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The international scoring system radiologists use to say how likely a prostate MRI finding is to be a serious cancer, from 1 (very unlikely) to 5 (very likely). British NHS reports usually do not use it: NICE asks for a 5-point Likert score instead, and the two look identical on the page and are not the same thing.
PI-RADS is a structured reporting system for multiparametric prostate magnetic resonance imaging, developed jointly by the American College of Radiology, the European Society of Urogenital Radiology and the Admetech Foundation. It assigns each suspicious finding an assessment category from 1 to 5 expressing the probability that it is clinically significant cancer, from very low at 1 to very high at 5, and it derives that overall category from scores given separately to the individual sequences: diffusion-weighted imaging with its apparent diffusion coefficient map is the determining sequence in the peripheral zone, T2-weighted imaging is dominant in the transition zone, and dynamic contrast enhancement acts as a tie-break that can move a peripheral-zone 3 to a 4. Clinically significant cancer, in the system's own definition, is Gleason score 7 or above including 3+4 with a prominent but not predominant pattern 4 component, or tumour volume above 0.5 cubic centimetres, or extraprostatic extension. The current version is 2.1, published in 2019 as a consensus revision of version 2 intended to reduce inter-reader variability, with changed guidance on the T2-weighted acquisition plane, diffusion b-values, dynamic contrast temporal resolution, the anterior fibromuscular stroma and the central zone, and the scoring of transition zone category 2.
A reader in Britain will meet a different word for the same job. NICE NG131 recommendation 1.2.2 offers multiparametric magnetic resonance imaging as the first-line investigation for suspected clinically localised prostate cancer and asks that the result be reported on a 5-point Likert scale. Recommendation 1.2.3 offers magnetic resonance imaging-influenced biopsy at a Likert score of 3 or more, and 1.2.4 allows omitting biopsy at Likert 1 or 2 after discussing the risks and benefits and reaching a shared decision, with systematic biopsy offered to anyone who still wants one. The Likert scale is the radiologist's overall subjective judgement of the probability of clinically significant cancer using all available information, including the prostate-specific antigen and the clinical picture; PI-RADS is a prescriptive algorithm that combines per-sequence scores by fixed rules and is intended to be read from the images alone. They share a 1 to 5 range and the same clinical thresholds in practice, and they are not interchangeable scores.
The number a man is given is therefore a probability statement and not a diagnosis. NG131 sets out what the threshold means in the two directions: between 11 and 28 out of 100 people with a low-risk magnetic resonance imaging result turn out to have clinically significant cancer, and between 18 and 23 out of 100 with a low-risk result who are biopsied receive a diagnosis of clinically insignificant cancer. And the score is a per-patient triage judgement, not a map: the same scan that is 93 percent sensitive for whether a man has clinically significant cancer detects only 65 percent of individual clinically significant lesions against whole-mount pathology.
Showing the technology this term belongs to: MRI.
This is the paper Europe PMC returns for registry id ISRCTN94604465 with the most citations, so it is the natural first reading for anyone following the trial. Read the abstract above alongside the trial page and the registry entry; the record was linked automatically and its figures have not been checked by hand.
The limit of what a prostate scan can be asked to do. It is a good triage test for whether to biopsy and a poor map of where every tumour is, which matters for anyone being offered treatment to part of the gland or follow-up by imaging alone.
Pre-biopsy MRI with targeted sampling is now the standard diagnostic pathway for suspected prostate cancer, reducing overdiagnosis of low-risk disease.
The evidence that put a scan in front of the biopsy. It reduces the number of men who are biopsied at all, reduces the number of harmless cancers found, and increases the number of dangerous ones, which is the only combination that improves a screening pathway on both sides at once.
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