Background: Recent trials have provided robust evidence demonstrating that endocrine therapy with/without targeted therapy, such as cyclin-dependent kinase 4/6 inhibitors or mTOR (mammalian target of rapamycin) inhibitors, effectively halts disease progression in hormone receptor (HR)-positive and human epidermal growth factor receptor 2 (HER2)-negative metastatic breast cancer. We investigated the survival impact of local treatment of metastases as a first-line treatment after metastasis in HR-positive and HER2-negative breast cancer patients with a very low metastatic volume. Materials and Methods: From a retrospectively constructed database for three institutes, we identified HR-positive and HER2-negative breast cancer patients with recurrent distant oligometastatic disease after initially curative treatment. De novo stage 4 patients were excluded, and only those with recurrent metastatic disease were included. Oligometastatic disease was defined as follows: (1) ≤2 metastatic lesions in a single organ, (2) a maximal diameter ≤3 cm, and (3) organ involvement, including the lung, liver, adrenal gland, bone, or distant lymph nodes. Local treatment comprised surgery or radiotherapy. Progression-free survival (PFS) and overall survival (OS) were investigated. Results: Forty-nine patients were included; 33 underwent local treatment. Of these 33 patients, 5 underwent surgical resection and 27 received radiotherapy. One patient underwent both surgical resection and radiotherapy. Median PFS was significantly longer among the patients with local treatment than among the patients without local treatment (30.0 vs. 18.0 months, p = 0.049). In multivariate analysis, local treatment was shown to prolong PFS. However, median OS after metastasis did not differ with regard to local treatment (72.3 vs. 91.0 months, p = 0.272). Conclusion: We showed that local treatment could positively affect disease progression in HR-positive and HER2-negative oligometastatic breast cancer.
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