Performance of Multiparametric MRI Appears Better when Measured in Patients who Undergo Radical Prostatectomy - Beyond the Abstract

The utility of mpMRI for prostate cancer diagnosis has been widely demonstrated. Still, considerable variation in accuracy has been reported in the literature. Such variation complicates patient counseling and clinical decision making about biopsy. Scoring systems such as PIRADS have been created to improve accuracy and reduce variability by standardizing mpMRI interpretation. However, prostate cancer detection rates can still vary widely even when stratified by suspicion score. For example, detection of clinically significant prostate cancer has been reported in between 21% to 98% for PIRADS 4 lesions.1-3 To optimize the usefulness of mpMRI, physicians should accurately quote patients a numerical risk of cancer based on their MRI. This requires physicians to recognize the factors beyond PIRADS score that can affect mpMRI accuracy. For example, several variables that affect MRI accuracy include variation in radiology interpretation,4 mpMRI protocols,5 PSA density,6 and even patient age.7

Another variable that is often mentioned but not previously quantified is the effect of patient cohort. Most studies evaluating mpMRI performance compare it to either radical prostatectomy histopathology or standard biopsy histopathology. Each cohort, however, carries its own biases. While whole mount histopathology from radical prostatectomy specimens allows pixel-by-pixel assessment for the presence of cancer, these cohorts have 100% prevalence of cancer and are biased towards higher-grade disease. On the other hand, while studies using TRUS biopsy histopathology include many men without cancer and thereby better represent the cancer prevalence in the target population, the measurement of mpMRI diagnostic accuracy based on biopsy as the gold standard is impaired by high rates of false-negative results in TRUS-biopsy.8

In our study, we addressed the impact of cohort selection bias on apparent MRI accuracy. We did this by retrospectively comparing the diagnostic accuracy of mpMRI in a biopsy cohort against a radical prostatectomy cohort, stratified by PIRADS score.

We found that for each PIRADS score, mpMRI detection of clinically significant cancer was higher in men who underwent radical prostatectomy than those who underwent biopsy only. While this finding is intuitive, it is important to recognize and quantify (Table 1). For example, cancer is present in 58% of PIRADS 4 lesions in the prostatectomy population, but only 31% of PIRADS 4 lesions in the biopsy population. This finding has important clinical implications. Clinicians should take great caution in counseling a man with an elevated PSA and abnormal MRI considering biopsy using data derived from a study using prostatectomy pathology as the gold standard. Instead, we advise using data from studies of more representative cohorts such as men without known cancer undergoing conventional biopsy, targeted biopsy, or perineal template mapping biopsy9.

Table 1: Demographic, clinical and biopsy results for men who underwent multiparametric MRI and targeted biopsy, separated into a biopsy-only cohort and radical prostatectomy cohort
BTA biopsy only cohort and radical prostatectomy cohort
Written by: Nancy N Wang, MD MPH1, Geoffrey A Sonn, MD1
1. Department of Urology, Stanford University School of Medicine, Stanford, California

References:
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9. Ahmed HU, Bosaily AES, Brown LC, Gabe R, Kaplan R, Parmar MK, Collaco-Moraes Y, Ward K, Hindley RG, Freeman A, Kirkham AP, Oldroyd R, Parker C, Emberton M, PROMIS study group. Diagnostic accuracy of multi-parametric MRI and TRUS biopsy in prostate cancer (PROMIS): a paired validating confirmatory study. Lancet. 2017;389(10071):815-22

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