An Enlarging Mass Superimposed on a Giant Congenital Melanocytic Nevus: A Case Report and Review of Literature.

Congenital melanocytic nevi (CMN), arising from the proliferation of melanocytes, manifest at birth and are colloquially termed as bathing trunk, coat-sleeve, or stocking nevi. These lesions, occurring in approximately 1 to 2% of neonates, predominantly localize over the trunk and thigh regions. Giant congenital nevi, characterized by a size exceeding 20 cm, often present with pigmentation and hairiness. Herein, we present the case of a 14-year-old patient diagnosed with a giant congenital nevus (bathing trunk nevus) accompanied by lipomatosis, while concurrently reviewing the literature to delineate this rare association. Our investigation identified a total of six cases, comprising five females and one male. Notably, half of the cases (3/6) exhibited multiple lipomatous lesions, with the back region emerging as the most prevalent site. In our presented case, progressive enlargement of a soft tissue mass in the right flank prompted excision, revealing a well-circumscribed lipomatous lesion infiltrated diffusely by benign-appearing melanocytes within fat lobules, as confirmed by immunohistochemistry. Specifically, positivity for HMB45 and S100 markers was observed. Beyond the rarity of such occurrences, our report underscores the infrequent concomitance of CMN with lipomatosis.