It is important to recognize the limitations of this study, including those inherent to retrospective studies. First, the follow-up period was not long enough to show all recurrences considering the long natural history of breast cancer. This might weaken the study's statistical power. Second, our study could not directly address the survival benefits in association with reductions in the LRR or AFR by PMRT, because our data regarding patient death included all-cause mortality rather than breast cancer mortality. Despite these limitations, our study is clinically valuable. Although breast-conserving treatment has been generalized for node-negative breast cancer over the past several years, some patients still undergo mastectomy for various reasons, such as the presence of multifocal breast cancer or the patient's preference for breast reconstruction after mastectomy. However, most such reference studies were performed from the 1960s to the 1990s. The present work was a multi-institutional study performed in the 2000s and included a large number of patients with pT1-2N0 breast cancer (n = 1,828) treated with mastectomy without PMRT. The conclusions that can be drawn from our analyses are more relevant to contemporary practice, in that we adopted current diagnostic and therapeutic strategies.