Adenocarcinoma of mammary gland type of the vulva is an exceedingly rare malignancy, believed to arise from mammary-like vulvar glands. This report details such a malignancy in a 38-year-old female patient who presented with an ulcerated lesion on the left labia majora. Immunohistochemical analysis revealed robust expression of keratin 7, p16, GATA3, TRPS1, HER2, and androgen receptor, while negative expression for p63, p40, estrogen receptor, and progesterone receptor. Molecular profiling through targeted DNA/RNA sequencing detected pathogenic missense variants in PIK3CA (exon 10; c.1633G>A; p.E545 K; 50.70% VAF) and TP53 (exon 6; c.659A>G; p.Y220C; 61.50% VAF). Copy-number analysis showed ERBB2 amplification and gains in CDKN2A and CDK4 genes. Given the rarity of the tumor, it is crucial to distinguish it from morphological mimics to inform appropriate patient management. Treatment approaches are typically based on protocols for primary breast cancer, given the histological and biological similarities between these tumors and breast tissue.
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