Metastatic Melanoma to a Neurofibroma

We present an extraordinary case of metastatic cutaneous melanoma to a pre-existing neurofibroma in a 75-year-old man with a history of primary invasive melanoma in an anatomically close vicinity. Histological examination of the metastatic melanoma showed a well-circumscribed intradermal nodule of frankly malignant epithelioid melanocytes without an intraepidermal component, surrounded and sharply demarcated from a diffuse spindle cell proliferation with morphological features of a neurofibroma. The spindle cell component showed bland cytologic features, with no mitotic activity or lymphocytic inflammation and no features of malignancy. By immunohistochemistry, both components expressed S100, while HMB45 positivity and complete loss of p16 were only observed in the epithelioid cells. The morphologically distinct areas were analyzed by fluorescent in situ hybridization, which demonstrated an abnormal profile (gain of RREB1 and homozygous loss of CDKN2A) in the epithelioid nodule; however, no abnormalities were detected in the spindle cell component. Next-generation sequencing showed somatic NRAS and PTEN mutations in the melanoma cells only. The overall molecular findings supported the immunomorphological diagnosis of metastatic melanoma within a neurofibroma over the potential differential diagnosis of melanoma with a neurofibroma-like spindle/desmoplastic component.