What is the PI-RADS Score?

By: Dan Sperling, MD

The tremendous international interest in 3T multiparametric MRI (mpMRI) brought with it the challenge of how to standardize the reporting of prostate image analysis among radiologists around the globe. The European Society of Urogenital Radiology (ESUR) proposed a numeric system called the Prostate Imaging Reporting and Data System, or PI-RADS, for prostate cancer detection. It is based in an earlier system for breast imaging.

First, a word about 3T mpMRI. A powerful 3 Tesla (3T) magnet is the hardware for capturing prostate images. Sophisticated software can amplify various features of these images in ways that emphasize certain tissue parameters. Each parameter shows a specific difference between normal tissue and prostate cancer. Here is a simple explanation of the four commonly used parameters:

  • T2 weighted images give excellent anatomic detail and thus show the location of a suspicious area.
  • Diffusion-weighted imaging (DWI) gives functional information about the movement of water molecules, which is different in healthy tissue than in tumors.
  • Dynamic contrast-enhancement (DCE) can point to a tumor by revealing abnormal blood flow from network of abnormal blood vessels that feed the tumor (angiogenesis)
  • MRI spectroscopy (MRS) is used to show concentrations of biochemicals called metabolites, since the presence of certain metabolites characterizes prostate cancer.

So, back to PI-RADS. You are probably familiar with the Gleason grade as a system for classifying prostate cancer cells according to aggression. The Gleason scale ranges from 1 to 5, where 1 indicates no cancer at all, and 5 indicates very aggressive disease. In that sense, PI-RADS is similar, but it’s an interpretation of images, not actual cells. Thus, it has to do with interpreting the likelihood of cancer depending on what the images show.

It, too, is based on a score from 1 to 5 but there are two levels to the system.

Level One: Each parameter that shows up in an image (T2, DWI, DCE and MRS) is assigned a numerical value, with 1 being most probably benign and 5 being highly suspicious of malignancy.

Level Two: The values are added together. In centers that don’t analyze for MRS, only T2, DWI and DCE are added together. In centers that analyze for all four parameters, those values are summed. The total determines whether the PI-RADS classification is Level I, II, III, IV, or V. The table below shows the probability range from benign to highly suspicious for cancer.

 PI-RADS Classification System

PI-RADS classification

Definition

Total T2, DWI, DCE

Total T2, DWI, DCE, MRS

I

Most probably benign

3, 4

4, 5

II

Probably benign

5, 6

6-8

III

Indeterminate

7-9

9-12

IV

Probably malignant

10-12

13-16

V

Most probably malignant

13-15

17-20

This system is useful in guiding decisions about whether to biopsy, how to optimize biopsy, and how to plan treatment. In a future blog I will describe the type of research that is being done to correlate the accuracy of PI-RADS with actual physical evidence.