Intramuscular Cellular Nodules in a Congenital Large Plaque-Like Blue Nevus: Melanoma or Not?

Melanoma arising in blue nevus (BN) is usually evident on histopathology. However, there are cases in the gray zone where neither morphology nor immunostains and molecular studies are conclusive.

A 33-year-old man presented with greenish discoloration of the abdominal skin at birth. Over time, the lesion increased in size to involve the entire left half of the abdomen, extending to the back and chest wall. He noticed nodules beneath the lesion, which were enlarging. Magentic resonanace imaging showed a lesion within the left external oblique measuring 8.3 × 6 × 4 cm and smaller lesions in the adjacent muscular planes. Gross examination of the excision showed an 8 × 6.5 × 5 cm brown-black, intramuscular nodule. Histopathology showed a BN of the overlying skin extending into the subcutis. The intramuscular nodule showed infiltrative, cellular areas with nonpigmented, plump spindle cells in nests and pseudorosettes with focal necrosis. There was minimal pleomorphism and prominent eosinophilic nucleoli. Mitoses were scarce. The neoplastic cells expressed Melan A, S100, and HMB45 with a low proliferative index (<1%). The lack of atypia and mitoses, despite clinically suspicious for melanoma, lead us to perform comparative genomic hybridization, which showed an abnormality suggesting malignancy. The patient remains with localized disease 6 years after surgery and no distant metastases.

Cellular nodules in a plaque-like blue nevus presenting as an intramuscular mass is hitherto unreported. It is prudent to follow-up such cases like a melanoma, despite lacking overt atypia and mutations, because metastases have been reported as late as 16 years after diagnosis.