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(adapt)), verification prior to (MRI(ver)) and after irradiation (MRI(post)) of 5 treatment fractions per patient, the primary tumor GTV (GTV(prim)) and mesorectum CTV (CTV(meso)) were delineated. The structures on MRI(adapt) were expanded to corresponding PTVs. We determined the required expansion margins such that on average over 5 fractions, 98% of CTV(meso) and 95% of GTV(prim) on MRI(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(ver) and MRI(post.) A threshold to assess the need for a secondary adaptation was determined by considering the overlap between MRI(adapt) and MRI(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(meso) and 5.0 mm isotropically for GTV(prim). A 2nd adaptation, applied for all fractions where the motion between MRI(adapt) and MRI(ver) exceeded 1 mm (36% of the fractions) would result in a reduction of the PTV(meso) margin to 3.2 mm/2.0 mm. For PTV(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(meso) and GTV(prim) respectively, could result in a 30–50% margin reduction with limited reduction of dose to the bowel.