ESTRO 2023 - Abstract Book

S358

Sunday 14 May 2023

ESTRO 2023

Toxicity was reported during FU using RTOG scoring criteria. The study endpoint was moderate-severe acute reactions (ACUG2+) developed within the first 3 months after RT completion. The NTCP model using logistic regression was created by combining dosimetry and clinical factors. A 5-fold cross-validation preserving the imbalance class distribution in each fold was applied. Results One-hundred-twenty-six patients representing 14.4% of the considered population (n=878 pts) reported ACUG2+ toxicity (13.2% of G2 and 1.2% of G3 events), manifesting at least bright erythema, patchy moist desquamation or moderate oedema. Average DVHs for patients with and without toxicity are depicted in Fig1. A significant association with ACUG2+ was found in univariate analysis for PTV volume, hypertension, diabetes, obesity, axillary lymph node dissection (ALND), and V5 to V34 Gy. We defined a multifactorial model including V20 Gy (OR=1.008,p<0.0001), hypertension (OR=1.54, p=0.030) and ALND (OR=1.58, p=0.045). We performed the final selection for the dosimetric variable based on correlations, the impact of the dose calculation algorithms and clinical application (V20 Gy represents 50% of the prescribed dose as other constraints in breast cancer RT, and it is an early point in the plateau of the dose distribution). AUC for the model was 0.63 (0.60-0.67, p<0.0001). The resulting dose-response curves are shown in Fig.2. A cut-off of 200 cc to Skin V20 Gy can be applied to limit the toxicity to 10%-15% in low and intermediate-risk classes.

Conclusion The association between moderate-severe acute skin reactions and skin DVH was quantified. For the first time to our knowledge, the current study identified dose-volume warnings for a structure usually not considered during plan optimization. Our findings should help planners to obtain more conformal dose distributions in high-risk pts with large breast and/or with tissue impaired by hypertension and/or previous removal of axillary lymph nodes. The study was funded by AIRC IG 25951 – 23150 and Fondazione Regionale per la Ricerca Biomedica project nr. 110-JTC PerPlanRT ERA PerMed, GA 779282.

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