Prostatic Adenosquamous Carcinoma With a Minute Squamous Component in the Initial Biopsy and Early Liver Metastasis: A Case Report

ABSTRACT Introduction We report a case of de novo prostatic adenosquamous carcinoma with a minute squamous component in the initial biopsy and early liver metastasis despite a marked prostate‐specific antigen response. Case Presentation A 72‐year‐old man was diagnosed with locally advanced prostate cancer with nodal and multiple bone metastases. Prostate biopsy initially showed adenocarcinoma with a Gleason score of 4 + 5. After leuprolide and abiraterone plus radiotherapy, prostate‐specific antigen decreased from 13.0 to 0.22 ng/mL; however, a liver metastasis developed. Re‐evaluation of the prostate biopsy revealed minute squamous differentiation, and liver biopsy showed metastatic squamous cell carcinoma. Immunohistochemistry showed retained MLH1 and PMS2 expression with loss of MSH2 and MSH6 in both specimens. Genomic profiling showed microsatellite instability‐high status. Conclusion Repeat biopsy and molecular‐pathological reassessment may be important when radiographic progression is discordant with prostate‐specific antigen kinetics.