Abstract
Small cell carcinoma (SmCC) of the prostate is an aggressive malignancy that is most frequently reported in patients with a history of androgen deprivation treatment (ADT) for high grade prostatic adenocarcinoma. Histologically, prostatic SmCC has similar features to SmCC of the lungs. We present a 72 -year-old who presented with painful priapism and a skin nodule of the distal penile shaft. Skin biopsy showed sheets of small basophilic cells with minimal cytoplasm, smudged hyperchromatic molded nuclei, and high mitotic rate. By immunohistochemistry, the tumor cells were diffusely positive for CK20, TTF-1, had a high proliferation index on Ki67 (>95%). Based on this evaluation, the diagnosis of Merkel Cell Carcinoma (MCC) was initially made by the outside pathologist, and the patient was referred to the multidisciplinary Merkel Cell Carcinoma Clinic. Additional workup subsequently demonstrated that the biopsy was negative for CK7, CD45, CD56, chromogranin, PSA, NKX3.1, CDX2, and MCPyV. Clinical evaluation demonstrated negative serology for Merkel Virus VP1 Capsid antibody. Further history was obtained, which was significant for high-grade prostatic adenocarcinoma (Gleason 4+5) with lymph node metastases treated with radical prostatectomy in 2016, followed by chemotherapy and radiation therapy for bone metastasis completed spring 2021. A recent significant rise in serum PSA level from 6 ng/mL in March to 27.5 ng/mL in June of 2021 was noted. Considering the patients history of prostate cancer, recent rise in PSA, and lack of supporting evidence for MCC, the diagnosis of prostatic SmCC was made. In the absence of an appropriate clinical history, prostatic SmCC can be mistaken for MCC, and should be included in the differential diagnosis for small blue tumors, particularly when occurring in the genitourinary area.
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