ESTRO 2021 Abstract Book

S527

ESTRO 2021

accounting for SNP-SNP interactions (PRSi) was developed; the PRSi shows which SNPs and alleles are included, whether they increase or decrease the risk of toxicity and their combined effect sizes. The added value of incorporating PRSi was evaluated (effects size of PRSi through Odds Ratios, AUC, p-value for increase in AUC). Internal validation was performed using bootstrapping (10000 resamples). Results pts had conventional fractionation (60-81 Gy), 25% received hypofractionation. 70% pts had VMAT, 12% static field IMRT, 18% 3DCRT. 30% had post-prostatectomy RT, 32% pelvic RT and 72% adjuvant/neo-adjuvant hormone therapy. Toxicity rates were 11.7% (grade≥1 rectal bleeding), 4.2% (grade≥2 urinary frequency), 5.5% (grade≥1 haematuria) and 17.8% (grade≥2 nocturia) and 17.0% (grade≥1 decreased urinary stream). Details on models including PRSi are given in Fig 1, ROC curves in Fig 2b. Only 13/43 SNPs validated but s ome common SNP-SNP interactions were found (i.e. SNPs with a frequency ≥10% in the population of pts with toxicity) and included in PRSi. PRSi included combinations of 8-15 different SNP-allele sets. Adding PRSi improved AUC for all endpoints, ranging from an increase of 5% (p=0.014) for rectal bleeding to an increase of 19% for urinary frequency (p<0.0001).