPT and IMRT plans by accumulating individual phase dose using an intensity‐based in‐house deformable image registration algorithm. The resulting 4D dose accumulation method was compared against current standard of three dimensional (3D) dose as calculated on the average CT. Results: Both proton and photon plans demonstrated dosimetric differences when accounting for respiratory motion using 4D dose calculation methods. Reported 4D‐3D values are mean plus one standard deviation. 4D‐3D difference in PTV 95% dose coverage was >2% for 5 IMRT (0.5±1.1Gy) and 4 PSPT plans (0.5±0.8Gy). The structural doses most affected by respiratory motion were normal structures distal to the tumor, such as esophagus V65 (IMRT: −0.5±1.2Gy, PSPT: −0.4±1.4), mean heart (IMRT: − 0.4±0.6 Gy, PSPT: −0.3±0.9 Gy) and spinal cord maximum (IMRT: 0.0±0.2Gy, PSPT: 1.5±2.9Gy). No correlations were found between tumor motion and the change in 4D dose difference for any lung, heart, or spine dose‐volume index. Conclusions: The use of 4D dose evaluation techniques at time of treatment planning revealed small but significant respiratory motion induced deviations from the 3D planned dose for both photon and proton therapy. In both modalities, differences between 3D and 4D dose distributions are patient dependent; no population trends were found correlating dose differences to the extent of tumor motion. The implementation of 4D dose calculation methods for proton and photon lung cancer radiotherapy plans may help identify patients for whom respiratory motion alters the planned dose distribution. This project is supported by grant P01CA021239 from the National Cancer Institute.