Which marker is NOT typically included in the IHC panel for primary prostatic adenocarcinoma?

• A. PAX8
• B. PSMA
• C. NKX3.1
• D. PAP

Answer: (A)

Immunohistochemistry for suspected prostatic adenocarcinoma uses markers that confirm prostatic tissue and help distinguish it from tumors that can look similar from other organs. PSMA is a membrane protein that is strongly expressed in most prostate cancers, making it a go-to marker for proving prostatic origin. NKX3.1 is a nuclear transcription factor with high sensitivity and specificity for prostate tissue, so its presence is a reliable indicator of prostatic differentiation. Prostatic acid phosphatase (PAP) is an older, still-used prostatic marker that supports the diagnosis when positive.

PAX8, on the other hand, is a marker of renal and Müllerian (ovarian/endometrial) lineage and some thyroid carcinomas; prostate tissue typically does not express PAX8. Because PAX8 would not support a prostatic origin and is more informative for ruling in metastasis from other primaries, it is not a standard part of the prostatic adenocarcinoma panel. It may be useful if the differential includes metastasis from a renal or Müllerian primary, but for primary prostatic lesions it’s not typical.