Retrospective study on the profile of patients undergoing intraoperative radiotherapy with INTRABEAM for breast cancer treatment at a Brazilian cancer center.

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e12538 Background: Conservative breast therapy combined with adjuvant radiotherapy has been the preferred treatment for early-stage breast cancer for many decades. Considering that most breast cancer recurrences occur near the original tumor site, partial breast radiotherapy has become an alternative for patients in this scenario, allowing for a single-dose radiation application directly to the tumor site. This technique has advantages such as reducing local toxicity and improving the quality of life for treated patients, in addition to decreasing the waiting time for radiotherapy treatment in developing countries like ours. Our case series is the largest in Latin America to date, and we believe that reporting the results of our experience will be valuable for the scientific community and aid in treatment decision-making in low- and middle-income countries. Methods: This is a retrospective study based on data from the medical records of 209 patients with early-stage breast cancer undergoing conservative surgery and intraoperative radiotherapy at A.C. Camargo Cancer Center (São Paulo, Brazil) from 2016 to 2023. Results: In our cohort, 88.5% of tumors were invasive ductal carcinomas. The majority were T1b (41.8%), had histological grade 2 (45.4%), nuclear grade 2 (62.7%), and showed no axillary involvement (87.6%). All patients received intraoperative radiotherapy in a single dose of 20 Gy radiation at the tumor site with a voltage of 50 kV. In our study, 17.7% of patients required additional external radiotherapy due to postoperative histopathological findings inconsistent with treatment indication criteria, including 35.3% for micrometastasis and 32.4% for nodal macrometastasis, 10.8% for narrow or compromised margins, 10.8% for histological types not covered in the final report, 2.7% for multifocality, 2.7% for extensive ductal carcinoma in situ, 2.7% for negative progesterone receptor, and 2.7% for the presence of lymphatic vascular invasion. The average follow-up time for patients was 36 months. The local recurrence rate was 0.5%. Conclusions: The results of this study support established data indicating the efficacy of intraoperative radiotherapy in early-stage breast cancer treatment. Therefore, for carefully selected patients, this technique with should be considered as a treatment option. We will continue to follow this patient cohort to identify the occurrence of late recurrences and publish long-term follow-up data.