Histology (histological subtype)

Pathologists classify tumours by histology using WHO classification books updated every few years, increasingly merged with molecular features ('histomolecular' classification of brain tumours). Histology determines chemotherapy choice (pemetrexed for non-squamous lung cancer only), eligibility for targeted testing (EGFR in adenocarcinoma), staging systems and prognosis. Rare variant histologies (sarcomatoid, micropapillary, signet-ring, plasmacytoid) often carry worse outcomes and are under-represented in trials; histologic transformation (adenocarcinoma to small-cell under EGFR inhibitors) is a recognised resistance mechanism.