Which patients don't need a tumor-bed boost after whole-breast radiotherapy?

Retrospective dose-response data suggest that a boost of about 15 Gy to the tumor bed following whole-breast radiotherapy reduces the risk of local recurrence (IBTR) by as much as 2-fold. Even if this benefit is confirmed by prospective trials, boost irradiation may not be considered cost-effective in patients having an IBTR risk of less than 1% per annum.

Published prospective trials of invasive breast cancers (National Surgical Adjuvant Breast and Bowel Project; Stockholm Adjuvant Tamoxifen Trial) in which patients received 50 Gy whole-breast irradiation, without a boost, were analyzed to identify subgroups with IBTR risk less than 1% per year. All studies were based on lumpectomy (rather than segmental or quadrant excision) and eligibility required the absence of cancer cells at the margins. It was assumed that clinical and pathological factors, other than those defining trial eligibility, were of negligible importance regarding IBTR risk, with the exception of young age.

All patients not receiving adjuvant systemic therapy, including tumors < 1 cm, have IBTR risk justifying boost irradiation. Of patients receiving systemic therapy, only patients with node-negative, estrogen receptor-positive tumors have low IBTR risk (3% at 10 years), provided that tamoxifen is administered. Of patients receiving only adjuvant chemotherapy, low IBTR risk seems to be associated with administration concomitantly with radiotherapy. IBTR risk in patients receiving chemotherapy sequentially with respect to radiotherapy probably is high enough to justify a boost. A boost is probably indicated in all patients younger than 40 years, regardless of other factors.