ESTRO 2021 Abstract Book

S971

ESTRO 2021

registration software, was used as planning scan. CT scans of patients scanned in arms-up position were augmented with artificial arms (water density) to approximate our MR-linac treatment position. An initial set of IMRT plans was created using Monaco v5.40.01 TPS (Elekta AB) following the EORTC 22113-08113- LungTech- inspired departmental guidelines for MR-linac lung SBRT. Manual delineations of the CS were added to the existing clinical OAR delineations under supervision of a senior cardiothoracic radiologist. Eleven CS were delineated including the four chambers and the ascending aorta. Dose to CS in the initial plans was then evaluated against 5 CS constraints derived from the literature (Fig. 1). Plans that violated one or more CS constraints were replanned. Cardiac sparing plans (CSP) were generated by adding CS constraints to the optimization and actively avoiding CS through manual beam angle optimization. All CSPs were reviewed by an experienced dosimetrist. Results CS constraints were violated in 16 out of 34 initial plans. Standard OAR constraints could be met in 14 of these 16 cases, comprising 8 central and 6 ultra-central cases. A CSP proved feasible for 9 out of remaining 14 cases (64%), 4 of the feasible CSPs belonged to ultra-central cases and 5 to central cases. Infeasibility of a CSP was most often attributed to exceeding the constraint on the left atrium, this was more often the case for larger tumors. Average PTV size was 37 cc for the successful sparing cases relative to 136 cc for the unsuccessful sparing cases. A comparison between an initial plan, in which the constraint on the base of the heart is violated, and corresponding CSP is shown in Fig. 2.

Conclusion