Immunohistochemistry of p16 in nevi of pregnancy and nevoid melanomas.

Abstract

Hormonal changes in pregnancy are known to alter melanocytic lesions, with some nevi noted to have increased mitotic figures and increased Ki-67 proliferation index. Additionally, cytomorphologic changes have also been noted, referred to as superficial micronodules of pregnancy. These changes may alarm the pathologist for malignancy, particularly nevoid melanoma. Immunohistochemistry for p16 has been recently utilized to distinguish benign nevi from melanoma. We assessed the use of p16 immunohistochemistry for distinguishing melanocytic nevi of pregnant patients from nevoid melanomas. Fourteen nevomelanocytic lesions were obtained from pregnant or postpartum patients along with 20 nevoid melanomas for comparison. Immunohistochemistry with p16 was performed on each melanocytic lesion. The percentage of nuclear p16 staining of dermal melanocytes was grouped on a scale of <5%, 5% to 25%, >25% to 50%, and >50%. The majority of nevi from pregnant patients (81%) showed staining of >5% for p16. In contrast, the majority of nevoid melanomas (65%) had staining of <5% for p16. The application of p16 as a potential immunohistochemistry diagnostic marker to distinguish nevi from pregnant patients vs nevoid melanomas may be useful.