RT EFFECT FOR BREAST CANCER SUBTYPES

In an analysis of the Swedish Breast Cancer Group 91 Radiotherapy trial reported in the Journal of Clinical Oncology, Sjöström et al found that adjuvant radiotherapy vs no radiotherapy had an increased benefit in triple-negative disease and little effect on HER2-positive disease in a population of patients with node-negative stage I or II breast cancer who mostly did not receive systemic adjuvant therapy.

Study Details

The study involved tumor tissue from 1,003 patients who were randomized to receive breast-conservation surgery with radiotherapy or no radiotherapy between 1991and 1997. Overall, only 8% of the patients received systemic adjuvant treatment.

Subtyping with immunohistochemistry and in situ hybridization on tissue microarrays was performed for 958 tumors (481 in the radiotherapy group, 477 in no-radiotherapy group). Tumors were classified as luminal A–like in 554 patients, luminal B–like in 259, triple-negative in 81, and HER2-positive in 66 (estrogen receptor–negative in 20, estrogen receptor–positive in 44).

Recurrence and Survival

The cumulative incidence of ipsilateral breast tumor recurrence as a first event within 10 years for the radiotherapy vs no-radiotherapy group was 9% vs 19% (P = .001) for luminal A–like tumors, 8% vs 24% (P < .001) for luminal B–like tumors, 6% vs 21% (P = .08) for triple-negative tumors, and 15% vs 19% (P = .6) for HER2-positve tumors; however, evidence of an overall difference in radiotherapy effect across subtypes was weak (P = .21). Radiotherapy was associated with a reduced risk of death from breast cancer for triple-negative tumors (hazard ratio = 0.35, P = .06) but not for other subtypes. Radiotherapy was not associated with reduced overall mortality in any subtype. In a clinical low-risk subgroup of luminal A–like tumors, radiotherapy was associated with a reduced risk of ipsilateral breast tumor recurrence within 10 years (6% vs 20%, P = .008) but not with a reduced risk of death from breast cancer or any cause.

The investigators concluded: “Subtype was not predictive of response to [radiotherapy], although, in our study, human epidermal growth factor receptor 2–positive tumors seemed to be most radioresistant, whereas triple-negative tumors had the largest effect on [breast cancer death]. The effect of [radiotherapy] in the presumed low-risk luminal A–like tumors was excellent.”

The study was supported by the Swedish Breast Cancer Association, Swedish Cancer Society, and others.

Martin Sjöström, MD, of Lund University, is the corresponding author of the Journal of Clinical Oncologyarticle.