ESTRO 2023 - Abstract Book

S1786

Digital Posters

ESTRO 2023

Conclusion We have identified a low level of grade 2 acute adverse events in the two treatment regimens, which allows us to validate our dose constraints in terms of isotoxicity. By adopting a CHHIP treatment scheme in hypofractionation, it has led to a decrease in the doses received by the OARs. Thus, it may favor tolerance to treatment.

PO-2012 In-silico feasibility study for short course MR-guided radiotherapy in rectal cancer with boost

H. Harford-Wright 1 , B. George 2 , J. Drabble 2 , A. Martin 1 , K. Owczarczyk 1 , J. Good 2

1 GenesisCare UK, Cromwell Hospital, London, United Kingdom; 2 GenesisCare UK, Oxford, Oxford, United Kingdom

Purpose or Objective There is considerable interest in developing primary organ preservation strategies in rectal cancer aiming to preserve the rectum and its function while minimizing long term pelvic toxicity. MR-guided Radiotherapy (MRgRT) offers enhanced soft tissue contrast providing excellent tumour and OAR visualisation, the ability to adapt to anatomy of the day and monitor intra fraction motion. We carried out a planning study to determine the benefit of using MRgRT in delivering short course radiotherapy to spare organs at risk (OAR) and allow for safe dose escalation to primary and involved nodes. Materials and Methods Retrospective planning study. Target volumes (GTVp, GTVn, CTVelective) were created in reference to published atlases [1]. 3mm GTV-PTV and CTV-PTV margins were applied with knowledge a clinical protocol would utilize daily adaption and beam gating. The PTV was prescribed 25Gy in 5 fractions with 30Gy Simultaneous Integrated Boost (SIB) to primary tumour and 35Gy to involved nodes. OARs were defined as bladder, uninvolved colon, small bowel, sacral nerves, vagina, and neurovascular bundle (NVB). Dosimetric analysis of test plans created using fixed field IMRT technique. Target coverage and OAR doses were reported. Results Plans achieved clinically acceptable PTV V(100%) and V(95%) coverage. For PTV2500 without SIB, MRgRT achieved a reduction in Dmax and Dmean to bladder, bowel, vagina and NVB as compared to PTV planned using conventional expansions. MRgRT plans allowed for SIB to both primary and nodal targets to be covered by 95% of prescribed dose while remaining within all mandatory OAR dose constraints.