Background: Occult breast cancer (OBC) is classified as carcinoma of an unknown primary site, and the adequate therapy for OBC remains controversial. This retrospective study aimed to reveal the transition in breast cancer therapy and the frequency of primary breast tumors after resection in clinical OBC (cT0N+) patients using the Japanese Breast Cancer Registry database. Methods: We enrolled OBC patients with cT0N+ from the registry between 2010 and 2018. On the basis of the period of diagnosis, OBC patients were divided into the following two groups: 2010–2014 and 2015–2018. We described the transition in treatments and tumor characteristics. After breast resection, the frequency of pathological identification of primary tumors and tumor sizes was assessed. Results: Of the 687,468 patients registered, we identified 148 cT0N+ patients with a median age of 61 years. Of these patients, 64.2% (n = 95) received breast surgery (2010–2014: 79.1%, 2015–2018: 50.0%). Axillary lymph node dissection was performed in 92.6% (n = 137, 2010–2014: 91.6%, 2015–2018: 93.4%). The breast tumor size in the resected breast was 0–7.0 cm (median: 0 cm, 2010–2014: 0–7.0 cm [median: 0 cm], 2015–2018: 0–6.2 cm [median: 0 cm]). The pathological identification rate of the primary tumor was 41.1% (n = 39, 2010–2014: 40.4%, 2015–2018: 42.1%). Conclusions: Breast surgery for cT0N+ decreased between 2010 and 2018. Despite the high identification rate of primary tumors, most tumors were small, and there was no significant change in the identification rate or invasive diameter of the identified tumors after 2010.
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