IORT with electrons as boost strategy during breast conserving therapy in limited stage breast cancer: Long term results of an ISIORT pooled analysis

Purpose: Linac-based intraoperative radiotherapy with electrons (IOERT) was implemented to prevent local recurrences after breast conserving therapy (BCT) and was delivered as an intraoperative boost to the tumor bed prior to whole breast radiotherapy (WBI). A collaborative analysis has been performed by European ISIORT member institutions for long term evaluation of this strategy. Material and methods: Until 10/2005, 1109 unselected patients of any risk group have been identified among seven centers using identical methods, sequencing and dosage for intra- and postoperative radiotherapy. A median IOERT dose of 10 Gy was applied (90% reference isodose), preceding WBI with 50– 54 Gy (single doses 1.7–2 Gy). Results: At a median follow up of 72.4 months (0.8–239), only 16 in-breast recurrences were observed, yielding a local tumor control rate of 99.2%. Relapses occurred 12.5–151 months after primary treatment. In multivariate analysis only grade 3 reached significance (p = 0.031) to be predictive for local recurrence development. Taking into account patient age, annual in-breast recurrence rates amounted 0.64%, 0.34%, 0.21% and 0.16% in patients <40 years; 40–49 years; 50–59 years and P60 years, respectively. Conclusion: In all risk subgroups, a 10 Gy IOERT boost prior to WBI provided outstanding local control rates, comparing favourably to all trials with similar length of follow up.