Individual Sensitivity for Radiotherapy-related Adverse Tissue Reactions in Patients Treated Twice for Metachronous Cancers.

The role of genetics in susceptibility to radiotherapy-induced toxicities is unclear. A strong impact of genetics should cause correlated toxicities in patients with metachronous double radiotherapy. We ascertained information about demographics, lifestyle, radiotherapy and early toxicities in irradiated tissues for a retrospective cohort of 98 patients from 2 hospitals who underwent two metachronous radiotherapeutic treatments (2000-2022) of different anatomical regions. European Organisation for Research and Treatment of Cancer/Radiation Therapy Oncology Group (EORTC/RTOG) toxicity scores per organ system were combined to a single mean score. We considered as genetic component the variation of toxicity not explained by radiation dose to the tumor, age at radiotherapy, sex, smoking status, and surgery. Variance components of toxicity were evaluated by ordinal logistic regression with random intercept. Common site combinations were breast/contralateral breast (N = 16), breast/endometrium (N = 6), and cervix/breast (N = 5). Mean toxicity over exposed tissues was 0.70 (range, 0-3). Prescribed radiation dose was significantly associated with mean toxicity, with a 5% (95% CI 3-8) increase of the odds for a higher toxicity level per Gy. Sex, surgery, age and smoking were not. There was no genetic contribution to risk of toxicities after adjustment. Toxicity levels were not more similar within patients than between patients, suggesting a negligible impact of genotype on radiotherapy-related toxicities.