Answer: Infiltrating sweat gland carcinoma with apocrine features and associated Paget’s disease

Histology: Within the dermis there is an infiltrating carcinoma composed of both cribriform glands as well as individual and poorly formed glandular structures. The cells have a modest amount of eosinophilic cytoplasm. Nuclear features are overtly malignant with prominent nuclei and numerous mitotic figures and moderate nuclear pleomorphism. Focal necrosis is identified. The overlying squamous epithelium shows a prominent pagetoid proliferation of cells with large nuclei, prominent nuclei, and abundant pale cytoplasm.

Discussion: Morphologically, Paget’s disease of the anogenital region is similar to Paget’s disease seen within the breast. Within breast carcinomas, Paget’s disease typically reflects the spread of intraductal carcinoma to the skin’s surface. There may be an associated infiltrating carcinoma deeper within the breast. In contrast, Paget’s disease of the anogenital region is associated more frequently than breast with an underlying infiltrating sweat gland cancer within the immediately adjacent dermis or deeper subcutaneous fat. Consequently, following the diagnosis of Paget’s disease of the anogenital region on a superficial skin biopsy, it is recommended that a deep biopsy or excision be performed to rule out an underlying sweat gland cancer. One must also distinguish anogenital Paget’s disease from spread of urothelial cancer or colorectal cancer onto the anogenital surface. Rarely, vulvar squamous cell carcinoma may also manifest as a pagetoid process which needs to be excluded from anogenital Paget’s disease. In contrast to mammary Paget’s disease, extramammary Paget’s disease cells are negative for estrogen and progesterone receptors. In approximately half the cases, they are positive for gross cystic duct protein. As with mammary Paget’s disease, mucicarmine stains are positive in extramammary Paget’s disease.