Amelanotic melanoma: a detailed morphologic analysis with clinicopathologic correlation of 75 cases

Abstract

Amelanotic melanoma can have a varied appearance both clinically and microscopically. Here, we present our experiences with 75 cases of amelanotic melanoma defined clinically as a non-pigmented lesion and histopathologically as a tumor lacking significant melanization. We evaluated microscopic features such as morphology, mitotic count, nuclear atypia and presence of solar elastosis. Our amelanotic melanomas exhibited the following morphology: epitheloid (72%), spindled (18.7%) or desmoplastic (5.3%). In addition, we obtained patient information and clinical presentations on most of the cases (74/75; 98.7%) and follow-up data on 40% (30/75) of the cases. The majority of amelanotic melanomas in men were found on the trunk (13/45; 29%), head and neck (12/45; 26.7%), and lower limb (13/45; 29%) and in women were found on the lower limb (12/30; 40%), upper limb (10/30; 33.3%) and head and neck (6/30; 20%). In addition, we found that an increase in mitotic index correlated with worse survival (p < 0.026), whereas there were no differences in survival for other pathological features, such as nuclear atypia or solar elastosis. Furthermore, in cases with available tissue, all amelanotic melanoma expressed microphthalmia-associated transcription factor and tyrosinase, suggesting that the tumor cells retained melanocytic lineage and an enzyme in melanin formation, respectively. As the occurrence of amelanotic melanoma and the expression melanoma markers were similar to pigmented melanoma, we favor that amelanotic melanoma represents a subtype of melanoma rather than poorly differentiated or de-differentiated melanoma.

Comparison of Clinicopathologic Features and Survival of Histopathologically Amelanotic and Pigmented Melanomas

“…Other studies have variably reported predilection of amelanotic melanoma for females, 6,7 males 14,30 or neither sex. 31 In unadjusted analyses, we observed that younger (< 50 years) females were more likely than males of a similar age to have amelanotic than pigmented melanoma but there was little difference at older (>50 years) ages.…”

Cytodiagnosis of epithelioid malignant melanoma (amelanotic) and diagnostic dilemmas

“…The amelanotic lesions are usually of the nodular type and are highly undifferentiated. [67] For the above-mentioned reasons, most cases of amelanotic melanomas present at a later stage, though early lesions have also been described. [8] In men, these are commonly found in the trunk while in women these are found commonly in lower limbs.…”

Formas cutáneas y mucosas infrecuentes de melanoma. Parte II.

“…Al igual que el MM convencional, su ubicación varía según género, siendo más común en el tronco en hombres y extremidades en mujeres. 8 Se clasifican en tres grupos según la cantidad de pigmento observado clínicamente en su superficie: amelanótico, parcialmente pigmentado (<25%) y ligeramente pigmentado (>25%). Se cree que la falta de pigmento traduce una pobre diferenciación.…”