Dose adaptation to compensate for cumulative intra-fraction motion effects in online adaptive radiotherapy

<i>Objective.</i>The objective of this work was to investigate the feasibility of using 0 mm PTV margin in online adaptive radiotherapy for the first fractions, in combination with treatment-specific local compensation of accumulated underdosage to the target in the last fraction.<i>Approach.</i>Intrafraction motion patterns and delineations of twelve patients with prostate cancer were selected to cover a range of observed systematic and random inter- and intrafraction motion patterns. Treatment plans with 0 and 3 mm margins were created and dose was accumulated rigidly using the observed motion patterns. For the dose-adaptation approach a plan was created for the last treatment fraction locally compensating for dose missed in the previous fractions. Robustness of the accumulation was estimated by simulating treatments with random registration errors added to the observed registrations, with standard deviations of 0.5 and 1.0 mm.<i>Main results.</i>Target coverage of the dose-adaptive workflow was not-significantly below the standard approach, and at the desired level but for the two patients with the largest systematic prostate motion. The near-maximum dose to the organs at risk is lowered for all patients with a median of 1.5 Gy. The total volume receiving 95% of the prescribed dose was reduced by 15% to 1.6 times the clinical target volume indicating better conformity, at the cost of an increased near-maximum dose to the target. However, the dose-adaptive plan was less robust leading to a median 0.5% decrease in dose to the target also with decreasing robustness with larger motion patterns.<i>Significance.</i>The results demonstrate that a post-hoc correction of missed dose leads to an overall lower dose to nearby organs at risk at the cost of target dose near-maximum dose, making it a feasible approach for consideration.