Effect of intrafraction adaptation on PTV margins for MRI guided online adaptive radiotherapy for rectal cancer

Purpose To determine PTV margins for intrafraction motion in MRI-guided online adaptive radiotherapy for rectal cancer and the potential benefit of performing a 2nd adaptation prior to irradiation. Methods Thirty patients with rectal cancer received radiotherapy on a 1.5 T MR-Linac. On T2-weighted images for adaptation (MRI.sub.adapt), verification prior to (MRI.sub.ver) and after irradiation (MRI.sub.post) of 5 treatment fractions per patient, the primary tumor GTV (GTV.sub.prim) and mesorectum CTV (CTV.sub.meso) were delineated. The structures on MRI.sub.adapt were expanded to corresponding PTVs. We determined the required expansion margins such that on average over 5 fractions, 98% of CTV.sub.meso and 95% of GTV.sub.prim on MRI.sub.post was covered in 90% of the patients. Furthermore, we studied the benefit of an additional adaptation, just prior to irradiation, by evaluating the coverage between the structures on MRI.sub.ver and MRI.sub.post. A threshold to assess the need for a secondary adaptation was determined by considering the overlap between MRI.sub.adapt and MRI.sub.ver. Results PTV margins for intrafraction motion without 2nd adaptation were 6.4 mm in the anterior direction and 4.0 mm in all other directions for CTV.sub.meso and 5.0 mm isotropically for GTV.sub.prim. A 2nd adaptation, applied for all fractions where the motion between MRI.sub.adapt and MRI.sub.ver exceeded 1 mm (36% of the fractions) would result in a reduction of the PTV.sub.meso margin to 3.2 mm/2.0 mm. For PTV.sub.prim a margin reduction to 3.5 mm is feasible when a 2nd adaptation is performed in fractions where the motion exceeded 4 mm (17% of the fractions). Conclusion We studied the potential benefit of intrafraction motion monitoring and a 2nd adaptation to reduce PTV margins in online adaptive MRIgRT in rectal cancer. Performing 2nd adaptations immediately after online replanning when motion exceeded 1 mm and 4 mm for CTV.sub.meso and GTV.sub.prim respectively, could result in a 30-50% margin reduction with limited reduction of dose to the bowel.