• Crowded sheets of tumour cells with overlapping nuclei
• Acinar formation is discernible within the sheets
• Round to oval eccentric nuclei with mild nuclear pleomorphism
• Fine powdery chromatin and inconspicuous nucleoli
• Moderate to abundant pale cytoplasm
• Clean background – no necrosis

Commentary:

Provisionally, the diagnosis is metastatic adenocarcinoma. Given the clinical history of an elderly male, primary sites to consider include lung, upper gastrointestinal tract, pancreatobiliary system, lower gastrointestinal tract and prostate.  

There are some morphologic clues to the site of origin, can you identify them?

• Tumour fragments with prominent acinar formation
• Round to oval nuclei with slightly irregular nuclear contours
• Relatively  fine chromatin with discernible small nucleoli
• Moderate amounts of amphophilic cytoplasm

• Diffuse positivity for PSA and NKX3.1 confirms the diagnosis of metastatic prostatic adenocarcinoma

Diagnosis

Metastatic adenocarcinoma of prostate origin.

Take home points

• Prostatic adenocarcinoma is one of the most common cancers in men
• Prostatic adenocarcinoma was commonly reported as a "cancer of unknown primary" in older publications
• Maintain a high index of suspicion if a well differentiated adenocarcinoma presents at a metastatic site in an elderly male 
• The morphologic clues on cytology include tight acinar formation and mild nuclear pleomorphism
• The diagnosis is not difficult if cell block, immunohistochemistry or concurrent biopsy are available; as well as a history of known prostatic carcinoma