Online Adaptive MRI-Guided Radiotherapy for Primary Tumor and Lymph Node Boosting in Rectal Cancer.

The purpose of this study was to characterize the motion and define the required treatment margins of the pathological mesorectal lymph nodes (GTV_ln) for two online adaptive MRI-guided strategies for sequential boosting. Secondly, we determine the margins required for the primary gross tumor volume (GTV_prim). Twenty-eight patients treated on a 1.5T MR-Linac were included in the study. On T2-weighted images for adaptation (MRI_adapt) before and verification after irradiation (MRI_post) of five treatment fractions per patient, the GTV_ln and GTV_prim were delineated. With online adaptive MRI-guided radiotherapy, daily plan adaptation can be performed through the use of two different strategies. In an adapt-to-shape (ATS) workflow the interfraction motion is effectively corrected by redelineation and the only relevant motion is intrafraction motion, while in an adapt-to-position (ATP) workflow the margin (for GTV_ln) is dominated by interfraction motion. The margin required for GTV_prim will be identical to the ATS workflow, assuming each fraction would be perfectly matched on GTV_prim. The intrafraction motion was calculated between MRI_adapt and MRI_post for the GTV_ln and GTV_prim separately. The interfraction motion of the GTV_ln was calculated with respect to the position of GTV_prim, assuming each fraction would be perfectly matched on GTV_prim. PTV margins were calculated for each strategy using the Van Herk recipe. For GTV_ln we randomly sampled the original dataset 20 times, with each subset containing a single randomly selected lymph node for each patient. The resulting margins for ATS ranged between 3 and 4 mm (LR), 3 and 5 mm (CC) and 5 and 6 mm (AP) based on the 20 randomly sampled datasets for GTV_ln. For ATP, the margins for GTV_ln were 10-12 mm in LR and AP and 16-19 mm in CC. The margins for ATS for GTV_prim were 1.7 mm (LR), 4.7 mm (CC) and 3.2 mm anterior and 5.6 mm posterior. Daily delineation using ATS of both target volumes results in the smallest margins and is therefore recommended for safe dose escalation to the primary tumor and lymph nodes.