Long-term follow-up after intraoperative radiotherapy (IORT) for breast cancer

10594 Background: IORT for breast cancer with new treatment modalities is becoming increasingly important. In our department IORT is delivered by a mobile x-ray source (Intrabeam) producing isotropic 50 kV x-ray fields. Because this treatment modality includes single high doses to the normal breast tissue, careful evaluation of early and late toxicity is mandatory. Methods: Between 2/02 and 12/05 161 patients received IORT either as a tumour-bed boost (103) before 46 Gy external beam radiotherapy (EBRT) or as partial breast irradiation (58). Median age was 64.6 years (35–95), median tumour size was 15 mm (1–45). Spherical applicators (2.0 -5.0 cm diameter) were inserted into the tumourbed, giving a single dose of 20 Gy. Follow-up (f/u) included physical examination and ultrasound 1 week, 2 months, then every 6 months after therapy and mammography at yearly intervals. Radiobiological modelling of the expected spatial distribution of late reaction was done for typical cases using the linear-quadratic model with special consideration of relative biologic effectiveness, spatial dose distribution and dose rate. Results: Mean (f/u) time was 22 months. Minor postoperative side effects included erythema (5%), delayed wound healing (4%), mastitis (1%) and hematoseroma (4%). A fibrotic induration of the tumour bed was observed in 9, 18, 22, 23 and 26% of the patients at 2, 6 12, 24 and 36 month (f/u). Reoperation was required in 2 patients after 10 and 12 months, one mastectomy due to a fibrotic change of the whole breast and one local drainage of a fat necrosis. Conclusions: Although 2/3 of the patients were treated with a combination of IORT and EBRT, acute reactions in all patients were mild. Predominant late effect was an induration of the tumour bed which slowly began to resolve 1–3 years after therapy. The low level of clinically late reactions observed so far is in agreement with radiobiologic predictions from radiobiological modelling. No significant financial relationships to disclose.