Section III: Assessment and Reporting
A major objective of a prostate MRI exam is to identify and localize abnormalities that correspond to clinically significant prostate cancer, and mpMRI may be able to detect tumor volumes <5mm, depending on the location and background tissue within the prostate gland. However, there is no universal agreement of the definition of clinically significant prostate cancer.
In PI-RADS v2, the definition of clinically significant cancer is intended to standardize reporting of mpMRI exams and correlation with pathology for clinical and research applications. Based on the current uses and capabilities of mpMRI and MRI-targeted procedures, for PI-RADS v2 clinically significant cancer is defined on pathology/histology as Gleason score > 7 (including 3+4 with prominent but not predominant Gleason 4 component), and/or volume > 0.5cc, and/or extraprostatic extension (EPE).
PI-RADS v2 assessment uses a 5-point scale based on the likelihood (probability) that a combination of mpMRI findings on T2W, DWI, and DCE correlates with the presence of a clinically significant cancer for each lesion in the prostate gland.
PI-RADS v2 Assessment Categories
PIRADS 1 – Very low (clinically significant cancer is highly unlikely to be present)
PIRADS 2 – Low (clinically significant cancer is unlikely to be present)
PIRADS 3 – Intermediate (the presence of clinically significant cancer is equivocal)
PIRADS 4 – High (clinically significant cancer is likely to be present)
PIRADS 5 – Very high (clinically significant cancer is highly likely to be present)
Assignment of a PI-RADS v2 Assessment Category should be based on mpMRI findings only and should not incorporate other factors such as serum prostate specific antigen (PSA), digital rectal exam, clinical history, or choice of treatment. Although biopsy should usually be considered for PIRADS 4 or 5, but not for PIRADS 1 or 2, PI-RADS v2 does not include recommendations for management, as these must take into account other factors besides the MRI findings, including laboratory/clinical history and local preferences, expertise and standards of care. Thus, for lesions with PIRADS Assessment Category 2 or 3, biopsy may or may not be appropriate, depending on factors other than mpMRI alone.
It is anticipated that, as evidence continues to accrue in the field of mpMRI and MRI-targeted biopsies and interventions, specific recommendations and/or algorithms regarding biopsy and management may be included in future versions of PI-RADS.
When T2W and DWI are of diagnostic quality, DCE plays a minor role. Absence of early enhancement within a lesion or diffuse enhancement not localized to a specific T2W or DWI abnormality usually adds little information and can be seen in the setting of prostatitis. Moreover, DCE does not contribute to the overall assessment when the finding has a low (PIRADS 1 or 2) or high (PIRADS 4 or 5) likelihood of clinically significant cancer. However, when DWI is PIRADS 3 in the PZ, a positive DCE may increase the likelihood that the finding corresponds to a clinically significant cancer and may upgrade the Assessment Category to PIRADS 4.
PI-RADS Assessment
Peripheral Zone (PZ)
DWI T2W DCE PIRADS
1 Any* Any 1
2 Any Any 2
3 Any -‐ 3
4 Any Any 4
5 Any Any 5
Transition Zone (TZ)
T2W DWI DCE PIRADS
1 Any Any 1
2 Any Any 2
3 ≤4 Any 3
3 5 Any 4
4 Any Any 4
5 Any Any 5
A. Reporting (see Appendix I: Report Templates)
Measurement of the Prostate Gland The volume of the prostate gland should always be reported. It may be determined using manual or automated segmentation or calculated using the formula for a conventional prolate ellipse (maximum AP diameter x maximum transverse diameter x maximum longitudinal diameter x x 0.52). Prostate volume may also be useful to calculate PSA density (PSA/prostate volume).
Mapping Lesions Prostate cancer is multifocal. The largest tumor focus usually yields the highest Gleason score and is most likely to contribute to extraprostatic extension (EPE) and positive surgical margins.
For PI-RADS v2, up to four findings with a PIRADS Assessment Category of 3, 4, or 5 may each be assigned on the Sector Map (Appendix II), and the index (dominant) intraprostatic lesion should be identified. The index lesion is the one with the highest PIRADS Assessment Category. If the highest PIRADS Assessment Category is assigned to two or more lesions, the index lesion should be the one that shows EPE. Thus, a smaller lesion with EPE should be defined as the index lesion despite the presence of a larger tumor with the identical PIRADS Assessment Category. If none of the lesions demonstrate EPE, the largest of the tumors with highest PIRADS Assessment Category should be considered the index lesion.
If there are more than four suspicious findings, then only the four with the highest likelihood of clinically significant cancer (i.e. highest PIRADS Assessment Category) should be reported. There may be instances when it is appropriate to report more than four suspicious lesions.
