Immunohistochemical and molecular analysis of spitzoid neoplasms with pulverocyte subclones

Clonal naevi are characterized by a focal proliferation of pigmented melanocytes in an otherwise banal naevus. These subclones are often composed of aggregates of larger, epithelioid melanocytes with nuclear atypia and dusty-grey cytoplasmic pigmentation, which are referred to as 'pulverocytes', and this finding may lead to a misdiagnosis of malignant melanoma (MM).

To characterize the significance of subclones of dusty-grey pigmented epithelioid melanocytes within spitzoid neoplasms.

We studied the histological and molecular features of a series of 20 spitzoid neoplasms with pulverocyte subclones encountered in our practice, including both atypical Spitz tumours (ASTs) and invasive MMs.

Pulverocytes were predominantly dermal, and the percentage of subclones ranged from 2% to 40%, with a median of 10% in ASTs and 25% in lesions we classified as MM. In cases with > 10% subclones, there was an increased odds of fluorescence in situ hybridization positivity (OR = 12; 95% CI 1.2-293.4; P = 0.03) and an increased odds of homozygous 9p21 deletion (OR = 3.6; 95 CI 0.28-89.82; P = 0.33), although the latter did not reach statistical significance.

We consider spitzoid lesions with a small subclone population to be a variant of a clonal naevus with indolent behaviour, whereas lesions with larger pulverocyte populations are more likely to have chromosomal copy number aberrations and in some cases may represent malignant transformation.