Plantar melanoma that mimics melanocytic nevi: a report of 4 cases with lymph node metastases and with review of positive and negative controls.

We report 4 cases of melanoma localized on the sole with some striking histological similarities to the compound nevi. One case showed inguinal lymph node metastases after a diagnosis of compound nevus made 5 years earlier. The other 3 cases were sent to us in consultation as suspected plantar lesions; a diagnosis of melanoma mimicking compound nevus was proposed: the sentinel lymph node technique was accordingly performed and resulted positive. These 4 index cases were compared with 26 cases of ordinary plantar melanoma and with 117 cases of benign compound plantar nevi. Histologically, the similarity of areas of these 4 cases of plantar melanoma to compound plantar nevi (namely Miescher, Clark, or acral lentiginous nevus) is so close that it may prove misleading in the diagnostic pathway. In such cases, an erroneous diagnosis of benign lesion may be made. The correct diagnosis of melanoma can, however, be done by combining the clinical and pathological findings. In fact, this deceptive form of melanoma mimicking a compound nevus is characterized by patients' advanced age (>45 years), large lateral diameters (>10 mm), and significant depth of the lesion (>1 mm). In our 4 cases, such features were combined with one or more of the following histological clues: lentiginous (ie, continuous) proliferation of enlarged and hyperchromatic melanocytes aligned in single units at the dermoepidermal junction; extensive eccrine duct melanocytic infiltration; dermal melanocytes with slight nuclear enlargement, nuclear membrane thickening and folding, and conspicuous central nucleoli; and cellular pleomorphism of the epidermal and dermal component. Moreover, mitotic figures were an inconstant but decisive indication of malignancy. In conclusion, a small group of plantar melanoma has striking similarities with compound nevi. Only the correlation of clinical data (old age of the patient and large size of the lesion) with subtle architectural and cytological alterations (depth of the lesion, lentiginous melanocytic epidermal and eccrine proliferation, and cellular alteration and mitoses) allows a correct diagnosis.