In the evaluation of metastatic tumors, which statement is true about immunohistochemistry?

• A. IHC cannot help determine site of origin
• B. IHC can help differentiate primary from metastatic tumors
• C. IHC is only used for primary tumors
• D. IHC is not useful in cytology

Answer: (B)

Immunohistochemistry uses antibodies to detect tissue-specific proteins in tumor cells, giving clues about where a tumor comes from. In metastatic workups, a panel of stains acts like a fingerprint, helping you distinguish whether a tumor in one site is a metastasis from another primary tumor or a new primary tumor arising there. By comparing the staining pattern to known profiles of possible origins, pathologists can pinpoint the site of origin or confirm metastasis, which directly guides treatment. IHC can be used on cytology specimens as well, such as cells from fine-needle aspirations or body fluids, to aid diagnosis. This makes it a powerful tool for differentiating primary from metastatic tumors.