Predictive IHC at the Microscope: Scoring Markers That Choose Drug

Immunohistochemistry no longer only names a tumour - increasingly, it chooses the drug. A membrane score gates an antibody-drug conjugate; a percentage of stained nuclei opens or closes a line of therapy; a pattern of nuclear loss unlocks immunotherapy across the body.Yet the threshold that decides treatment is not a fact about the tumour. It is a decision rule tied to a particular trial, a particular drug, and a particular assay. For every predictive marker, this pan-tumour guide sets out what to score, where the cutoff came from, how reproducibly it can be read, and how to report it in language the oncologist can act on.Markers covered: PD-L1 - HER2 and HER2-low/ultralow - MMR/MSI - ER, PR and Ki-67 - Trop-2 - Nectin-4 - FRα - Claudin 18.2 - HER3 - B7-H3 - quantitative and AI-assisted scoring.Inside this edition: fifteen focused chapters across four parts; the trial-validated cutoff behind every marker; reproducibility and the faint-end problem, marker by marker; pitfalls at the bench, controls, and external quality assurance; assay, clone and platform specifics throughout; and synoptic reporting templates.Written for practising pathologists, trainees, and tumour-board participants.Expression is not eligibility.