The result of a prostate magnetic resonance imaging (MRI) scan often includes a rating from the Prostate Imaging-Reporting and Data System (PI-RADS). This standardized scoring system provides physicians with a structured way to assess how suspicious a detected lesion is for containing prostate cancer. Receiving a PI-RADS 4 score represents a high degree of suspicion for disease. This article clarifies the meaning of a PI-RADS 4 score, explaining the difference between high suspicion and certainty, and detailing the necessary subsequent diagnostic steps.
Understanding the PI-RADS System
The PI-RADS classification was developed by international radiology and urology groups to standardize the reporting of multiparametric MRI (mpMRI) findings. This system assigns a score from 1 to 5, reflecting the likelihood that a lesion represents clinically significant prostate cancer—a potentially aggressive tumor requiring treatment.
The score is derived from the radiologist’s evaluation of multiple, specialized MRI sequences, including T2-weighted imaging, diffusion-weighted imaging (DWI), and dynamic contrast-enhanced (DCE) imaging. T2-weighted images provide anatomical detail of the prostate, while DWI assesses the movement of water molecules, which is restricted in dense cellular environments like aggressive tumors. DCE imaging involves injecting a contrast agent to observe how blood flows into the suspicious area, as cancers often have a distinct, rapid blood supply.
The current iteration, PI-RADS version 2.1, uses a zone-dependent approach, meaning the primary determining sequence changes based on where the lesion is located within the prostate gland. For instance, in the peripheral zone where most cancers occur, the DWI sequence is the dominant factor in determining the score. This systematic evaluation using various imaging characteristics allows for a more precise estimation of malignancy risk than traditional diagnostic methods alone.
The full scale ranges from PI-RADS 1 (very low likelihood of clinically significant cancer) to PI-RADS 5 (very high likelihood). PI-RADS 2 indicates the lesion is unlikely to be cancerous, while PI-RADS 3 presents an intermediate or equivocal risk.
Defining a PI-RADS 4 Score
A PI-RADS 4 score places a lesion into the “High risk” or “Highly Suspicious” category, indicating that clinically significant prostate cancer is likely to be present. This designation means the radiologist has observed a combination of imaging features that strongly suggest malignancy. The findings are too concerning for an intermediate risk classification but are not definitive enough for a PI-RADS 5 rating.
In the peripheral zone, a PI-RADS 4 lesion typically demonstrates marked restriction of water diffusion on DWI, though perhaps slightly less pronounced than a PI-RADS 5 lesion. It may also show a moderately low signal intensity on T2-weighted images, which is characteristic of abnormal tissue. If the lesion is in the transition zone, the score is primarily driven by the T2-weighted appearance, such as a mass-like, non-encapsulated region with indistinct margins.
The PI-RADS 4 designation serves as a clear alert to the referring physician that the suspicious area requires immediate and targeted tissue sampling. While the score is a strong prediction of cancer, it remains an imaging assessment and not a final, confirmed diagnosis.
Cancer Probability Versus Certainty
The central question for anyone receiving a PI-RADS 4 score is whether it confirms a cancer diagnosis, and the answer is definitively no; it is not always cancer. The score represents a high probability, not a certainty, and is considered a false positive in a significant number of cases. The risk of finding clinically significant prostate cancer for a PI-RADS 4 lesion is estimated to be in the range of 40% to 70%.
This range means that between 30% and 60% of PI-RADS 4 lesions that undergo biopsy will ultimately be found to be benign or contain low-risk, non-aggressive cancer. Several benign conditions can mimic the appearance of a malignant tumor on mpMRI, leading to a false-positive PI-RADS 4 score. Severe or chronic inflammation of the prostate, known as prostatitis, can create areas of restricted water diffusion and low T2 signal that closely resemble cancer.
Benign prostatic hyperplasia (BPH) nodules, particularly those with a dense stromal component, can also exhibit highly suspicious features on imaging. These benign findings can sometimes produce the same signal intensity patterns that characterize cancer, meaning the PI-RADS 4 score cannot replace the definitive tissue analysis provided by a biopsy.
The Next Steps After a PI-RADS 4 Finding
A PI-RADS 4 finding almost universally triggers the recommendation for a prostate biopsy to obtain a definitive diagnosis. The standard procedure is a targeted biopsy, often performed using an MRI-ultrasound fusion technique. This advanced method overlays the MRI images showing the PI-RADS 4 lesion onto a real-time ultrasound image during the procedure.
This fusion technology allows the urologist to guide the biopsy needle with high precision directly into the suspicious area. Targeted biopsy significantly increases the chance of sampling the most aggressive part of the tumor, which is a major improvement over older, systematic biopsy techniques.
Once the tissue is collected, it is sent to a pathologist who examines the cells under a microscope and assigns a Gleason score. This score, which is the sum of the two most common cell patterns, determines the cancer’s Grade Group, indicating its aggressiveness. The pathology result is the final piece of information that confirms whether the PI-RADS 4 lesion is truly cancer and guides the treatment decision.