Unfortunately, cancer has often spread before it is diagnosed. In such cases, it can be a diagnostic challenge to determine the site of the primary neoplasm. In some cases, an extensive search for the primary may prove fruitless, leading to a diagnosis of carcinoma with unknown primary or CUP. It is in such cases that immunohistochemistry comes into its own.
Immunohistochemistry (IHC) is the application of a large panel of monoclonal antibodies directed to a plethora of tissue specific antigens to the putative cancer tissue, followed by the use of immunofluorescence to identify stained areas.
Every tissue has its own signature antigens. Because such antigens will often be present in other tissues, they can be rather non-specific and no one antigen should be relied upon while looking at a biopsy sample. Rather, pathologists rely on a panel of antigens while performing IHC.
Carcinomas, i.e cancers of epithelial origin will demonstrate cytokeratins (CK). CK 8 and 18, and CK 1-3 are universally present in all carcinomas and are initially looked for to differentiate carcinomas from tumours of mesenchymal origin, i.e. sarcomas.
Sarcomas themselves will always have a protein called vimentin. Unfortunately, vimentin can also be produced by the occasional carcinoma.
The most useful cytokeratins are CK7 and CK20. Presence or absence of one or both of these cytokeratins is initially employed to narrow down the possible range of neoplasms.
Thus CK7+ and CK20+ will include transitional cell carcinoma, ovarian mucinous carcinoma and pancreatic carcinoma. CK7+ but CK20- will identify breast, lung, thyroid, gallbladder and cholangiocarcinoma. CK20+ and CK7- comprises gastrointestinal, particularly colon carcinoma and Merkel cell carcinoma. CK7- and CK20- narrows the field down to prostate, renal, adrenal and hepatocellular carcinomas.
Here I will give examples of some of the commonest clinical challenges faced by pathologists and how they are resolved.
1.Tumour deposits in pleura. Primary may be lung, breast, thyroid or pleura itself. Stain for TTF1, CK5/6, WT-1, calretinin, CK7, CK20, CEA, PAX 8, gross cystic disease fluid protein 15 (GCDFP15), mammaglobin, ER, PR.
Thus, TTF-1 positive- lung or thyroid. Thyroid will demonstrate thyroglobulin and PAX 8, lung will not. If lung, stain for BER-EP4, BG8 and MOC-31. All three are seen only with adenocarcinomas.
TTF1 negative- GCDFP15+, mammaglobin+, CEA positive, ER+, PR+ means breast cancer.
2. Deposits in axillary nodes. Cytokeratin negative, but positive for Melan A, HMB 45, S100. Diagnosis is metastatic melanoma.
3. Liver mass- primary or secondary? CK7-, CK20-, Hep-par 1+, alpha-fetoprotein positive, CEA (p), in situ hybridisation positive for albumin-primary hepatocellular Ca.
4. Liver deposit- CK20+, CK7-, CEA+, CDX2+, villin+ denotes metastatic colon cancer.
5. Peritoneal deposits- CD10+, RCA+, CK7-, CK20-, PAX2+ will mean this is a Clear cell carcinoma of the Kidney.
6. Lymphadenopathy- CD5+, Cyclin D1 positive, CD10-, CD23-, CD19, 20, 22 and 79A+ (pan B cell antigen) denotes Mantle cell lymphoma.
7. Adrenal deposits- CK7-, CK20-, simple CK (1, 8, 18+), EMA-, Melan A+, S100+, AdCB4P means adrenocortical carcinoma
8. Liver deposits- CK5/6+, CK7+, CK20+, uroplakin+, thrombomodulin+ denotes TCC of urothelial tract.
9. Pleural tumour- CK5/6+, CK7+, CK20-, TTF1-, BER-EP4-,WT-1+, calretinin+ denotes mesothelioma.
10. Vulval tumour- CK7+, CK20-, GCDFP15+, CEA+, ER+, HER2- is consistent with Pagets disease of vulval appendage- usually sweat duct.
