ESTRO 2025 - Abstract Book

S1720

Clinical - Sarcoma & skin cancer & malignant melanoma

ESTRO 2025

3942

Mini-Oral Preliminary dose-volume constraints predicting subcutaneous tissue fibrosis for limb sarcoma using the VorteX and IMRiS trials. Rita Simoes 1,2,3 , Beatrice Seddon 4 , Martin Robinson 5 , Shumona Shelly 6 , Temi Adedoyin 6 , Hakim-Moulay Dehbi 7 , Piers Gaunt 8 , Ana Hughes 8 , Elizabeth Miles 9 , Peter Hoskin 10 , Kevin Harrington 1,11 , Sarah Gulliford 9,12 , Aisha Miah 2 1 Radiotherapy and Imaging, The institute of Cancer research, London, United Kingdom. 2 Sarcoma Unit, The Royal Marsden Hospital, London, United Kingdom. 3 RT and PBT department, University College Hospitals London, London, United Kingdom. 4 Sarcoma Unit, University College Hospitals London, London, United Kingdom. 5 Radiotherapy, University of Sheffield, Sheffield, United Kingdom. 6 Cancer Research UK & UCL Cancer Trials Centre, University College London, London, United Kingdom. 7 Comprehensive Clinical Trials Unit, University College London, London, United Kingdom. 8 Cancer Research UK Clinical Trials Unit (CRCTU), University of Birmingham, Birmingham, United Kingdom. 9 Radiotherapy Physics, National Radiotherapy Trials Quality Assurance Group, Northwood, United Kingdom. 10 Division of Cancer Sciences, University of Manchester, Manchester, United Kingdom. 11 Head and neck Unit, The Royal Marsden Hospital, London, United Kingdom. 12 Dept of Med Phys & Biomedical Eng, University College London, London, United Kingdom Purpose/Objective: Subcutaneous tissue fibrosis (STF) is common in patients following RT for soft tissue sarcoma of the extremities (STSE). Grade 2 or above (grade2+). STF has been previously reported in 31% and 48% of patients, respectively, for pre- and post-operative 3DCRT and 11% for pre- and post-operative IMRT. 1 Traditionally, normal tissue-sparing for STSE is achieved by adding a normal tissue corridor for RT plan optimisation. However, the corridor is not anatomically defined. 2 This work aims to predict the incidence of grade2+ STF by deriving novel dose-volume constraints for the muscle compartments of the thigh (MCT). Material/Methods: Patients in the UK trials VorteX (NCT00423618) or IMRiS (NCT02520128) were analysed. The VorteX phase III trial tested if reduced radiotherapy volumes resulted in lower toxicity rates for post-operative RT to 66Gy. The IMRiS phase II trial studied the feasibility of IMRT for pre- (50Gy) and post-operative RT (60 or 66Gy). Patients had the anterior, posterior and adductor MCT (MCTA, MCTP and MCTAD, respectively) retrospectively outlined. Descriptive statistics were calculated. Dose-volume thresholds were derived using univariate (UVA), multivariate analysis (MVA) and receiver-operator characteristics (ROC).3 The validity of the dose-volume thresholds was explored using odds ratio (OR) in a small holdback cohort. Results: The model development and validation cohorts included 118 and 24 patients, respectively. Patient characteristics are presented in Table 1. STF Grade2+ incidence was 50% ( pre-op 53.4% vs post-op 46.6%) and 62.5% (pre-op 20.8% vs post-op 79.2%) in the model and validation cohorts. UVA shows that 38, 29 and 16 dose levels correlated with grade 2+ STF, respectively, for the MCTA, MCTP and MCTAD. Smoking status together with the dose-volume endpoints was associated with grade2+ STF. Five, six and three statistically significant dose-volume constraints were derived using ROC analysis. Even though all were validated (Figure 1), the dose-volume constraints are best summarised into a maximum dose (Dmax). We propose that this is Dmax<63Gy for MCTA and MCTAD and Dmax<62Gy for MCTP for patients receiving post-operative RT only.