Reporting of additional findings with PIRADS Assessment Category 2 or definitely benign findings (eg. cyst) is optional, but may be helpful to use as landmarks to guide subsequent biopsy or for tracking lesions on subsequent mpMRI exams.
If a suspicious finding extends beyond the boundaries of one sector, all neighboring involved sectors should be indicated on the Sector Map (as a single lesion).
Measurement of Lesions With current techniques, mpMRI has been shown to underestimate both tumor volume and tumor extent compared to histology, especially for Gleason grade 3. Futhermore, the most appropriate imaging plane and pulse sequence for measuring lesion size on MRI has not been definitely determined, and the significance of differences in lesion size on the various MRI pulse sequences requires further investigation. In the face of these limitations, the PI-RADS Steering Committee nevertheless believes that standardization of measurements will facilitate MR-pathological correlation and research and recommends that the following rules be used for measurements:
The minimum requirement is to report the largest dimension of a suspicious finding on an axial image. If the largest dimension of a suspicious finding is on sagittal and/or coronal images, this measurement and imaging plane should also be reported. If lesion is not clearly delineated on an axial image, report the measurement on the image which best depicts the finding.
Alternatively, if preferred, lesion volume may be determined using appropriate software, or three dimensions of lesions may be measured so that lesion volume may be calculated.
In the PZ, lesions should be measured on ADC. In the TZ, lesions should be measured on T2W.
If lesion measurement is difficult or compromised on ADC (for PZ) or T2W (for TZ), measurement should be made on the sequence that shows the lesion best.
In the mpMRI report, indicate the image number(s)/series and sequence used for measurement.
Caveats for Overall Assessment
• In order to facilitate correlation and synchronized scrolling when viewing, it is strongly recommended that imaging plane angle, location, and slice thickness for all sequences (T2W, DWI, and DCE) are identical.
• Changes from prostatitis (including granulomatous prostatitis) can cause signal abnormalities in the PZ with all pulse sequences. Morphology and signal intensity may be helpful to stratify the likelihood of malignancy. In the PZ, mild signal changes on T2W and/or DWI that are not rounded but rather indistinct, linear, lobar, or diffuse are less likely to be malignant.
• For the PZ, DWI is the primary determining sequence (dominant technique). Thus, if the DWI score is 4 and T2W score is 2, PIRADS Assessment Category should be 4.
• For the TZ, T2W is the primary determining sequence.. Thus, if the T2W score is 4 and DWI score is 2, PIRADS Assessment Category should be 4.
• Since the dominant factors for PIRADS assessment are T2W for the TZ and DWI for the PZ, identification of the zonal location of a lesion is vital. Areas where this may be especially problematic include the interface of the CZ and PZ at the base of the gland and the interface of the anterior horn of the PZ with TZ and anterior fibromuscular stroma.
• Currently, the capability of reliably detecting and characterizing clinically significant prostate cancer with mpMRI in the TZ is less than that in the PZ
• Homogeneous or heterogeneous nodules in the TZ that are round/oval, well-circumscribed, and encapsulated are common findings as men aged 40 and above. Often, they demonstrate restricted diffusion and/or focal contrast enhancement, but they are considered to be benign BPH. These do not have to be assigned a PIRADS Assessment Category. Although such nodules may on occasion contain clinically significant prostate cancer, the probability is very low.
• Bilateral symmetric signal abnormalities on any sequence are often due to normal anatomy or benign changes.
• If a component of the mpMRI exam (T2W, DWI, DCE) is technically inadequate or was not performed, it should be assigned PIRADS Assessment Category “X” for that component. This occurs most commonly with DWI. Since DWI is often crucial for diagnosis of clinically significant cancers in the PZ, inadequate or absent DWI data should usually prompt repeat of this component of the mpMRI examination if the cause of failure can be remedied. If this is not possible, assessment may be accomplished with the other pulse sequences that were obtained using the tables below. However, this is a serious limitation, and it should be clearly acknowledged in the exam report, even if it applies to only one area of the prostate gland.
Assessment Without Adequate DWI
Peripheral Zone (PZ) and Transition Zone (TZ)
T2W DWI DCE PiRADS
1 x any 1
2 x any 2
3 x – 3
3 x + 4
4 x any 4
5 x any 5
Assessment Without Adequate DCI
Peripheral Zone (PZ): Determined by DWI
Assessment Category Transition Zone (TZ)
T2W DWI DCE PiRADS
1 any x 1
2 any x 2
3 <4 x 3
3 5 x 4
4 any x 4
5 any x 5
If both DWI and DCE are inadequate or absent, assessment should be limited to staging for determination of EPE.