SU‐E‐J‐29: Evaluation of Effectiveness of Immobilization Devices for Radiation Therapy with Repeated CBCT Studies

Purpose: To evaluate the effectiveness of three different immobilization devices for lung radiation therapy with daily and/or weekly cone‐beam CTs (CBCT). Methods: Three different immobilization devices, short (IM1) and long (Im²) Vac‐lok cushion with wing boards (Civco), BodyFix (Im³, Elekta), were investigated. Repeated CBCT images for 3 SBRT patients immobilized with Im³, and 6 IMRT patients immobilized with IM1 and Im² (3 each) were acquired. Each SBRT patient received 70 Gy in 10 fractions and setup daily with CBCT, while IM1 and Im² patients were setup daily with orthogonal kV, and weekly CBCT (week 0–6). The day 1, 6 and 10 CBCT for Im³ were considered week 0–2 CBCT. Patient setup was first based on skins markers then shifts were made based on bony landmarks in kV and GTV with CBCT. Skin contours, which are further away from the isocenter and thus more sensitive to rotation and skin folding, were extracted in each CBCT data set and the difference on skin contours were investigated for both before and after shifts. Results: Over 2 weeks of treatment, Im³ has the smallest standard deviation for setup shifts (0.09, 0.16, 0.20 cm on LR, UD and IO directions) compared to IM1 (0.29, 0.35, 0.34 cm) and Im² (0.11, 0.19, 0.40 cm), which indicates setup based on skin markers were the most reproducible for Im³. Over 6 weeks of treatment, Im² (0.16, 0.18, 0.42 cm) is more reproducible than IM1 (0.44, 0.45, 0.35 cm). All 3 patients with IM1 and 2 patients with Im² were found to have >1 cm difference in skin contour even after shifts were made. 1 patient with IM1 was found to have non‐reproducing skin folding of >3 cm difference. Conclusions: Information derived from repeated CT images could be used to evaluate the effectiveness of the immobilization devices.