Histology

Thin slices of fixed tissue are stained, classically with haematoxylin and eosin (H&E), so that nuclei show purple and the surrounding tissue pink; the pathologist reads cell type, architecture, invasion, grade, and margins, then adds immunohistochemistry and molecular tests as needed. Histology defines the cancer's name (adenocarcinoma, squamous, small cell, sarcoma subtype) and, with stage, is the foundation of treatment; a change in histology at progression, such as an EGFR-mutant adenocarcinoma transforming into small-cell lung cancer, changes therapy entirely. Digital slide scanning and AI models now assist with grading, counting and predicting molecular features from the image alone.