A Prostate Imaging Reporting and Data System, or PI-RADS, score is a standardized method used by radiologists to interpret the findings of a prostate Magnetic Resonance Imaging (MRI) scan. When a patient receives a PI-RADS 5 score, it naturally causes significant concern, as this designation represents the highest level of suspicion on the scale. The score itself is a radiological assessment of probability and is not a definitive diagnosis of cancer.
Decoding the PI-RADS System
The PI-RADS scale is a five-point system designed to stratify the risk of finding clinically significant prostate cancer. Clinically significant cancer is generally defined as a high-grade or high-volume cancer that requires active treatment. Scores range from 1 to 5, with each number corresponding to an increasing likelihood of aggressive disease.
A PI-RADS 1 indicates a very low likelihood of clinically significant cancer being present, while a score of 2 suggests a low probability. Scores 1 and 2 often mean that a biopsy is not immediately necessary unless other clinical factors suggest a high overall risk. The intermediate score is a PI-RADS 3, which means the presence of clinically significant cancer is equivocal, being neither likely nor unlikely. A PI-RADS 4, however, indicates a high probability that clinically significant cancer is present in the identified area of the prostate.
What the PI-RADS Score of 5 Specifically Means
A PI-RADS 5 score signifies a very high probability that clinically significant prostate cancer is present within the lesion being evaluated. This numerical designation typically correlates to a likelihood greater than 80% that a biopsy of the area will reveal aggressive disease.
The term “clinically significant” generally refers to cancer that has a high Gleason score, specifically a Gleason Grade Group 2 or higher, which includes Gleason scores of 3+4=7 or greater. While the probability is high, it remains a radiological finding, and a final cancer diagnosis can only be made through a biopsy.
Despite the strong predictive power, a PI-RADS 5 lesion is not a guarantee of cancer, as a small percentage of these findings may turn out to be benign upon biopsy. Causes for a false positive result can include benign conditions that mimic cancer on the MRI, such as chronic inflammation, prostatitis, or atrophy.
How PI-RADS 5 Lesions Are Identified
The PI-RADS score is determined using a multiparametric MRI (mpMRI), which combines several different imaging techniques to assess the prostate tissue. The three main components of the mpMRI sequence are T2-weighted imaging, Diffusion-Weighted Imaging (DWI), and Dynamic Contrast Enhancement (DCE). A radiologist assigns a PI-RADS 5 based on a specific set of characteristics that suggest highly restricted cellular movement and an abnormal tissue structure.
For lesions located in the peripheral zone of the prostate, the DWI sequence is the dominant factor in determining the final score. A PI-RADS 5 lesion in this zone typically demonstrates severely restricted diffusion, meaning it is very bright on high b-value DWI images and very dark on the corresponding Apparent Diffusion Coefficient (ADC) map.
In addition to the DWI findings, a PI-RADS 5 lesion often appears as a markedly low signal intensity area on the T2-weighted images. The lesion is typically large, often measuring greater than 1.5 cm in its largest dimension, and may show signs of extending beyond the prostate capsule. While Dynamic Contrast Enhancement (DCE) is also part of the multiparametric study, the high DWI and T2-weighted scores are the primary drivers for assigning the highest PI-RADS category.
Next Steps Following a PI-RADS 5 Finding
The discovery of a PI-RADS 5 lesion immediately necessitates a targeted biopsy to confirm the presence and grade of cancer. Because the mpMRI provides precise location data, the standard procedure is a targeted or fusion biopsy. This technique uses the MRI images, sometimes fused with real-time ultrasound, to guide the biopsy needle directly into the suspicious area.
Targeted biopsy ensures that the most concerning part of the lesion is sampled, which increases the accuracy of diagnosing a clinically significant cancer. This targeted approach is often combined with a systematic biopsy, which samples other areas of the prostate to ensure no other cancer foci are missed. Following the biopsy, a pathologist will analyze the tissue samples and provide a definitive diagnosis, including the Gleason score.
Consultation with a multidisciplinary team is a necessary step after receiving a PI-RADS 5 result. This team usually includes a urologist, the interpreting radiologist, and potentially a radiation oncologist, all of whom collaborate to review the imaging and biopsy plan. Definitive treatment planning, such as surgery or radiation, is only determined after the biopsy results are available and the tumor’s grade and stage are fully understood.