Hidden Malignancy: Predominantly InSitu Porocarcinoma Arising in a Poroma.

Porocarcinoma is a rare malignant adnexal tumor which can originate either de novo or-much more rarely-from a preexisting eccrine poroma. Given its aggressive clinical behavior, a timely diagnosis is essential for optimal patient's management. We report the case of a 79-year-old woman presenting with a well-circumscribed exophytic mass located on her right calcaneal region, which had been present for 2 years but had recently shown progressive enlargement. An excisional diagnostic biopsy was performed, and histopathological examination showed a poroid lesion with no malignant features. A diagnosis of eccrine poroma was then considered. However, deeper sections revealed a focal intraepidermal proliferation of markedly atypical cells and a small intradermal nest suggestive of early dermal invasion. The atypical population was characterized by overexpression of both p53 and p16, an increased ki-67 proliferation index, and preserved mismatch repair proteins. The diagnosis was then changed into a predominantly in situ, focally invasive porocarcinoma arising in an eccrine poroma. The patient underwent electrocautery of the wound margins and an echographic examination of pelvic nodes-which were negative for metastases-and was free of recurrence at a 6-month follow-up. This case provides evidence that benign-looking eccrine poromas may harbor microscopic foci of malignant transformation, thus emphasizing the need for a careful and thorough histopathological evaluation of these lesions.