ESTRO 2024 - Abstract Book

S3552

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2024

1299

Poster Discussion

Individualised isotoxic radiotherapy for re-irradiation in recurrent pelvic cancers

Christopher J. H. Pagett 1 , John Lilley 1 , Ane Appelt 1,2 , Louise Murray 3,2 , Stina Svensson 4 , Kjell Eriksson 4 , Rasmus Bokrantz 4 , Jakob Ödén 4 , Christopher Thompson 1 1 Leeds Teaching Hospitals NHS Trust, Department of Medical Physics, Leeds, United Kingdom. 2 University of Leeds, Leeds Institute of Medical Research at St James's, Leeds, United Kingdom. 3 Leeds Teaching Hospitals NHS Trust, Department of Clinical Oncology, Leeds, United Kingdom. 4 RaySearch Laboratories, Research, Stockholm, Sweden

Purpose/Objective:

Re-irradiation is highly individualised due to the variation in the previous dose received by organs at risk (OARs) and their subsequent proximity to re-irradiation planning target volumes (PTVs). This can make re-irradiation planning aims harder because cumulative OAR constraints are more likely to limit PTV coverage. This complex situation means that a standard re-irradiation prescription is unlikely to be optimal for the majority of patients, making an isotoxic approach more attractive. This work uses an isotoxic planning approach that optimises the cumulative equieffective dose in 2Gy (EQD2) to OARs whilst maximising PTV dose with the aim to maintain or improve PTV coverage (D95%) without compromising OAR dose constraints.

Material/Methods:

Ten patients with recurrent pelvic cancer were included, encompassing various levels of re-irradiation complexity, including differing PTV/OAR proximity and 1-3 PTVs per patient. The isotoxic planning approach was applied in a research version of RayStation treatment planning system (TPS) (RayStation V11A, RaySearch, Stockholm). Treatment planning used a novel (and non-clinical) module for re-irradiation plan optimisation which takes previously delivered dose into account during inverse plan optimisation (described in previous publications). The cumulative EQD2 OAR constraints used were (all D0.1cc) 89.9Gy (colon, rectum); 78.8Gy (small bowel); 67.2Gy with 33% recovery on background dose (sacral plexus, cauda equina); 80.6Gy (bladder) and 144.2Gy (vessels). The bladder and vessels limits were considered optimal only but kept as low as reasonably practicable (ALARA). An α/β of 3Gy was used for all OAR except sacral plexus and cauda equina where it was 2Gy. The EQD2 optimisation method used trusted the DIR for most OARs but for some (small bowel, colon, and rectum) the maximum point dose (D0.1cc) in the original plan within 2cm of re-irradiation PTV was used as the background dose for all voxels in these OAR in the optimisation. The remaining OAR were optimised using voxel-by-voxel background dose. The creation of both the standard and isotoxic plans followed a standard methodology, with each planned using a 360° VMAT arc. The optimisation process was as follows: (1) optimise normally using standard objectives, (2) if any OAR dose constraint was exceeded, the relative weight of PTV objectives was dropped until the OAR passed, (3) further adjustment of the relative weight on OAR constraints. After creating the standard 30Gy in 5 re irradiation plan, the prescription dose was iteratively adjusted, increasing, or decreasing in major increments of 5Gy and minor increments of 1Gy until the isotoxic plan was achieved.