The Prostate Imaging Reporting and Data System (PIRADS) is a standardized scoring system used to interpret the results of a multiparametric Magnetic Resonance Imaging (mpMRI) scan of the prostate gland. PIRADS creates a common language for radiologists and urologists, ensuring consistent assessment and reporting of suspicious findings. The system classifies lesions based on the likelihood of them representing clinically significant prostate cancer, which is a tumor likely to grow and require treatment. This standardized scale aims to improve diagnostic accuracy, reduce unnecessary prostate biopsies, and guide treatment decisions effectively.
The Imaging Components Used to Determine the Score
The PIRADS assessment uses a combination of three image sequences from the mpMRI scan: T2-weighted imaging (T2WI), Diffusion-Weighted Imaging (DWI), and Dynamic Contrast-Enhanced (DCE) imaging. The current version, PIRADS v2.1, utilizes an algorithmic approach where the contribution of each sequence is weighted based on the lesion’s location within the prostate.
T2WI provides high-resolution anatomical details, allowing radiologists to assess the size, shape, and internal structure of the prostate zones and any suspicious lesions. DWI measures the random movement of water molecules within the tissue. Cancer cells are denser and more tightly packed, restricting this movement. DWI is typically the dominant sequence for assessing lesions found in the peripheral zone, where most cancers originate.
DCE imaging involves injecting a gadolinium-based contrast agent to observe how blood flows into and out of the prostate tissue. Cancerous tumors often have an increased and rapid blood supply, causing them to “enhance” quickly, which indicates malignancy. While DCE plays a secondary role in most cases, it can be used to upgrade the risk score of an otherwise indeterminate lesion in the peripheral zone.
Interpreting Each PIRADS Category of Risk
The PIRADS scale uses five categories, ranging from 1 to 5, to communicate the probability of clinically significant prostate cancer being present at a specific location. Clinically significant cancer is generally defined as high-grade disease (Gleason score \(\geq\) 7) or disease with a significant volume (greater than 0.5 mL). A score of PIRADS 1 indicates a very low likelihood that clinically significant cancer is present.
A PIRADS 2 score suggests a low probability of clinically significant cancer, representing findings that are often benign or non-aggressive. The PIRADS 4 category signifies a high probability that clinically significant cancer is present, making the lesion highly suspicious.
PIRADS 5 represents a very high probability that clinically significant cancer is present, based on imaging features typical of aggressive tumors, such as being large or extending beyond the prostate capsule. Studies show that PIRADS 4 and 5 lesions have a high correlation with cancer confirmation upon biopsy, with detection rates often exceeding 35% and 75%, respectively.
The PIRADS 3 category is often described as indeterminate or equivocal, meaning the probability of clinically significant cancer is neither likely nor unlikely. This intermediate score is challenging because it represents a significant range of risk, with the probability of finding clinically significant cancer typically falling between 10% and 20%. A PIRADS 3 lesion has features that are not definitively benign but also lack the strong indicators of high-grade cancer.
Clinical Management and Next Steps
The assigned PIRADS score directly influences the recommendation for follow-up care. For lesions scored as PIRADS 1 or 2, the likelihood of an aggressive tumor is low, and a biopsy is not recommended. Patients with these low scores are often recommended for routine monitoring, which may include periodic PSA blood tests.
A score of PIRADS 3 places the patient in an equivocal risk group, and the decision for further action is individualized. The urologist will consider other clinical data, such as the patient’s prostate-specific antigen (PSA) level, the trend of the PSA over time, and the patient’s family history of cancer. A patient with a PIRADS 3 lesion and a low PSA may be recommended for a short-term follow-up MRI, while a patient with a rapidly rising PSA may be advised to proceed with a targeted biopsy.
Lesions categorized as PIRADS 4 or 5 require a prostate biopsy to confirm the diagnosis. Because the mpMRI provides precise location information, the biopsy can be targeted directly to the suspicious area, often using MRI-fusion biopsy. This approach helps ensure that the most concerning areas are sampled accurately. Ultimately, the PIRADS score serves as a guide, and the final clinical management plan must be determined through discussion between the patient and their urologist.