Histology and Immunohistochemistry in Mesothelioma Diagnosis
Mesothelioma is hard to name from imaging alone. Fluid and scans can suggest a tumour. Only tissue usually confirms it. Pathologists use two steps. They study the slide under the microscope. They then apply immunohistochemistry, or IHC, to mark proteins in the cells.
Histology sorts the tumour into three patterns. Epithelioid cells look like glands or sheets. Sarcomatoid cells look spindle-shaped and aggressive. Biphasic tumours mix both. The mix matters. Epithelioid disease often behaves less aggressively than sarcomatoid disease. A small biopsy can miss the second pattern, so sampling has to be adequate.
Looks are not enough. Lung adenocarcinoma and other cancers can mimic mesothelioma on a slide. IHC separates them. Pathologists apply a panel, not a single stain. Positive mesothelial markers often include calretinin, WT1, cytokeratin 5/6, and D2-40. Carcinoma markers such as CEA, TTF-1, and napsin A usually stay negative in mesothelioma. A matching pattern across several stains is stronger than one bright cell.
BAP1 loss and MTAP loss have become useful extra clues. Nuclear BAP1 loss supports mesothelioma in the right setting. It is not present in every case. CDKN2A deletion, often tested by FISH rather than a routine stain, can add support. These tests help most when the morphology is already suspicious.
Sarcomatoid tumours are harder. They may lose the usual mesothelial stains. Broad cytokeratin then becomes important so the tumour is not mistaken for a sarcoma. Clinical history of asbestos exposure can guide the work-up. It cannot replace the stains.
Cytology from pleural fluid is a start. It often cannot prove invasion. Guidelines therefore favour a biopsy when mesothelioma is likely. IHC on a cell block can still narrow the list while tissue is arranged.
A correct panel changes treatment and prognosis. It also avoids treating a lung cancer as mesothelioma, or the reverse. The diagnosis rests on pattern plus a disciplined set of stains, read with the scan and the clinical story. This overview is general information. It is not medical advice.