Fusion guided prostate biopsies are the new darling of urologic diagnostic procedures. For decades, the transrectal ultrasound (TRUS) guided 12-plus needle sampling of the gland was the prostate cancer (PCa) diagnostic gold standard, despite its widely recognized shortcoming:
- Inaccuracy rates of at least 30% on average
- Overdetecting insignificant PCa while often missing significant PCa
Tends to miss PCa at the gland apex
- Can be uncomfortable to painful
- PCa is the only cancer subject to such random, blind needle sampling
- Side effect risks include bleeding, infection and sexual dysfunction, among others.
These downfalls are not surprising. TRUS lacks the visual definition to differentiate cancerous tissue from healthy tissue. Thus, needle samples are taken according to a systematic template, but are essentially blind to cancer and random in terms of hitting it.
The introduction of MRI-guided transrectal biopsies radically altered everything. Thanks to the ability of real-time multiparametric MRI (mpMRI) to immediately identify suspicious lesions, it enables precisely targeted biopsies directly into the core of the lesion most likely to harbor aggressive cells. This means fewer needles and higher accuracy, especially in the hands of an experienced radiology team.
As published data began to reveal the diagnostic advantages of real-time MRI-guided targeted biopsies, patient interest grew. The problem was, urologists themselves can’t perform MRI scans; only imaging radiologists have access to the magnets and are trained to use the equipment and interpret the results.
Fusion puts MRI into the hands of urologists… sort of
It wasn’t long before engineers and researchers began seeking ways to fuse high resolution MRI images with real-time ultrasound. From a practical standpoint, adding hardware with fusion software doesn’t take up much more space in a urology suite, which already has portable ultrasound equipment and monitors. All that’s needed is to send a patient for a prostate mpMRI, and then get those images downloaded into the fusion software. Now the urologist has the MRI images, and simply needs to fuse them with real-time TRUS while the patient is on the table. But is it the true fidelity of real-time MRI?
The fusing is done by a process called co-registration. The urologist manually creates a point-by-point prostate outline on both the MRI plus real-time ultrasound images, and a point-by-point suspicious area as seen on the MRI. The software then integrates the two based on the demarcated points. The result, as seen on the monitor, is a Pixar-like 3D model of the gland with the suspicious area highlighted within it. This provides the urologist with a target for placing needles, and of course the fusion imaging helps calculate the placement trajectory and confirms the urologist’s aim.
Sounds good, right? But here’s the problem: There is never 100% MRI fidelity. Dr. Laurence Klotz (Sunnybrook Health Sciences Centre, Toronto) points out the vulnerabilities of the fusion process:
At every step of a fairly complex algorithm, there can be errors in the quality of the MRI, the interpretation of the MRI, the registration of the suspicious area, and the fusion. Then, there is the targeting by the biopsy needle, and in every one of these steps there is room for error, and you need the whole thing to be done in a quality way to get an excellent outcome.[i]
In short, this is vastly different than a real-time, in-bore MRI guided biopsy. For example, patient positioning differs: he lies on his back in the bore (tunnel) of the magnet, but in the urology examining room, he’s on his side, knees drawn up. The fusion software must be very smart in order to translate the size and shape of the gland from one view to another, and integrate them.
In addition to the software is the crucial human factor. Dr. Klotz points to the learning curve that inexperienced urologists go through to become adept and accurate at using fusion. In fact, an Israeli study published in Dec. 2019 addresses what it takes to attain efficiency from the beginning of anesthesia to the procedure’s end, and accuracy (in this study, the rate of correctly identifying PI-RADS 3 lesions). The authors studied a total of 779 fusion biopsies (523 transrectal, 256 transperineal). For the transrectal approach alone, it took 109 fusion biopsy cases to decrease procedure time from 45 minutes to 15 minutes, and 104 procedures to increase accuracy from 35% to 50%. Thus, on average, it takes a urologist roughly 110 procedures to acquire proficiency.[ii]
Fusion does not truly put MRI into the hands of a urologist using fusion to guide transrectal biopsies, and as such they should not be called “MRI-guided biopsies.” This is very misleading. Well-designed studies show that fusion guidance is superior to TRUS alone, but there are equally well-designed studies showing that real-time in-bore MRI guidance is superior to both. Given that fusion guidance is not in-bore MRI guidance, accept no substitutes for the real thing.
NOTE: This content is solely for purposes of information and does not substitute for diagnostic or medical advice. Talk to your doctor if you are experiencing pelvic pain, or have any other health concerns or questions of a personal medical nature.
[i] Scheiszer, J. “Prostate MRI Improves Confirmatory Biopsy Sensitivity.” Cancer Therapy Advisor, Mar. 24, 2020. https://www.cancertherapyadvisor.com/home/cancer-topics/prostate-cancer/prostate-mri-improves-confirmatory-biopsy-sensitivity/
[ii] Halstuch D, BAniel J, Lifshitz D, Sela S et a. Characterizing the learning curve of MRI-US fusion prostate biopsies. Prostate Cancer Prostatic Dis. 2019 Dec;22(4):546-551.