Amelanotic lentigo maligna melanoma: Beware the wolf in sheep’s clothing

Case report: A 69 year old woman presented with an ill-defined, erythematous patch on the left nasal tip. She had Fitzpatrick skin type I. No other suspicious lesions were present on full skin examination.The erythematous patch persisted despite treatment with topical steroids. A 3mm punch biopsy was obtained to characterise the lesion, in particular to out rule non-melanoma skin cancer. Histopathological examination revealed an atypical junctional melanocytic proliferation consistent with lentigo maligna. Punch biopsy was negative for invasive melanoma.
 Subsequent wide local excision showed invasive Amelanotic Lentigo Maligna Melanoma.
Discussion: Lentigo maligna is a melanocytic neoplasm that originates on chronically sun exposed skin, especially the head and neck. It is almost universally pigmented. Amelanotic lentigo maligna is extremely rare and is often mistaken for other conditions such as non-melanoma skin cancer and dermatitis. On review of the literature there have been 32 reported cases of amelanotic lentigo maligna melanoma (ALMM). 

ALMM typically presents as a non-pigmented, erythematous macule with irregular, indistinct borders. It may have a slightly scaled surface. As in this case, the lesion is almost always asymptomatic. 

The prognosis of primary cutaneous amelanotic melanoma is no different than that for its pigmented counterpart as determined by tumour thickness, location, age and sex. However, as the diagnosis is clinically challenging ALMM is often more advanced at time of diagnosis.
Summary: We present a rare case of amelanotic lentigo maligna melanoma. This case illustrates the need for a low threshold for biopsy of an atypical, persistent, erythematous lesion.

Dr. Leonard Timoney