A PI-RADS score is assigned by radiologists using a multiparametric Magnetic Resonance Imaging (mpMRI) scan to evaluate prostate gland findings. This system provides a score reflecting the likelihood that a lesion represents clinically significant prostate cancer. If results show an intermediate score of PI-RADS 3, it signals an area of uncertainty that requires further clarification to determine the next steps in management. This information explains the meaning of this score and the subsequent diagnostic path, but it is not a substitute for professional medical advice.
What is the PI-RADS Scoring System?
The Prostate Imaging-Reporting and Data System (PI-RADS) was developed to standardize the interpretation and reporting of prostate MRI results globally. This structured scheme assigns a numerical score from 1 to 5 to prostate lesions, representing the probability of finding clinically significant prostate cancer (CS-PCa). The current iteration, PI-RADS version 2.1, guides physicians in making risk-adapted decisions about whether a biopsy is necessary.
Clinically significant cancer is generally defined by specific pathological features, typically including a Gleason score of \(3+4=7\) or higher, or a smaller volume of high-grade cancer. The scoring scale is anchored by the extremes: a score of 1 indicates that CS-PCa is highly unlikely to be present, and a score of 5 means CS-PCa is highly likely to be present.
The scoring relies on combining images from different MRI sequences, including T2-weighted imaging, diffusion-weighted imaging (DWI), and dynamic contrast-enhanced (DCE) imaging. For instance, in the peripheral zone of the prostate, the DWI sequence is the dominant factor in determining the overall PI-RADS score. This standardized system ensures that imaging findings are communicated clearly and consistently between the imaging specialist and the treating physician.
The Ambiguity of a PI-RADS 3 Finding
A PI-RADS score of 3 is officially categorized as indeterminate or equivocal, placing the lesion in an uncertain risk group. This score means the imaging findings are neither clearly benign nor clearly malignant. The presence of clinically significant cancer is thus neither likely nor unlikely.
The probability of finding CS-PCa upon biopsy of a PI-RADS 3 lesion is variable across different studies and patient populations. Published literature suggests the detection rate for clinically significant disease in these intermediate lesions typically falls within a range of approximately 11% to 21%. This rate is high enough to warrant serious consideration for further diagnostic steps.
The score of 3 is assigned based on the lesion’s characteristics across the different imaging sequences. In the peripheral zone, for example, a score of 3 may be assigned if the DWI findings are mildly suspicious, but there is no clear enhancement on the DCE sequence. Because of this inherent ambiguity, the PI-RADS 3 score requires careful integration with other clinical factors before a definitive decision on biopsy can be made.
Diagnostic Follow-up After a PI-RADS 3 Score
The indeterminate nature of a PI-RADS 3 finding means that a definitive diagnosis relies on obtaining tissue samples. The primary follow-up is a targeted prostate biopsy. The current standard of care often involves an MRI-transrectal ultrasound (TRUS) fusion biopsy.
This procedure uses the MRI images to create a three-dimensional map of the prostate, allowing the urologist to precisely guide the biopsy needle to the specific PI-RADS 3 lesion. In addition to targeting the suspicious lesion, many physicians also perform a systematic, 12-core biopsy of the rest of the prostate gland at the same time. This combination helps ensure that any clinically significant cancer located outside the visible lesion is also detected.
The decision to proceed directly to biopsy for a PI-RADS 3 lesion is heavily influenced by supplementary clinical markers. One important factor is Prostate-Specific Antigen Density (PSAD), calculated by dividing the PSA blood level by the volume of the prostate gland. A PSAD value of \(0.15\) \(\text{ng/mL/mL}\) or greater is often predictive of clinically significant disease, even with a PI-RADS 3 score, and strongly favors proceeding with a biopsy. Other factors, such as the patient’s age, family history, and the rate of PSA level increase over time, are also considered to construct an individualized risk profile.
Prognosis and Treatment Pathways
The PI-RADS score itself does not have an associated survival rate, as it is an imaging risk assessment, not a cancer diagnosis. Prognosis is determined after a biopsy confirms the presence of cancer and provides a pathological grade. If a biopsy performed after a PI-RADS 3 finding reveals cancer, the survival rate is directly linked to the cancer’s Grade Group, a simplified scale based on the Gleason score.
For the vast majority of prostate cancers detected in the local or regional stages, the prognosis is highly favorable, with a 5-year relative survival rate approaching 100%. The cancers most commonly found after a PI-RADS 3 workup tend to be Grade Group 1 (Gleason \(3+3=6\)) or Grade Group 2 (Gleason \(3+4=7\)), which are considered low- or intermediate-risk, respectively. These categories have positive long-term outcomes.
The treatment pathway for these lower-risk cancers often involves Active Surveillance (AS). AS is a management strategy where treatment is intentionally deferred for cancers that are not immediately life-threatening, thereby avoiding the side effects of immediate surgery or radiation. Patients on AS undergo a monitoring protocol that includes regular PSA testing, periodic repeat MRIs, and scheduled confirmatory biopsies.
This approach is suitable for Grade Group 1 cancers, and sometimes for favorable Grade Group 2 cancers, provided other risk factors are low. If the biopsy reveals a higher-grade cancer, such as Grade Group 3 or above, or if the disease progresses while on active surveillance, the patient is typically transitioned to definitive treatment. Definitive options include radical prostatectomy, which is the surgical removal of the prostate, or various forms of radiation therapy.