WHO Classification

The WHO Classification is the World Health Organization's official system for naming and categorizing tumors based on their cell of origin, microscopic appearance, molecular features, and clinical behavior. For asbestos-related cancers, the WHO Classification of Tumours of the Pleura, Peritoneum, and Lung defines how mesothelioma and lung cancer are diagnosed and subtyped. It is updated periodically and serves as the international reference standard used by pathologists worldwide.

Origin

The WHO began publishing its "Blue Books" series of tumor classifications in 1967 to standardize cancer diagnosis across countries. The classification covering mesothelioma has been revised multiple times, with major updates in 2004, 2015, and 2021 reflecting advances in immunohistochemistry and molecular pathology.

Significance in Asbestos Context

The WHO Classification determines exactly how your mesothelioma or asbestos-related lung cancer is labeled on your pathology report, for example, epithelioid, sarcomatoid, or biphasic mesothelioma. These subtype labels directly affect prognosis, eligibility for clinical trials, and treatment decisions, and they are also central to legal claims because defendants and their experts often scrutinize whether the diagnosis meets current WHO criteria. The 2021 edition reclassified "well-differentiated papillary mesothelioma" as a tumor of uncertain malignant potential and emphasized BAP1 and MTAP testing to distinguish malignant mesothelioma from benign mimics, so the edition used in your diagnosis matters.

Example Sentence

Her pathology report cited the 2021 WHO Classification in confirming epithelioid mesothelioma, which her oncologist explained carries a better prognosis than the sarcomatoid subtype.

Related Terms

Epithelioid Mesothelioma, Sarcomatoid Mesothelioma, Biphasic Mesothelioma, Immunohistochemistry, BAP1, Histologic Subtype

Notes

• Ask your pathologist or oncologist which edition of the WHO Classification was used (2015 vs. 2021). Criteria have changed and may affect diagnostic certainty.
• Request a copy of the full pathology report and any immunohistochemistry results; these documents are critical for both treatment planning and legal claims.
• If the diagnosis is borderline or unusual, ask whether a second opinion from a mesothelioma reference pathology center (such as the U.S.-Canadian Mesothelioma Reference Panel) would be appropriate.
• Confirm whether BAP1 and MTAP testing were performed, as these are now part of standard WHO-recommended workup for suspected mesothelioma.