Primary Mucosal Melanoma of the Penile Urethra: A Case Report of an Extremely Rare Disease with a Unique Molecular Profile

Abstract

Primary melanomas of the penis are extremely rare, accounting for 0.18% of all melanomas and less than 2% of all primary penile malignancies. We present a case of primary mucosal melanoma of the penile urethra in an 82-year-old man. His partial penectomy revealed sheets of spindling and epithelioid tumor cells with pale eosinophilic granular cytoplasm and indistinct cell borders, which invaded into the corpus spongiosum. Multi-foci of melanoma in situ were identified at the mucosal surface of the urethra meatus. Both positron emission tomography (PET) and magnetic resonance imaging (MRI) scan one month after the partial penectomy showed no evidence of metastatic disease. Five months later, an F18-fluorodeoxyglucose-PET/computed tomography scan demonstrated mildly increased F18-fluorodeoxyglucose avidity along the ventral penis and a marked avidity of a right inguinal lymph node. Subsequent excision confirmed an ulcerated melanoma and a metastatic melanoma in one inguinal lymph node, respectively. Molecular analysis revealed a unique BRAF c.1780G>A mutation, resulting in the D594N alteration, which is the first report in penile urethral melanoma. The patient was miserable from the first infusion of immunotherapy (Keytruda), and a PET scan showed that the tumor continued to grow, with extensive metastatic pulmonary disease leading to massive pleural effusion. Unfortunately, the patient died of disease 18 months after his first presentation.