Primary prostatic malignant melanoma

Primary melanoma of the prostate is rare, and usually cannot be diagnosed on imaging alone. In many cases, it is believed that in fact the tumour represents prostatic involvement by melanoma of the urethra.

Epidemiology

Primary malignant melanoma of the prostate represent both a tiny fraction of all malignant melanomas and a tiny fraction of all prostatic malignancies ^1-2. Unlike prostate cancer which is a disease usually of patients over the age of 50, melanoma of the prostate can affects younger adult males ². 

Clinical presentation

Patients either present with urinary symptoms, attributable from outflow obstruction, or with metastases. It is rare for the diagnosis to be made clinically or on imaging, although suspicion that an aggressive prostate mass is not the more common prostate cancer is usually raised due to the patients age or absence of raised PSA ². 

Radiographic features

Unfortunately there are no findings on imaging to suggest the diagnosis. 

All modalities typically demonstrates a prostatic mass, often large and heterogeneous, with evidence of extracapsular spread into bladder base and seminal vesicles. 

Metastases to local pelvic lymph nodes, bone and lung are frequently seen. 

Treatment and prognosis

Aggressive surgical management often with adjuvant chemotherapy is the standard of care, however despite this prognosis is very poor ^1-2. These tumours are highly aggressive, resulting in direct local extension and distant metastases early, especially to bone ². 

The overall reported 5 year survival of genitourinary melanoma is only 30% ³, however due to the rarity of prostatic melanoma no figures are available.  

Differential diagnoses

The differential depends on the size of the mass, and the degree of local invasion. For small masses the differential can usually be limited to: 

• adenocarcinoma of prostate
• benign prostatic hypertrophy (BPH)
• prostate sarcoma

Larger masses often have their epicentre still centered on the prostate. Other adjacent tumours to be considered however include:

• tumours of the base of bladder
• tumours of the seminal vesicles
• tumours of the rectum
• tumours of the pelvis (e.g. sarcomas)