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.