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112 Background: Intraoperative boost irradiation as part of breast-conserving therapy is a perfect method to adequately capture the high risk tumor relapse area. The most homogeneous dose distribution is achieved with electrons. Intraoperative radiotherapy (IOERT) as a boost for breast cancer releases a high single dose of radiation to the breast tissue; therefore acute toxicity is of particular attention. To date there is only inadequate information available on breast cancer patients treated with IORT using electrons applied as a boost. We therefore analyzed the acute toxicity after radiotherapy with 10 Gy as a boost with a minimum follow-up of 3 months. Methods: A total of 69 patients treated with IOERT (10Gy with 5, 7 and 9 MeV electrons) with a dedicated robotic linac (NOVAC 7, New Radiant Technology, Aprilia, Italy) to the tumor bed during breast-conserving surgery as a boost followed by whole-breast radiotherapy (WBRT, 50.4 Gy; 1.8 Gy per fraction) were included in this study. All patients underwent a retrospective follow-up (median, 8 months; range 1-17 months) regarding acute side effects within the first three months. Toxicities were documented using the common toxicity criteria (CTC 4.0 of the European Organization for Research and Treatment of Cancer). Results: The IOERT was well tolerated. As a side effect there was one patient with seroma. Two patients developed chronic pain in the irradiated breast. Two patients developed a secondary wound healing. The remaining patients did not develop any grade 3 or 4 side effects. The observed toxicity rates were not influenced by age, tubus size, electron energy or systemic therapy. Conclusions: After IOERT of the breast using electrons we did not find any unexpected acute toxicity rates.