Post-Treatment MRI Assistant

Name: Post-Treatment MRI Assistant - PI-RR and PI-FAB
Creator: Nick Shaheen, MD

A prostate MRI recurrence helper that routes cases to PI-RR after radiation/prostatectomy or PI-FAB after focal ablation.

Educational tool · PI-RR / PI-FAB

1. Choose prior treatment

The pathway changes the scoring logic.

Overall assessment

PI-RR

Select DWI and DCE scores

T2W is included for localization and descriptive support, but does not directly drive the final PI-RR score.

PI-RR quick glance

PI-RR 1Very low likelihood of local recurrence
PI-RR 2Low likelihood of local recurrence
PI-RR 3Equivocal
PI-RR 4High likelihood of local recurrence
PI-RR 5Very high likelihood of local recurrence

PI-RR is intended for local recurrence assessment after whole-gland treatment, such as radiation therapy or prostatectomy. Focal therapy assessment is better handled with PI-FAB.

Post-Treatment Prostate MRI: PI-RR and PI-FAB Scoring Explained

Standard PI-RADS v2.1 scoring is designed for the treatment-naive prostate. Once a patient has undergone radiation therapy, radical prostatectomy, or focal ablation, the post-treatment tissue changes (fibrosis, atrophy, edema, and altered vascularity) substantially alter the MRI appearance and make PI-RADS scoring unreliable. Two separate systems address this: PI-RR for whole-gland treatment and PI-FAB for focal ablation.

PI-RR: scoring local recurrence after radiation or prostatectomy

PI-RR (Prostate Imaging Reporting and Data System for Recurrence) provides standardized scores for local recurrence after external beam radiation therapy, brachytherapy, or radical prostatectomy. The system scores T2-weighted morphology, DWI/ADC restriction, and DCE enhancement on separate 1–5 scales, then synthesizes these into an overall PI-RR score. A score of 1–2 indicates low suspicion, score 3 is equivocal, and scores 4–5 indicate increasing suspicion for local recurrence. In the post-prostatectomy bed, the scoring focuses on the vesicourethral anastomosis and seminal vesicle remnants.

PI-FAB: scoring in-field recurrence after focal ablation

PI-FAB is a DCE-driven scoring system for suspected residual or recurrent tumor at the focal therapy treatment zone or treatment margin. DCE is the primary driver because post-ablation fibrosis predictably suppresses T2 and DWI signal, making enhancement the most reliable marker of active disease. PI-FAB scores run from 1 (no suspicious enhancement) to 3 (enhancing focus greater than 3 mm or with suspicious DWI/T2 correlate). Lesions clearly outside the ablation zone in untreated prostate should be assessed with PI-RADS v2.1, not PI-FAB.

Post-Treatment MRI: Frequently Asked Questions

Why can't standard PI-RADS be used after treatment?

PI-RADS v2.1 was calibrated for the treatment-naive prostate. Post-treatment changes, including radiation-induced fibrosis, post-prostatectomy anatomic distortion, and ablation zone edema, alter T2 signal, DWI/ADC values, and enhancement patterns in ways that make PI-RADS thresholds unreliable. Using PI-RADS after whole-gland radiation or prostatectomy leads to high false-positive rates from fibrosis and poor sensitivity for true recurrence. Purpose-built systems like PI-RR and PI-FAB account for these tissue changes.

What is the key difference between PI-RR and PI-FAB scoring?

PI-RR applies after whole-gland treatments (radiation therapy, brachytherapy, radical prostatectomy) and uses a multi-sequence approach combining T2, DWI, and DCE scores. PI-FAB applies after focal ablation (HIFU, cryotherapy, IRE) and is primarily DCE-driven, because ablation zone fibrosis reliably suppresses T2 and DWI signal, making enhancement the most specific marker. PI-FAB also has a narrower score range (1–3) compared to PI-RR (1–5).

What does PI-FAB 2 mean and when is biopsy recommended?

PI-FAB 2 indicates a tiny enhancing focus measuring 3 mm or less at the original tumor site, which is considered equivocal. Management is driven by PSA kinetics and comparison to prior MRI. If PSA is rising or the focus was not present on a prior post-treatment MRI, targeted biopsy may be warranted. PI-FAB 3 (enhancing focus greater than 3 mm, or interval growth, or suspicious DWI/T2 correlate) is suspicious for residual or recurrent disease and usually warrants targeted biopsy.

How should an out-of-field lesion be scored in a focal ablation follow-up?

If a suspicious finding is clearly outside the ablation zone in untreated prostate tissue, it should be assessed with PI-RADS v2.1, not PI-FAB. PI-FAB is specifically designed for the treated ablation zone and its margin. The ablation zone itself should still be described separately. Both findings should be clearly delineated in the report so the referring clinician understands which lesion is in-field recurrence versus a de novo or residual untreated lesion.

Quick reference

PI-FAB after focal ablation: what the score means

PI-FAB is meant for suspected residual or recurrent tumor at the focal therapy treatment zone or treatment margin. It is not meant to score a separate lesion in untreated prostate tissue. Those out-of-field lesions should be assessed with PI-RADS v2.1 while the ablation zone is described separately.

1PI-FAB is DCE driven.
Start with enhancement at or near the treatment zone, then use DWI/ADC and T2W as supporting features.

2The practical question is local recurrence.
The score is asking whether the ablation zone or margin looks benign, equivocal, or suspicious.

3Out-of-field findings are different.
Do not force an untreated gland lesion into PI-FAB. Use PI-RADS and describe the ablation zone separately.

PI-FAB decision logic

Score	Typical pattern	Practical interpretation
PI-FAB 1	No suspicious enhancement, or linear/non-masslike enhancement favoring fibrosis, vessel, or inflammation.	Low suspicion for in-field or marginal recurrence.
PI-FAB 2	Tiny enhancing focus measuring 3 mm or less at the original tumor site.	Equivocal. Correlate with PSA kinetics and prior MRI; biopsy may be considered if concern is rising.
PI-FAB 3	Enhancing focus greater than 3 mm within/at the treatment margin, interval growth of a prior PI-FAB 2 focus, or focal enhancement with suspicious DWI/T2 correlate.	Suspicious for residual or recurrent disease; targeted biopsy is usually the next step.

How this assistant uses PI-FAB

The focal ablation pathway first asks whether the finding is in-field or at the ablation margin. It then uses the DCE pattern to assign the primary PI-FAB score. DWI/ADC and T2W morphology are included because they can increase confidence, especially when a focal enhancing abnormality also restricts diffusion or has a masslike T2 correlate.

DCE
Primary driver after focal therapy. Look for focal enhancement at the treated site or margin.

DWI / ADC
Supportive. Matched high b-value DWI signal and low ADC at the enhancing focus increases concern.

T2W
Supportive. Helpful for morphology, ablation cavity/scar, masslike tissue, and comparison with the original tumor site.

PI-RR reminder

Radiation therapy and radical prostatectomy use PI-RR logic rather than PI-FAB. After radiation, DWI and DCE drive the score. After prostatectomy, DCE is the dominant sequence, with DWI used as an upgrade pathway when reliable. T2W remains important for localization and morphology, but it is usually descriptive rather than the main score driver. Read the Understanding PI-RR article.

All about PI-RR A short guide to local recurrence scoring after radiation therapy or radical prostatectomy.

About this article This educational article was written by Nicholas H. Shaheen, MD for the Prostate MRI Toolkit. It is intended for teaching and reference, is not peer reviewed, and should not replace local practice standards, institutional protocols, or formal society guidelines. Last updated: June 2026

Reviewed by Nick Shaheen, MD, board-certified radiologist