Early initiation of external beam radiotherapy (EBRT) may increase the risk of long-term toxicity in patients undergoing intraoperative radiotherapy (IORT) as a boost for breast cancer.

Background: Intraoperative radiotherapy (IORT) during breast-conserving surgery is increasingly used. We analyzed the influence of the interval between an IORT boost and external beam radiotherapy (EBRT) on late toxicity.

Methods: Forty-eight patients received 20 Gy IORT (50 kV X-rays (Intrabeam, Carl Zeiss, Oberkochen, Germany) followed by 46-50 Gy EBRT with a median interval of 36 days (14-197).