# Efficient strategy for magnetic resonance image-guided adaptive radiotherapy of rectal cancer using a library of reference plans

## TL;DR

A new method using a library of reference plans improves the efficiency and effectiveness of MRI-guided radiotherapy for rectal cancer.

## Contribution

A novel adaptive radiotherapy strategy using a library of reference plans that improves plan quality and reduces treatment time.

## Key findings

- LoRP achieved 92% clinical acceptability compared to 74% with conventional methods.
- Treatment time was reduced by over 20 minutes compared to fully adaptive methods.
- LoRP provided 94% target coverage matching fully adaptive strategies.

## Abstract

•Magnetic resonance-guided adaptive radiotherapy benefits from a plan library.•Plans meeting criteria increased to 92%, up from 74% with the conventional method.•Treatment time decreased from 55.1 to 34.9 min compared with fully adaptive method.

Magnetic resonance-guided adaptive radiotherapy benefits from a plan library.

Plans meeting criteria increased to 92%, up from 74% with the conventional method.

Treatment time decreased from 55.1 to 34.9 min compared with fully adaptive method.

Adaptive radiotherapy for patients with rectal cancer using a magnetic resonance–guided linear accelerator has limitations in managing bladder shape variations. Conventional couch shifts may result in missing the target while requiring a large margin. Conversely, fully adaptive strategy is time-consuming. Therefore, a more efficient strategy for online adaptive radiotherapy is required.

This retrospective study included 50 fractions from 10 patients with rectal cancer undergoing preoperative radiotherapy. The proposed method involved preparing a library of reference plans (LoRP) based on diverse bladder shapes. For each fraction, a plan from the LoRP was selected based on daily bladder filling. This plan was compared with those generated by conventional couch shift and fully adaptive strategies. The clinical acceptability of the plans (i.e., per protocol, variation-acceptable, or unacceptable) was assessed.

In per protocol criterion, 44 %, 6 %, and 100 % of the plans for LoRP, conventional couch shift, and fully adaptive strategies were achieved, respectively. In variation-acceptable criterion, 92 % of LoRP plans and 74 % of conventional couch shift plans were achieved. LoRP demonstrated 94 % target coverage (100 % prescription dose) in the fully adaptive strategy compared with 91 % in conventional couch shift strategy. The fully adaptive strategy had the best performance in sparing the intestine and colon. LoRP reduced the treatment session duration by more than a third (>20 min) compared with the fully adaptive strategy.

LoRP achieved adequate target coverage with a short treatment session duration, potentially increasing treatment efficiency and improving patient comfort.