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

Abstract

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 D-99% 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 cm(3)) on average. Conclusion: Online DART is technically feasible, achieving adequate delivered accumulated CTV dose, while substantially sparing the healthy tissue.