Prostate magnetic resonance imaging (MRI) has become a valuable tool in the detection and characterization of prostate cancer. This imaging technique offers detailed views of the prostate gland, helping healthcare providers identify suspicious areas. To standardize the interpretation of these MRI findings, a system known as PI-RADS was developed. This system aids in assessing the likelihood of clinically significant prostate cancer and guides subsequent diagnostic steps.
Understanding the PI-RADS Scoring System
PI-RADS stands for Prostate Imaging-Reporting and Data System. It is a standardized scoring system used by radiologists to evaluate prostate MRI scans and communicate findings consistently.
The PI-RADS scale ranges from 1 to 5, with each number indicating a different level of suspicion. A PI-RADS 1 score signifies a very low likelihood of clinically significant cancer. PI-RADS 2 also indicates a low likelihood. Moving up the scale, PI-RADS 3 represents an intermediate or equivocal finding, where the presence of clinically significant cancer is neither likely nor unlikely.
A PI-RADS 4 score suggests a high likelihood of clinically significant prostate cancer. Finally, a PI-RADS 5 score indicates a very high likelihood that clinically significant prostate cancer is present.
What a PI-RADS 5 Score Indicates
A PI-RADS 5 score on a prostate MRI signifies a very high probability of clinically significant prostate cancer. While the likelihood is substantial, often cited as exceeding 80-90% for clinically significant cancer, it is not an absolute certainty of 100%.
Radiologists assign a PI-RADS 5 score based on specific characteristics observed on the multiparametric MRI (mpMRI). These characteristics typically include a lesion’s large size, markedly restricted water diffusion as seen on diffusion-weighted imaging (DWI), and early and strong contrast enhancement after injection of a contrast agent. The presence of features like capsular abutment or suspected invasion into surrounding tissues also contributes to a PI-RADS 5 assessment.
Next Steps After a PI-RADS 5 Finding
Following a PI-RADS 5 finding, the immediate and most common next step is a prostate biopsy to obtain tissue for definitive diagnosis. The biopsy aims to confirm the presence of cancer, determine its aggressiveness, and inform treatment decisions.
Several types of biopsies may be performed to target the suspicious lesion identified on the MRI. MRI-guided fusion biopsy is a commonly preferred method, where the MRI images are fused with real-time ultrasound images to guide the biopsy needle precisely to the suspicious area. Another approach is a cognitive fusion biopsy, where the urologist mentally correlates the MRI findings with the live ultrasound images to direct the biopsy. In some cases, an MRI-guided in-bore biopsy may be performed directly within the MRI scanner, offering direct visualization of the target. Discussing these targeted biopsy options with a urologist or oncologist is important to determine the most suitable approach for individual circumstances.
Interpreting Biopsy Results and Beyond
Once the prostate biopsy is performed, the tissue samples are sent to a pathologist for microscopic examination. Even with a PI-RADS 5 score, the biopsy results might occasionally show benign tissue or a lower-grade cancer than initially suspected. While less common, factors like inflammation, atrophy, or benign prostatic hyperplasia can sometimes mimic cancerous lesions on MRI, leading to a PI-RADS 5 score without actual clinically significant cancer. Studies indicate that a percentage of PI-RADS 5 lesions may result in a negative biopsy or show non-clinically significant cancer, ranging from approximately 13% to over 30% in some cohorts.
If cancer is confirmed, the pathologist will assign a Gleason score, a grading system used to assess the aggressiveness of prostate cancer cells. The Gleason score is determined by adding two numbers, each ranging from 1 to 5, representing the two most prevalent patterns of cancer cells observed in the biopsy samples. A lower Gleason score generally indicates a less aggressive cancer, while a higher score suggests a more aggressive form.
For instance, a Gleason score of 6 (3+3) is considered low-grade cancer, while scores of 7 (3+4 or 4+3) are intermediate-grade, and 8, 9, or 10 indicate high-grade cancer. Following the confirmation of cancer and determination of the Gleason score, further discussions with the healthcare team will involve staging the cancer and exploring appropriate treatment options.