Melanoma Incognito: A Case Series

Summary: Malignant melanoma-in-situ (MMIS) is typically poorly circumscribed, asymmetric, and irregularly pigmented clinically. Histologically, MMIS is characterized by nests and single atypical melanocytes with confluent growth and pagetoid spread. Most malignant melanomas (MM) are diagnosable on hematoxylin and eosin (H&E) sections and with immunohistochemical (IHC) stains in challenging cases. We present nine cases where H&E sections lacked characteristic features of MM requiring IHC to render the correct diagnosis. There were 6 male and 3 female patients, ranging from 53 to 80 years of age, with lesions on the arm or head. Clinically the lesions were biopsied to rule out melanoma or dysplastic nevus versus seborrheic keratosis/lentigo. Histologically, all biopsies demonstrated macular seborrheic keratosis/lentigo changes with background solar elastosis. Scattered intraepidermal cells with grayish cytoplasm were noted in all cases, however nests or pagetoid spread of atypical melanocytes were not apparent in H&E-stained sections. SOX10 and PRAME immunostains demonstrated melanocytes with confluent growth and pagetoid spread, consistent with MMIS. The re-excised specimens demonstrated residual MMIS similar to the corresponding biopsy. These nine cases highlight melanomas having unconventional histologic characteristics. Awareness of this phenomenon with clinical suspicion of an atypical melanocytic lesion will aid in the assessment of these cases to avoid misdiagnosis.