ESTRO 2022 - Abstract Book

S418

Abstract book

ESTRO 2022

treatment. In this study, we develop a method for markerless lung tumor localization in cine MV images of IMRT treatment fields and demonstrate use of the method to determine the tumor position error during DIBH treatments.

Materials and Methods

25 lung cancer patients were treated with IMRT in DIBH in 20-33 fractions with daily cone-beam CT (CBCT) guided setup based on the primary tumor. Cine MV images were recorded for all treatment fields at most fractions. Post-treatment, the shift of the tumor position in the MV images relative to the intended position from the CBCT setup procedure was found as described in the following and summarized in Fig 1. First the individual cine MV frames with IMRT segments were pieced together to form a composite MV image that covered the entire MV beam aperture (L1 in Fig 1). Here, the possible tumor motion during the DIBH field delivery was neglected. Next, an in-house developed ray tracing algorithm was used to generate digitally reconstructed radiographs (DRRs) based on the entire CBCT (DDR total ), a slab of the CBCT near the isocenter plane containing the tumor volume (DDR tumor ), and the entire CBCT except for the tumor containing slab (DDR non- tumor ) (L2). All DRRs were rescaled to have similar contrasts as the composite cine MV image (L3). DDR non-tumor was then subtracted from the composite cine MV image to generate a tumor-enhanced cine MV image (L4) in which the tumor position was found by template-based segmentation (L6) using the projected tumor shape in DDR tumor as template (L5). The tumor segmentation has been performed for one patient at the time of abstract deadline and is currently being extended to all 25 patients.

Results