Online dose-adaptive radiotherapy considerably reduces irradiated volume in rectal cancer radiotherapy

Background and purpose: Online adaptive radiotherapy (OART) allows for a reduction of geometric uncertainties and therefore smaller PTV margins. In this work we take the next step and introduce online dose-adaptive radiotherapy (DART), where the treatment plan is adapted not only to the geometry of the day but also to the accumulated dose delivered in previous fractions. Materials and methods: 14 rectal cancer patients that were treated with OART with 5x5Gy on a 1.5 T MR-Linac were retrospectively included. For fractions 1–4, a daily treatment plan using 1 mm PTV margin was generated. Actual delivered dose was estimated by recalculation of this plan on the post-treatment scan. By means of deformable image registration, the delivered doses were accumulated on the adaptation scan of fraction 5. For fraction 5, a plan was optimized to the prescribed dose, using the accumulated dose of fractions 1–4 as background. Target coverage and irradiated volume of the final accumulated delivered dose was compared with the accumulated delivered dose from conventional OART using clinical PTV margins. Results: After dose accumulation, DART led to a mean decrease in CTV D99% of 0.5 Gy compared to conventional OART, however, conform protocol, target coverage was sufficient in >90 % of patients. The median dose in the 1 cm ring around the clinical PTV was reduced by 3.6 Gy and the patient volume that received 95 % of the prescribed dose decreased by 32 % (221 cm3) on average. Conclusion: Online DART is technically feasible, achieving adequate delivered accumulated CTV dose, while substantially sparing the healthy tissue.