Quantitative imaging biomarkers (RECIST, PERCIST, SUV, ADC)

What they measure. A scan is a picture, but a decision needs a number. Response criteria turn measurements into categories: RECIST 1.1 sums the longest diameters of up to five target lesions on CT or MRI and calls response, stable disease or progression from the percentage change; PERCIST does the same with the peak standardised uptake value of FDG on PET; the Deauville five-point scale reads FDG uptake in lymphoma against liver and blood pool; PI-RADS, LI-RADS and BI-RADS grade the probability of prostate, liver and breast cancer on multiparametric imaging; the apparent diffusion coefficient on diffusion MRI measures how freely water moves, which falls in dense tumour and rises when cells die; and dynamic contrast measures such as Ktrans track blood vessel leakiness. Radiomics adds hundreds of texture and shape features on top of these.

Who uses them and what changes. Every oncology trial defines its endpoints with these criteria, so a RECIST progression call ends a treatment on trial and usually in clinic too. Deauville scores after two cycles decide escalation and de-escalation in Hodgkin lymphoma; PI-RADS decides who is biopsied; the Lugano criteria decide remission in lymphoma; PSMA-RADS and PERCIST are being written into radioligand and immunotherapy trials. Immunotherapy needed its own variant, iRECIST, because tumours can swell before they shrink.

What limits them. Diameters ignore necrosis and cavitation; standardised uptake values shift with scanner, reconstruction, glucose level and time after injection; and readers disagree, which is why pivotal trials use blinded central review. The RSNA Quantitative Imaging Biomarkers Alliance publishes profiles setting the acquisition and analysis standards under which a measurement can be trusted to a stated precision. None of these measures needs new equipment; the cost is in standardising protocols and training readers.