Background: The effectiveness of postmastectomy radiotherapy (PMRT) in terms of survival for breast cancer patients with American Joint Committee on Cancer (AJCC) pT1-2 and one to three tumor positive lymph nodes is controversial, especially in this era of more effective systemic treatment.
Methods: Using data from the National Cancer Database (NCDB) and the Surveillance, Epidemiology, and End Results (SEER) program between 1998 and 2008, this study respectively identified 93,793 and 36,299 women with AJCC pT1-2pN1 breast cancer who underwent mastectomy. The association of PMRT use with overall and cause-specific survival was examined using multivariable Cox models in subgroups defined by tumor stage.
Results: In the NCDB cohort, 21.5 % of the patients (n = 20,236) received PMRT, and a very similar percentage (21.9 %, n = 7939) received PMRT in the SEER cohort. In the NCDB cohort, PMRT was associated with a 14 % relative risk reduction in all-cause mortality among the patients with two positive lymph nodes and tumors 2-5 cm in size or three positive nodes [hazard ratio (HR), 0.86; 95 % confidence interval (CI), 0.81-0.91; p < 0.0001], but PMRT had no beneficial effect for the patients with one positive node or two positive nodes and tumors 2 cm in size or smaller. Analysis of the SEER cohort confirmed this heterogeneous effect, showing PMRT to be associated with a 14 % relative risk reduction in breast cancer cause-specific mortality among the patients with two positive nodes and tumors 2-5 cm in size or three positive nodes (HR 0.86; 95 % CI 0.77-0.96; p = 0.007) but not in the other subgroup.
Conclusion: The effectiveness of radiotherapy depends on the combination comprising the number of positive lymph nodes and tumor size, which may enable more precise patient selection for PMRT.