ESTRO 2021 Abstract Book

S933

ESTRO 2021

To assess acute skin toxicity in locally advanced breast cancer patients treated with hypofractionated or normofractionated radiotherapy after modified radical mastectomy. Materials and Methods We retrospectively evaluated 221 patients, treated with adjuvant radiotherapy after modified radical mastectomy for breast cancer between August 2017 and December 2019. Sixty-eight patients received conventional fractionation radiotherapy (CFRT), 50 Gy in 25 fractions and 153 patients received hypofractionated RT (HFRT), 40 Gy in 15 fractions. Skin toxicity was evaluated using the CommonToxicity Criteria for Adverse Events (CTCAE) v. 4.0 scale. Data were analyzed on SPSS version20. Results Acute skin toxicity was observed in 74% (n=113) for HFRT and 72% (n=49) for CFRT (P=0.8). No grade 3-4 erythema was observed in the two groups. Radiation dermatitis grade 0,1 and 2 were noted in 25%, 71% and 4% in the HFRT group and in 28%, 62% and 10 % in the CFRT group (p=0.6) Conclusion The results confirm that hypofractionated radiation therapy had similar results in terms of acute skin toxicity and tolerance as conventional fractionation radiation therapy; Grade2 erythema was even lower in HFRT (4% vs 10% respectively [p=0.6]). PO-1121 Polymorphisms in DNA repair genes and radiotherapy late adverse events in breast cancer patients T. Marinko 1,2 , F. Dugar 2 , V. Dolžan 3 , K. Goričar 3 1 Institute of Oncology Ljubljana, Radiation oncology department, Ljubljana, Slovenia; 2 University of Ljubljana, Faculty of Medicine, Ljubljana, Slovenia; 3 Institute of Biochemistry and Molecular Genetics, Faculty of Medicine, University of Ljubljana, Pharmacogenetics Laboratory, Ljubljana, Slovenia Purpose or Objective HER2-positive early breast cancer (BC) patients are treated with radiotherapy (RT) due to its survival improvement and better local disease control. However, adverse events may occur as a result of radiation damage to cell DNA. The most harmful type of DNA damage are double-strand breaks, which are repaired by homologous recombination. DNA repair capacity may be altered by genetic factors. The aim of the study was to evaluate the impact of polymorphisms in genes, involved in homologous recombination repair, on the occurrence of late adverse events of RT in BC patients. Materials and Methods Genomic DNA was isolated from buccal swabs. All patients were genotyped for NBN rs1805794, rs709816, and rs1063054, RAD51 rs1801320, rs1801321, and rs12593359, as well as XRCC3 rs1799794 and rs861539 polymorphisms using competitive allele-specific PCR. Logistic regression was used to assess the association with skin RT adverse events. Results Altogether, 101 HER2-positive early BC patients, treated with adjuvant RT, trastuzumab and chemotherapy were enrolled in the study. 58 (57.4%) patients received taxanes, 29 (28.7%) patients had arterial hypertension (AH). Most (84, 83.2%) patients were irradiated with 25 x 2 Gy, others with 17/18 x 2.5 Gy. Median time after RT was 4.0 (2.6-5.4) years. After RT, 12 (11.9%) patients experienced grade 2 late skin toxicity according to CTC criteria, while 33 (32.7%) experienced grade ≥ 2 late adverse events according to LENT-SOMA criteria. Among clinical parameters, AH and treatment with taxanes were associated with increased risk for grade ≥ 2 late adverse events (P<0.001 and P=0.001, respectively) or grade 2 skin toxicity (P=0.016 and P=0.012, respectively). Late adverse events were significantly more common in carriers of two polymorphic XRCC3 rs1799794 (c.-316A>G) alleles (OR=5.83; 95% CI=1.22-28.00, P=0.028), and the difference was even more pronounced after adjustment for clinical variables (OR=10.90; 95% CI=1.61-73.72, P=0.014). On the other hand, late adverse events were less common in carriers of at least one polymorphic XRCC3 rs861539 (p.Thr241Met) allele (OR=0.43; 95% CI=0.18-1.00, P=0.050), but the difference was no longer significant after adjustment for clinical variables (OR=0.45; 95% CI=0.17-1.16, P=0.097). Conclusion Genetic variability of DNA repair gene XRCC3 can contribute to the occurrence of late skin adverse effects of RT, alone or in combination with clinical parameters. Polymorphisms in homologous recombination genes could therefore be used as biomarkers contributing to the personalization of RT in breast cancer patients. Research grants: ARRS J3-1753, P1-0170 and P3-0321.

PO-1122 Early and intermediate treatment outcome after external beam accelerated partial breast irradiation

A. Stenger-Weisser 1 , O. Unterkirhere 1 , C. Glanzmann 1 , G. Studer 1 1 Cantonal Hospital Lucerne, Radiation Oncology, Lucerne, Switzerland

Purpose or Objective To evaluate intermediate outcome of accelerated partial breast irradiation (APBI) in early stage breast cancer patients. Materials and Methods Inclusion criteria were defined according to the APBI ASTRO Consensus Statement . Prescribed dose was 26-28 Gy in 5 fractions on 5 consecutive days. Regular follow-up visits at our institution with objective and subjective assessment of treatment tolerance (patient reported outcome measures, PROMs) were prospectively performed after 0 and 2 weeks, 6 months and in annual intervals. Results Between 02/2017-05/2020, 198 patients met the inclusion criteria for APBI. 8% of patients were diagnosed with DCIS, 5% with Tis, 5/35/36% with T1a/b/c, 11% with T2 stages, respectively. Mean PTV volume was 115 cc