Clinicopathologic and molecular characteristics of 44 patients with pure secretory breast carcinoma

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.