A male patient aged 48 years with type 2 diabetes presented with a painless nonhealing ulcer of 18 months duration under his right first metatarsal head. The ulcer was not a typical-appearing neuropathic foot ulcer and had mushrooming granulation tissue and areas of intact epidermis in a lenticular fashion over the wound bed (Fig. 1). The patient also complained of a “knot” in his right inguinal area. An incisional biopsy was taken from the foot lesion, which revealed a poorly differentiated melanoma covered by an intact epidermis and granulation tissue. The incisional biopsy was 0.8-cm thick, and melanoma extended to the deep margin. At presentation, the size and poor differentiation of the tumor made it impossible to assess the subtype of the original melanoma. The S-100 and HMB-45 stains (positive in melanoma cases) were strongly positive. A computed tomography of the chest, abdomen, and inguinal areas revealed metastasis to the inguinal lymph nodes and liver. The patient died 6 months later.
Although rare, melanomas can present as neuropathic foot ulcers in individuals with diabetes (1,2). Melanomas are located on the plantar surface in ∼7% of cases (3) with the exception of Japanese patients, in whom the plantar surface is the most common location (4). Acral lentiginous melanoma is the most common melanoma type that presents on the plantar aspect of the foot (3). This type of melanoma is commonly amelanotic, frequently ulcerates (5), and does not exhibit the classic signs of malignant melanoma associated with the mnemonic aid “ABCD” (asymmetry, border, color, diameter). In a review (6) of 53 lower extremity melanomas, 11 of 18 (61%) misdiagnosed cases were on the plantar foot. All misdiagnosed lesions were histopathologically acral lenginous melanomas. Initial misdiagnoses included nonhealing ulcer, wart, tinea pedis, and onychomycosis. Another retrospective review (7) of palmoplantar melanoma found that misdiagnosis led to a median delay of treatment for 12 months and was associated with increased tumor thickness (5.0 vs. 1.5 mm) and a lower 5-year survival rate (15.4 vs. 68.9%).
We are not supposing that plantar melanoma occurs more frequently in individuals with diabetes. However, we believe there is a greater chance of misdiagnosis given this population’s predilection toward plantar ulceration. An individual with peripheral sensory neuropathy is more likely to unknowingly ambulate on a plantar foot lesion, and this increased pressure and trauma can cause a lesion to initially resemble a diabetic foot ulcer. This case and short review emphasizes the importance of performing biopsies on chronic and atypical wounds early in the treatment algorithm of diabetic foot ulcers.
