# Isotoxic stereotactic reirradiation for recurrent pelvic cancers

## TL;DR

This study shows that personalized radiation doses for pelvic cancer can be safely increased using advanced planning techniques.

## Contribution

The study demonstrates the feasibility of isotoxic reirradiation planning in pelvic cancers using radiobiological voxel-based methods.

## Key findings

- Isotoxic reirradiation doses up to 50 Gy were safely achievable in 23 out of 25 cases.
- Median escalated dose was 42 Gy, with four patients reaching the maximum of 50 Gy.
- Vessels and the sacral plexus were the most common dose-limiting organs.

## Abstract

•Radiobiological voxel-based planning used to guide isotoxic pelvic reirradiation.•Dose from previous radiotherapy incorporated using deformable registration.•Dose escalation feasible in 23/25 cases without exceeding organ at risk limits.•It is feasible to safely personalise pelvic reirradiation dose between 30–50  Gy.•Median escalated dose was 42  Gy; four patients reached the 50  Gy maximum dose.

Radiobiological voxel-based planning used to guide isotoxic pelvic reirradiation.

Dose from previous radiotherapy incorporated using deformable registration.

Dose escalation feasible in 23/25 cases without exceeding organ at risk limits.

It is feasible to safely personalise pelvic reirradiation dose between 30–50  Gy.

Median escalated dose was 42  Gy; four patients reached the 50  Gy maximum dose.

Reirradiation is clinically challenging, requiring a balance between delivery of dose to tumour while respecting cumulative organ at risk (OAR) dose constraints. Standard prescriptions are often conservative, ignoring patient variability in achievable OAR doses. Isotoxic radiotherapy individualises treatment by delivering the highest equieffective dose in 2 Gy per fraction (EQD2Gy) while meeting OAR constraints. This technical feasibility study assessed isotoxic pelvic reirradiation using cumulative OAR constraints, the original dose distribution as background, and voxel-by-voxel EQD2Gy optimisation.

Data from 30 patients previously treated with pelvic stereotactic body radiotherapy (SBRT) at three UK centres were included. OARs were delineated on both previous and reirradiation image sets and deformably registered. Previous dose was mapped to the current image set and used as background dose for SBRT planning, following published methods. Initial 25 Gy in five fractions (25 Gy/5#) plans were generated for all patients, with further isotoxic dose escalation conducted up to a maximum of 50 Gy (fraction number fixed) until cumulative EQD2Gy constraints were reached.

For 25 of 30 patients, clinically acceptable isotoxic plans were obtained, with 23 exceeding the standard UK reirradiation prescription dose of 30 Gy/5#. The median isotoxic prescription was 42 Gy/5#, with four patient plans reaching the upper evaluated limit of 50 Gy. Vessels and the sacral plexus were most frequently dose limiting.

This study highlighted the feasibility of isotoxic pelvic reirradiation and supports further investigation into automation and prediction models to streamline implementation in clinical practice.