The effect of adjuvant radiotherapy on mortality differs according to primary tumor location in women with node-positive breast cancer.

PURPOSE: To evaluate the prognostic significance of primary tumor location and to examine whether the effect of adjuvant radiotherapy on survival varies according to tumor location in women with axillary node-positive (ALN+) breast cancer (BC). PATIENTS AND METHODS: Data were abstracted from the SEER database for 24,410 women aged 25-95 years, diagnosed between 1988-1997 with nonmetastatic T1-T2, ALN+ BC. Subgroup analyses were performed using interactions within proportional hazards models. Event was defined as death from any cause. Prognostic variables were selected using Akaike Information Criteria. Joint significances of subgroups were evaluated with Wald test. RESULTS: Median follow-up was 10 years. In joint models, statistically significant interactions were found between tumor location, nodal involvement, type of surgery, and radiotherapy. Factorial presentation of interactions showed consistent 13% proportional reduction of mortality in all subgroups, except in women with medial tumors with > or = 4 ALN+ treated with mastectomy. In this subgroup, use of radiotherapy was associated with a 16% proportional increase in mortality. CONCLUSION: Medial tumor location is a significant adverse prognostic factor that should be considered in treatment decision- making for women with ALN+ BC. Improved survival was observed with radiotherapy use in all subgroups, except in women with medial tumors with > or = 4 ALN+ treated with postmastectomy radiotherapy. These findings raise concern that the favorable effect of radiotherapy may be offset by excess toxicities in the latter subgroup.