Immunohistochemical and molecular analysis of spitzoid neoplasms with pulverocyte subclones

Background: 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). Aim: To characterize the significance of subclones of dusty-grey pigmented epithelioid melanocytes within spitzoid neoplasms. Methods: 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. Results: 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. Conclusions: 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.