A rare case of multifocal vulvar carcinoma of mammary gland type (AMGT) with mucinous features: Differential diagnosis and literature review.

Background: Primary vulvar adenocarcinoma of mammary gland type (AMGT) is a rare gynecologic cancer with only 54 reported cases to date. The tumor may have variety of histologic patterns and can be confused with other more common entities of vulvar tumors. Here, we report a rare mucinous variant of AMGT coexisting with ductal carcinoma in situ (DCIS) and benign mammary-like glands in the vulva. We further review the clinicopathological features of previously reported cases in the literature and discuss the differential diagnosis and treatment updates of this rare entity. Case presentation: A 78-year-old woman with a complex gynecological history who presented with gradually enlarging vulvar nodules and underwent wide local resection. Histological examination revealed mucinous adenocarcinoma and DCIS arising from benign mammary-like glands. The tumor shared histological and immunohistochemical features with invasive ductal carcinoma of the breast, including immunoreactivity to GATA binding protein 3 (GATA3), estrogen receptor (ER), progesterone receptor (PR), and gross cystic disease fluid protein 15 (GCDFP-15). Additional imaging did not reveal primary tumor in the breast, gastrointestinal tract, or other organs. Conclusion: Accurate diagnosis of AMGT relies on awareness of this rare entity, recognition of its typical "milk line" location, and familiarity with its morphological and immunohistochemical similarities to breast carcinoma. The diagnostic importance of identifying tumor-associated benign mammary-like glands or DCIS cannot be overemphasized. Immunohistochemistry is critical in identifying the "mammary-like" phenotype of AMGT and excluding other primary and metastatic tumors of vulva. The unique biologic profile of AMGT dictates a treatment strategy distinct from other primary vulvar tumors.