DP11 Primary cutaneous apocrine carcinoma presenting as scrotal skin tag in a patient with prostatic adenocarcinoma: diagnostic challenges

Abstract

Primary cutaneous apocrine carcinoma (PCAC) of the scrotum is rare. Histologically, tumours are often poorly differentiated and can demonstrate pagetoid spread, closely mimicking extramammary Paget disease (EMPD) or metastatic adenocarcinoma. We report a case of scrotal PCAC in a patient with history of prostatic adenocarcinoma, highlighting the diagnostic complexity. A 76-year-old man with a history of prostatic adenocarcinoma [Gleason score 7 (3+4), treated with radiotherapy 2 years earlier] presented with a 10-mm scrotal nodule, clinically diagnosed as a skin tag. Excision demonstrated a dermal-based, poorly differentiated carcinoma composed of sheets and nests of cells with abundant eosinophilic cytoplasm. No ductal differentiation or decapitation secretions were noted. There was focal pagetoid spread. Given the clinical history, an extensive immunohistochemical panel was performed. Tumour cells were positive for AE1/AE3, CK7, GATA3, GCDFP-15 and androgen receptor. CK20, CDX2, TTF-1, oestrogen receptor and progesterone receptor were negative. Prostate-specific antigen (PSA) showed focal cytoplasmic positivity, raising concern for metastatic prostatic adenocarcinoma; however, the staining was equivocal. P502S and NKX3.1 were completely negative. Radiological staging demonstrated no evidence of metastatic disease and serum PSA remained stable. Following clinicopathological correlation, a diagnosis of PCAC was made. A review of the published literature identified fewer than 20 reported cases. Scrotal PCAC frequently mimics benign entities clinically. Despite small tumour size, reported cases suggest an aggressive biological behaviour. Diagnosis of poorly differentiated PCAC is challenging, as typical decapitation secretions may not be seen due to solid growth pattern. In such cases, demonstration of an apocrine immunophenotype (GATA3+, GCDFP-15+) helps in supporting the diagnosis. However, this represents a significant diagnostic pitfall in patients with prostatic carcinoma as metastases from this can also show expression of GATA3 and GCDFP-15. Another differential to consider is an invasive EMPD. Scrotal PCAC deceptively combines benign clinical presentation and often absent apocrine histology, demanding comprehensive IHC and investigations.