An extrafacial amelanotic lentigo maligna of the leg: a case report

Dear Editors, Lentigo maligna (LM) and lentigo maligna melanoma (LMM) represent one of the four main subtypes of melanoma, presenting most commonly in chronically sun-exposed areas of the head and neck, with a predilection for the cheek and less commonly on arms, legs and trunk. Leg localisation is the most frequent extrafacial site. Amelanotic LM constitutes a clinical challenge, because it can be misdiagnosed as other benign skin conditions such as granuloma annulare, dermatomycosis, eczematous lesions, etc. Here we report a very rare case of amelanotic LM of the leg. A 58-year-old woman was referred for evaluation of an erythematous lesion on her left leg. The patient reported that the lesion had appeared 1 year previously and had gradually enlarged, without any pruriginous or painful symptoms. The lesion was treated with topical antibiotics and corticosteroids for 1 month without any benefit. Her past medical history was negative, except for a venous deficiency. On examination we observed, on the lateral aspect of the left leg, an isolated, erythematous lesion ∼1⋅5 cm diameter, with irregular outline, smooth raised borders, an erythematous periphery and a central acromic region. Differential diagnosis included basal cell carcinoma, granuloma annulare, dermatomycosis and eczema. Therefore, we performed a surgical excision of the lesion and submitted the same for histological analysis. At scanning magnification, the skin showed a mildly thinned epidermis showing ridge flattening. The underlying dermis demonstrated a mild-to-moderate actinic elastosis and a moderate perivascular chronic inflammatory infiltrate in the superficial dermis, predominantly composed of lymphocytes. At higher magnification, the epidermis demonstrated a continuous basal proliferation of melanocytes with cytoplasmic retraction artefacts and atypical hyperchromatic nuclei. An immunohistochemical evaluation with HMB-45 and MART-1 highlighted the lentiginous melanocyte proliferation, and revealed a diffuse pagetoid spread of melanocytes in the epidermis. The morphological and immunophenotypical features were consistent with a LM diagnosis. LM, with its invasive form LMM, represents one of the four main subtypes of melanoma. LMM comprises an estimated 4–15% of invasive melanoma types; LM presents most commonly in chronically sun-exposed areas of the head and neck, with a predilection for the cheek and less commonly on arms, legs and trunk (1–3). Leg localisation is the most frequent extrafacial site (4). LM affects middle aged and elderly, almost exclusively Caucasian and rarely Asian people, and is generally not associated with pre-existing melanocytic nevi. Over time the lesion may acquire a dermal component and is then reclassified as an LMM even if the exact percentage of LM cases that progress to LMM is unknown (5–7). Differential diagnoses with LM include solar lentigo, pigmented actinic keratosis and seborrheic keratosis, being typically smaller, more uniform in colour, well-circumscribed and raised (8). The features of this patient’s lesion that were suggestive for a benign lesion were the size, the smooth raised non-scaly borders and the apparent absence of pigment (9). Indeed, differential diagnosis with nummular eczema, granuloma annulare and tinea corporis or pityriasis rosea were possible, but were excluded for the presence of a single not pruriginous localised lesion, without scales at the borders and superficially infiltrated. Therefore, we considered it necessary to describe this unusual case in order to highlight the frequent difficulties in the clinical diagnosis of apparently benign lesions that may more often hide under serious clinical conditions. It is therefore essential to encourage patients to have regular follow-ups to avoid delayed diagnoses.