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

Purpose: Intraoperative radiotherapy (IORT) during breast conserving surgery is increasingly used. When given as a boost, the sequencing of IORT, systemic therapy and external beam radiotherapy (EBRT) may be of importance for normal tissue damage. We analyzed the influence of the interval between IORT and EBRT on the development of late toxicity in breast cancer patients receiving IORT as a boost prior to EBRT of the whole breast. Material and Methods: 91 patients were followed up for a median of 24 months (range 21 29 months) after IORT and EBRT. Twenty Gy IORT were given with 50 kV X-rays (INTRABEAM, Carl Zeiss Surgical, Oberkochen, Germany) followed by 46-50 Gy EBRT after a median interval of 37 days (range, 13-226 days). Toxicity was assessed with the modified LENT SOMA score. Results: 27 patients developed higher grade fibrosis (II-III degrees). There were 13 patients with retractions, 8 patients with pain II degrees, 8 with edema P, 4 patients with teleangiectases, 1 patient with lymphedema II degrees, and 1 patient with hyperpigmentation II degrees. No ulcers were seen. 54 patients had no higher grade chronic toxicity. A comparison of the patients showed that the median interval in the group of patients with higher grade toxicity (n=37) was significantly shorter than in the group of 54 patients without late effects (34 days vs. 40 days, p = 0.044, Mann-Whitney U-test). Conclusion: Starting EBRT about 5 weeks after IORT appears safe. A shorter interval between IORT and EBRT was found to be associated with an increased frequency of chronic late toxicity.