Retrospective Assessment of Surface Imaging Set up Accuracy for Intensity Modulated Proton Therapy of Comprehensive Breast Irradiation

Abstract

The purpose of this work is to retrospectively assess differences in pretreatment set-up between x-ray-based alignment (kV pair and/or CBCT) and the CatalystPT Surface Imaging System (C-RAD) to quantify accuracy and determine maximum deviation of intra-fractional motion in breast patients treated with proton therapy. Breast patient setup differences were evaluated for ninety-seven patients (n = 97) treated by proton therapy between 2019 and 2022 at our institution. Each patient was initially aligned using C-RAD followed by orthogonal planar x-rays and/or CBCT to finalize pre-treatment localization. The couch coordinates from C-RAD immediately after patient positioning were captured and compared with the finalized couch coordinates using x-ray-based alignment. Additionally, the maximum 6D deviation of patient position recorded by C-RAD during the course of treatment was captured. Patient BMI and age were correlated with setup differences between C-RAD and x-ray imaging and evaluated using Pearson Correlation Coefficients (PCC) and Analysis of Variance (ANOVA) test. The absolute mean displacement was calculated over the entire course of treatment and reported by comparing the absolute difference between C-RAD and x-ray setups for each patient. The absolute mean displacement difference calculated by the mean of all the patients was found to be 2.5±1.1 mm (lateral), 2.8±1.2 mm (longitudinal), 2.5±1.0 mm (vertical), 1.2±0.5° (Yaw), 1.0±0.5°(Pitch) and 1.0±0.5°(Roll). The relative mean displacement difference with respect to the couch coordinate finalized by x-ray imaging was calculated by the mean of all the patients and was found to be 3.9±1.8 mm (lateral), 4.8±1.7 mm (longitudinal), 3.1±1.6 mm (vertical), 1.2±0.7° (Yaw), 1.6±0.7°(Pitch) and 1.3±0.6°(Roll). The mean maximum deviation for the entire course of treatment reported by C-RAD for all the patients was calculated to be 3.3±0.9 mm. Statistically, there are no significant correlations between BMI or age for mean displacement difference (BMI: PCC = 0.18, p-value = 0.075, Age: PCC = 0.004, p-value = 0.80705) and maximum deviation (BMI: PCC = 0.023, p-value = 0.72, Age: PCC = -0.089, p-value = 0.37). C-RAD was able to position breast patients to within 3 mm of the isocenter and monitor the intra-fractional motion of patients which was found to be 3.3±0.9 mm.