Microscopic description and IHC:
Sections show a proliferative neoplasm within the dermis with apparent eccrine differentiation. It is composed of clusters of ductal structures lined by an inner layer of epithelial cells without significant cytologic atypia and an outer layer of myoepithelial cells. Some of the ducts have elongated contours, characteristically resembling a tadpole. Immunohistochemical stains are not contributory in making this diagnosis, however a PAS stain may highlight the eosinophilic material within the ductal lumina.
Discussion:
Syringoma is a benign neoplasm of eccrine ducts that may cause pruritus or be asymptomatic. It is most commonly seen in puberty aged females on the eyelids, neck and cheeks. It is very rare in the vulva. It can be very difficult to differentiate a syringoma from microcystic adnexal carcinoma (MAC), which is why complete excision of this lesion was recommended. MAC also has very bland cytology, however it is aggressively infiltrative and often shows perineural invasion, neither of which were seen on the resection specimen.
Hidradenoma papilliferum will usually have at least some component of papillary architecture, as well as more columnar type cells.
This is also unlikely to be metastatic endometrial cancer, given the lobular proliferation of the glands, morphology of the glands (round open lumina lined by two layered epithelium), bland cytology, and the lack of mitotic figures.
References:
Tiwana KK, Nibhoria S, Kaur H, Bajaj A, Phutela R. Benign Vulvar Adnexal Tumours - A 5 year Study in a Tertiary Care Hospital. J Family Reprod Health. 2015;9(4):199-201.
Fallaha A, Thuile T, Tappeiner L. Misdiagnosed microcystic adnexal carcinoma on the lateral forehead and challenges in reconstruction of a large and bone-deep defect. J Dtsch Dermatol Ges. 2016;14(1):86-90.