Abstract

Porocarcinoma is a rare malignant sweat gland neoplasm that represents the malignant counterpart of benign poroma. Histologic identification of porocarcinoma can be diagnostically challenging. The use of YAP1 and NUT immunohistochemistry for the diagnosis of porocarcinoma was recently described. In addition, CD117 (KIT) has been reported as a useful immunohistochemical marker in porocarcinoma. We present a case of a healthy 55-year-old Caucasian male who presented to dermatology for a suspected cyst on the right shoulder. The patient reported that a subcutaneous nodule had been present for over a year. The lesion had grown larger over time. The patient had reportedly previously expressed purulent fluid from the lesion. The area was tender to palpation. On exam a firm, 4 cm nodule with central erosion and surrounding erythema was appreciated. The patient was given a 14-day course of oral doxycycline and scheduled for excision of the lesion. Histopathology of the excision revealed a large tumor arising from the epidermis and extending deeply into the dermis and subcutis. The tumor was composed of round, uniform cuboidal epithelial cells. In some areas the cells were bland, but in other areas the nuclei were atypical and hyperchromatic with an irregular chromatin pattern. Many mitotic figures were present. Multiple sweat duct lumens were present throughout the lesion. On immunohistochemical staining, the tumor cells were diffusely positive for NUT, showed broad zones of CD117 expression, and contained zones of increased Ki-67 proliferative activity. A diagnosis of porocarcinoma was rendered. The lesion was treated with complete surgical excision.