PSMA-expressing metastases: melanoma or prostate cancer?

Is it stage III melanoma, stage IV prostate cancer, or stage IV melanoma with bone and lymph node metastases? This was the question doctors faced recently when an elderly man with PSMA-expressing metastases presented to a German clinic. He had previously been treated for prostate cancer and skin cancer.

Positron emission tomography (PET) with 68Ga prostate-specific membrane antigen (PSMA) and computed tomography (CT) can detect metastases in prostate cancer. However, PSMA expression can sometimes also occur in other tumors, making it difficult to establish a clear diagnosis.

In 2019, a patient over 70 years of age was diagnosed with melanoma of the left forearm (Breslow depth 1.4 mm, no ulceration [pT2a]). After complete resection, local re-excision was performed with a safety margin of 10 mm. At the patient's request, a sentinel biopsy was not performed at this time.
He was diagnosed with prostate cancer 13 years ago and was treated with radical prostatectomy and salvage radiotherapy due to disease recurrence 5 years after diagnosis.
During screening in 2022, an elevated serum level of prostate-specific antigen (PSA) was detected (0.58 ng/mL) and PSMA-PET/CT was ordered to detect a possible recurrence. This showed two lesions: a markedly enlarged PSMA-positive left axillary lymph node and a PSMA-positive bone lesion in the L5 lumbar vertebral body.