Setup accuracy for prone and supine whole breast irradiation.

Abstract

To evaluate cone-beam computed tomography (CBCT) based setup accuracy and margins for prone and supine whole breast irradiation (WBI). Setup accuracy was evaluated on 3559 CBCT scans of 242 patients treated with WBI and uncertainty margins were calculated using the van Herk formula. Uni- and multivariate analysis on individual margins was performed for age, body mass index (BMI) and cup size. The population-based margin in vertical (VE), lateral (LA) and longitudinal (LO) directions was 10.4/9.4/9.4mm for the 103 supine and 10.5/22.4/13.7mm for the 139 prone treated patients, being significantly (p < 0.01) different for the LA and LO directions. Multivariate analysis identified a significant (p < 0.05) correlation between BMI and the LO margin in supine position and the VE/LA margin in prone position. In this series, setup accuracy is significantly worse in prone compared to supine position for the LA and LO directions. However, without proper image-guidance, uncertainty margins of about 1cm are also necessary for supine WBI. For patients with a higher BMI, larger margins are required.