Introduction of pre-biopsy mpMRI and PI-RADS scoring along with MRI-targeting have improved prostate biopsy. However, while FUS-TB generally achieved the highest detection rate in past studies, others reported that COG-TB showed no significant difference in cancer detection on a per patient basis12-14. Furthermore, multiple studies have pointed out a decrease in cost-effectiveness with MRI-TB and FUS-TB15-17. MRI-TB, while having the most direct targeting of the specified lesion, is associated with longer operative time, increased costs, and patient discomfort. Inability to perform concomitant systematic biopsy has also been noted as a demerit10. FUS-TB, while delivering promising results, currently does not have a billable medical care code in our country, and additional cost of equipment is not compensated for. As of 2018, the Ministry of Health, Labor, and Welfare has not deemed the evidence sufficient to provide for the method under public health insurance. Our institution is one of many that have been unable to incorporate FUS-TB for this reason. In addition, our MRI scanner is 1.5T, inferior in resolution to the newer-generation 3T models. Although PI-RADS scoring has allowed urologists to incorporate mpMRI into patient selection for prostate biopsy far more easily, it was unclear whether pre-biopsy mpMRI would be beneficial under these circumstances. We conducted this retrospective study to analyze whether conducting 1.5T mpMRI imaging before biopsy and adding cognitive-targeted biopsy to systematic biopsy in a minimally-invasive protocol improved diagnostic yield, and to determine what measures we could take to further improve within the constraints of cost.
Conducting mpMRI before biopsy significantly improved the percentage of positive biopsies for both PCa and csPCa, consistent with previous reports comparing SB and TB. Past studies have compared COG-TB and SB cores in a single cohort and reported that the combination of COG-TB and SB was superior to SB or COG-TB alone18. Several groups have also applied this to FUS-TB and found the combination of SB and FUS-TB to be superior, especially in the detection of csPCa 19-21. In all studies except the PI-RADS score3 subgroup in Hansen et al. 21, TB alone showed a tendency to outperform SB alone. Although comparison between cohorts may be difficult, cancer detection rate for TB alone was 52.9% for all PCa, similar to those previously reported for COG-TB (36-59%) 13,14,18,22 and FUS-TB (36-77%)13,18,19,22. It is of note that SB alone outperformed TB alone in our cohort. 46.2% of TB cores were positive for PCa, similar to past reports (32-47%) 12-13, and approximately 3 times more likely than the SB core to be PCa-positive, so this difference most likely comes from the number of TB cores per biopsy. For comparison, the average number of COG-TB cores per patient in our study was 1.7, as opposed to 4 for Peuch et al.12 and 7 for Borkowetz et al.18. We failed to detect a correlation between the number of TB cores per lesion and cancer detection, probably due to small sample size with 2 or more cores taken per lesion in only 10 cases (4.1%). Although a single study reports that a second TB core per lesion adds minimal value in MRI-TB23, a statement from the American Urological Association recommends a minimum of two biopsy cores per suspicious lesion24. In prioritizing minimal invasiveness we initially adopted a one TB core per lesion protocol, but it seems likely that more TB cores per lesion would lead to higher accuracy moving forward.
Logistic regression analysis identified PI-RADS score and PSAD as independent predictors of csPCa. A few reports have incorporated ROC curve analysis to support this conclusion25-27. In these studies ROC for PSAD was presented as a continuous variable, but in a real-life setting, decision making for recommendation of biopsy would be better served with a designated cutoff value. We used Youden index to determine cutoffs and constructed a model that would be easy to implement in everyday practice; i.e., recommending biopsy to patients with either PSAD0.25 or PI-RADS score4. Venderlink et al.25 proposed a similar model in which a PSAD cut-off of 0.15 was proposed for patients with PI-RADS score3. Both diagnostic models showed high sensitivity and accuracy in our cohort, although specificity and NPV was higher in our model. NPV and specificity were generally low across all parameters, probably due to the low percentage of PI-RADS score 1-2 (11 of 255 patients, 4.3%) which decreased the true negative population. The value of the tradeoff between reducing biopsies by 23.1% while missing 9.7% of csPCa is debatable, but we feel the easy-to-implement model would be a feasible option going forward. Interestingly, in the sole report which states that PSAD did not improve PCa detection28, the ROC curve for PI-RADS score had an extremely high AUC of 0.92. A learning curve for PI-RADS evaluation has been detected in multiple studies, the ratio of score3 lesions progressively decreasing along with radiologist experience. This may indicate that modeling with variables such as PSAD would have less value in the future with the refinement of MRI evaluation.
Lastly, we evaluated the performance of MRI prostate imaging. Thompson et al. reported that magnet strength (1.5T vs. 3T) was not an independent predictor of csPCa detection29. While imaging resolution is obviously higher with 3T MRI, there may not be as much of a difference in the quality of its evaluation. In our cohort, PI-RADS was correlated with a higher presence of PCa, csPCa, and a higher ISUP score. These findings were consistent with previous studies using either 1.5T 19 or 3T 20. With our results reflecting traits from existing literature, we concluded that PI-RADS v2 scoring of 1.5MRI at our institution was of a comparable quality to scoring with newer methods of imaging.
This study is limited by its retrospective nature, especially by patient selection bias which is apparent in the small number of PI-RADS score 1-2. This directly led to lower specificity and NPV for diagnostic models. However, since this is a result of less patients with low PI-RADS score electing for biopsy, we feel it reflects real-life clinical practice. Multiple urologists were involved with patient selection and biopsy procedure, possibly leading to differing standards in recommending pre-biopsy MRI or levels of biopsy technique. We also acknowledge that defining csPCa as ISUP score≥2 regardless of core cancer volume is debatable. Unfortunately, core cancer volume is not currently calculated for all cores at our institution, therefore we were unable to incorporate it into our classifications.