Standard Biopsy Limitations

The prostate remains the only organ where a blind sampling technique is standard practice. Although convenient, transrectal ultrasound (TRUS) has poor cancer visualization. As a result, cancer may be missed entirely, or if detected, it is difficult to determine the extent and severity. This leads to under treatment of men with serious disease and overtreatment of men with indolent disease.1

The MRI-Targeted Advantage

Improve Diagnostic Pathway

TRUS vs MRI first workflow

Typically, an abnormal DRE and elevated PSA will warrant a referral by the GP. The screening methods, although useful, aren’t without their respective limitations. The digital rectal exam provides a quick assessment of the prostate, but it is limited to the posterior and lateral areas. It can detect palpable abnormalities, but these are typically clinically advanced. By definition, Stage 1 is non palpable. DRE has low sensitivity (51%) and specificity (59%).2 The PSA test has a high false positive rate as elevated levels aren’t always cancer specific. As a result, this ambiguity has blurred the lines between “normal” and “abnormal” levels. At a cutoff of 4.0ng/L, PSA had high sensitivity for any PCa, but low specificity. Any choice of a PSA cutoff involves a tradeoff between sensitivity and specificity.2 Despite these complications, most men are offered a standard TRUS biopsy based on an elevated PSA level or an abnormal DRE. 

Magnetic Resonance Imaging (MRI) offers an alternative diagnostic pathway in men with a clinical suspicion of cancer. MRI could be used as a triage test to avoid biopsy if the results are negative, whereas positive results could be used for targeting regions of interest during biopsy. Studies like the PRECISION trial1 and PROFUS study3 show that MRIs with or without targeted biopsies resulted in fewer unnecessary biopsies (up to 40%4), a higher detection rate of clinically significant cancers (92.3% vs. 57.7%3), and a decrease in the identification of clinically insignificant cancers (63.4% vs. 82.9%3), all requiring less biopsy cores than standard TRUS. Despite the fact that the majority of physicians involved in the trial had modest experience with MRI- targeted biopsies, relative to systematic, the results showed fusion biopsies outperformed standard TRUS.1

Improve Diagnostic Technology

Detection of clinically significant cancer depends on two diagnostic qualities:

  1. Accuracy- how close to the truth a diagnostic test is 
  2. Reliability- how reproducible a test is

Localization Accuracy

Detecting cancer requires high specificity; you need to know where the cancer is and more importantly, where it is not.2

Compared to ultrasound, MRI can be considered the “Gold Standard” for visualizing lesions.5 Acquiring a high quality MRI is the crucial first step in targeted procedures. Once regions of interest (ROIs) are identified, the MRI is fused with live ultrasound to create a 3D model of the prostate. Using the model as a map and the projected needle path as a compass, physicians can accurately target ROIs in real time.

Sampling Reliability

Since low-blind systematic biopsies sample less than 1% of the prostate, you cannot rely on taking more samples.6 

As the appearance of cancer is difficult to differentiate from normal tissue on ultrasound imaging, oversampling/ under sampling is a common occurrence.7 Oversampling can lead to detection of clinically insignificant cancers, repeat biopsy and overtreatment, while under sampling can miss clinically significant cancers. Interestingly, increasing the number of cores marginally improves diagnostic yield. Instead, it further escalates the risks of oversampling.8

Unlike systematic biopsies, fusion biopsies are targeted and repeatable. Because ROIs are targeted, procedures can be performed using fewer cores, preventing the adverse effects of oversampling/ under sampling. Fusion software also keeps a detailed record of core locations, beyond the site of entry,2 so ROIs can be accurately resampled. This reliability makes active surveillance a feasible option for low risk tumors.

  1. Kasivisvanathan, Veeru, et al. MRI-Targeted or Standard Biopsy for Prostate-Cancer Diagnosis.” New England Journal of Medicine. 2018; 378(19):1767–77. doi:10.1056/nejmoa1801993. ↩︎
  2.  Hoffman, R. M., MD, MPH. (2018, June 25). Screening for prostate cancer. Retrieved from https://www.uptodate.com/contents/screening-for-prostate-cancer#H17. ↩︎
  3. Bjurlin MA, Rosenkrantz AB, Taneja SS. MRI-fusion biopsy: the contemporary experience. Transl Androl Urol. 2017;6(3):483-489. doi:10.21037/tau.2017.04.30. ↩︎
  4. Klaassen, Z. (2018). EAU 2018: The role of multi-parametric MRI as triage test: A propensity-matched comparison of a MRI-triage and a TRUS-biopsy pathway. Retrieved from https://www.urotoday.com/conference-highlights/eau-2018/eau-2018-prostate-cancer/102894-eau-2018-the-role-of-multi-parametric-mri-as-triage-test-a-propensity-matched-comparison-of-a-mri-triage-and-a-trus-biopsy-pathway.html ↩︎
  5.  Wei, JT. Limitations of a contemporary prostate biopsy: the blind march forward. Urol Oncol. 2010;28(5):546-9. doi: 10.1016/j.urolonc.2009.12.022. ↩︎
  6. Weiss, B., & Loeb, S. (2015). MRI/Ultrasound Fusion Biopsy Versus Standard 12-Core Biopsy. Reviews in Urology, 17(2), 113–115. doi:10.3909/riu0670b. ↩︎
  7. Harvey CJ, Pilcher J, Richenberg J, Patel U, Frauscher F. Applications of transrectal ultrasound in prostate cancer. Br J Radiol. 2012;85 Spec No 1(Spec Iss 1):S3-17.doi: 10.1259/bjr/56357549. ↩︎
  8. Bjurlin MA, Taneja SS. Standards for prostate biopsy. Curr Opin Urol. 2014;24(2):155-61. doi: 10.1097/MOU.0000000000000031. ↩︎