Objective Secretory breast carcinoma (SBC) is a rare type of breast malignancy, accounting for less than 0.02% of all infiltrating breast malignancies. The pure SBC, a type of SBC without another type of breast malignant neoplasm, is particularly rare. This study aimed to investigate the clinicopathologic and molecular features of pure SBC. Methods The main pathological parameters such as estrogen receptor (ER), progesterone receptor (PR), and human epithelial growth factor receptor 2 (C-erbB-2) were detected by immunohistochemistry (IHC), and the clinicopathologic and prognostic difference were compared with invasive ductal carcinoma (IDC). Fluorescent in situ hybridization (FISH) and reverse transcription polymerase chain reaction (RT-PCR) was performed to identify the ETV6-NTRK3 rearrangement of SBC. Results We found that the positivity rates of ER, PR, C-erbB-2, p53, and S-100 were 47.7% (21/44), 52.3% (23/44), 36.4% (16/44), 27.3% (12/44), and 95.5% (42/44), respectively, which were higher than those reported in previous studies. Special periodic acid-Schiff analysis was performed in 36 patients, and the value of the Ki-67 index ranged from 1% to 50% (mean value: 10%). Interestingly, most patients with pure SBC harbored an ETV6-NTRK3 rearrangement with an 88.6% (39/44) expression rate. Compared with IDC, the tumor size of most patients with SBC was larger than 2 cm (P = 0.024). Ultrasound showed benign lesions, and the total misdiagnosis rate was higher (P = 0.020). Although the pathological classification was mostly triple-negative breast cancers (P = 0.036), there was less metastasis (P = 0.029), and the overall prognosis was better than that of the IDC group. Conclusions Although axillary lymph node metastasis, local recurrence, or distant metastasis may occur, SBC is also considered an indolent neoplasm with a good prognosis. Once diagnosed, surgical treatment should be performed as soon as possible, followed by appropriate adjuvant chemotherapy, irradiation, and endocrine therapies.
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