ESTRO 2023 - Abstract Book

S284

Sunday 14 May 2023

ESTRO 2023

Conclusion Risk factors such as large tumour at diagnosis, midline tumours, mean dose and V30Gy to the oral cavity and parotids mean dose increases the likelihood of needing enteral feeding during PBT. Therefore patients with these risk factors should be considered for prophylactic RIG placement. Further work is being carried out to explore other dosimetric parameters and risk thresholds, with multivariate analysis ongoing. MO-0383 Risk factors associated with acute skin toxicities in patients treated with Proton Beam Therapy K. Oguejiofor 1 , S. Gaito 2 , A. France 2 , S. Pan 2 , M. Aznar 3 , P. Sitch 2 , M. Merchant 4 , E. Smith 2 , J. Richardson 2 1 Christie Hospital Manchester, Proton Beam Therapy Centre , Manchester, United Kingdom; 2 Christie Hospital Manchester, Proton Beam Therapy Centre, Manchester, United Kingdom; 3 University of Manchester / Christie Hospital Manchester, Radiotherapy-Related Research Group, Manchester, United Kingdom; 4 University of Manchester / Christie Hospital Manchester, PRECISE group, Manchester , United Kingdom Purpose or Objective Acute radiotherapy induced skin toxicities (RIST) are reported to be higher in patients treated with proton beam therapy (PBT) due to loss of the build-up effect. Predictive factors of acute RIST in patients treated with PBT remains largely undefined. In this study, we retrospectively reviewed the acute RIST of patients treated with PBT to identify dose and volume thresholds predictive of acute RIST. Materials and Methods Patients treated with pencil beam scanning (PBS) at a National PBT centre between December 2018 to October 2022, who developed Grade 0 (G0) or Grade 2+ (G2+) acute RIST were included in this study. Anonymised demographic and clinical data were obtained retrospectively from electronic patient records. Dosimetric data required manual extraction from Eclipse v16.1. Skin structure is defined as 5mm rind grown as an inner margin from the patient contour. The following skin dose–volume statistics were collected: Dmax (maximum dose to any pixel inside the skin contour) and dose to skin volumes (cm3) in 5Gy increments (V5Gy, V10Gy etc). The RTOG criteria was used in grading acute RIST. Preliminary analyses of dosimetric data of patients with G0 vs G2+ acute RIST are presented, with significance assessed at the 5% level using t-tests and univariate logistic regression models, and risk thresholds found using receiver operating characteristic (ROC) curves. Results Of 893 patients treated with PBS PBT, 171 had G0 RIST and 342 had G2+ RIST. So far, of these, 191 patients have had their dosimetric information extracted for inclusion in the study, 46 G0 and 145 G2+ RIST. Of these 191 patients, the three major indications for PBT were head and neck (20%), sarcoma (28%) and chordoma (16%). The median age of patients treated was