T-staging of prostate cancer: Identification of useful signs to standardize detection of posterolateral extraprostatic extension on prostate MRI.

Objectives: To determine the prevalence and predictive value of a series of commonly used MRI criteria for posterolateral extraprostatic extension (EPE) of prostate cancer (PCa).

Methods: The presence of EPE in index lesions visible on prebiopsy mpMRI (T2w, DWI and DCE on a 3 Tesla-system) of biopsy-proven PCa patients was blindly assessed retrospectively by two radiologists with 8- and 17-years of experience on the basis of 8 commonly used staging criteria. Radical prostatectomy was used as standard of reference. The prevalences and positive predictive values (PPV) of all criteria were calculated for each reader separately and averaged for the two readers together. Cohen's K and percentage of agreement were used to assess the interobserver agreement.

Results: In 51 patients (mean age: 63 years; mean PSA: 17.2 ng/ml), tumor-capsule contact was the most prevalent sign (average 56,9%), but with the lowest PPV (average 51.9%), although increasing with broader capsular contact (56.5% if ≥10 mm; 87.5% if ≥20 mm; 100% if ≥25 mm). "Early signs" of EPE such as bulging, capsular disruption and unsharp prostatic margin showed a prevalence of 11.8%-18.6% on average, with 74.5%-86.3% of agreement; the average PPV range was 69.0%-75.0%. "Late signs" of EPE such as irregular prostatic contour, periprostatic fat infiltration, rectoprostatic angle obliteration and periprostatic mass showed a prevalence of 2.9%-8.8% on average, with 86.3%-94.1% of agreement; the average PPVs ranged between 85.7% and 100%.

Conclusions: "Early" signs of EPE show high prevalences but low PPVs, while "late" signs show lower prevalences but higher PPVs. MRI-staging following this chronological concept can standardize morphologic staging and decrease the existing multi-reader variability.