Salivary Duct Carcinoma and Invasive Ductal Carcinoma of the Breast: A Comparative Immunohistochemical Study.

Salivary duct carcinoma (SDC) is a high-grade salivary gland malignancy with great morphological resemblance to invasive ductal carcinoma (IDC) of the breast. Rarely, female patients may have a past history of both SDC and IDC. When these patients present with distant metastasis, accurate identification of the primary tumor is particularly difficult. Additionally, rare metastasis of SDC to the breast and IDC to the salivary (parotid) gland can also present a diagnostic challenge. Our aim was to develop an immunohistochemical panel that reliably distinguishes SDC from IDC. We included all SDCs diagnosed from 1989 to 2016 (23 cases) and 29 treatment naïve and histologically similar IDCs. All cases were stained with androgen receptor (AR), estrogen receptor-alpha (ER-α), progesterone receptor (PR), HER-2, CK5/6, p63, and beta-catenin. The great majority (> 90%) of both SDCs and IDCs reacted positively to AR. The main discrepancy in the immunohistochemical profiles was a distinctly different reactivity to ER-α, PR and HER-2. While 28 IDCs (96.6%) reacted positively to ER-α and/or PR, the majority expressing both (82.8%) with a moderate to strong staining intensity, only 2 SDCs expressed ER-α (8.7%) and 5 others expressed PR (21.7%) with only one case expressing both (P value < 0.05). On the other hand, 8 SDC (34.8%) were positive for HER-2 while none of the IDCs were positive (P value < 0.05). ER-α, PR, and HER-2 may be helpful to distinguish SDC from IDC. Positive reactivity to ER-α, PR or both and negative HER-2 favors a diagnosis of IDC while ER-α, PR negative, HER-2 positive tumors are more likely SDC.