In the world of prostate cancer (PCa), there is growing consensus that insignificant PCa (ISUP grade group 1) is indolent and unlikely to progress and spread; as such, immediate aggressive treatment may safely be deferred. However, significant PCa (ISUP grade group > 2) poses higher risk and thus requires immediate intervention. Therefore, accurate detection and diagnosis of clinically significant PCa (csPCa) is essential to identify which patients require immediate treatment.
The integration of multiparametric MRI (mpMRI) into the diagnostic pathway has revolutionized the detection and diagnosis of csPCa. mpMRI together with standardized PI-RADS (Prostate Imaging Reporting and Data System) scores, offers high rates of identifying csPCa while avoiding detection of indolent PCa. For this reason, utilization of MRI prior to biopsy is increasing dramatically in order to rule out unnecessary biopsy. If biopsy is indicated based imaging, the scan results facilitate selective biopsy directly into suspicious lesions. This approach overcomes the shortcomings of conventional TRUS-guided systematic biopsies.
Although concerns have been raised about the healthcare expense of MRI vs. conventional transrectal ultrasound (TRUS), economic models suggest that pre-biopsy MRI and MRI targeted biopsies offer larger scale cost effectiveness compared with systematic TRUS biopsies.
Three targeting methods
mpMRI produces high resolution 3-dimensional (3-D) images of the prostate gland, revealing any lesion(s) suspicious for csPCa. Studies demonstrate that mpMRI-guided targeted biopsies increase the yield and accuracy of diagnosis over standard TRUS biopsies, by capturing a higher percentage of cancer per needle core as well as the most aggressive cells. Generally, this procedure requires 2-4 needles, reducing risks associated with conventional prostate biopsy.
There are three methods for targeting needles based on mpMRI images:
- Cognitive fusion – Based on MRI images, the physician mentally calculates where to place TRUS-guided biopsy needles; while ultrasound cannot depict the tissue differences, it confirms needle placement as the doctor mentally calculates a pathway into the target. Benefit: Can be done in the urology office setting; inexpensive (no additional in-office device is needed). Disadvantages: Requires a separate visit to a radiology center for the MRI; the prostate seen by the doctor on real-time ultrasound will never exactly match the MRI scan, since different angles and patient movement can confound accuracy.
- MRI/ultrasound fusion (MR/US fusion or co-registration) – Previously captured MRI images are “fused” with real-time ultrasound images of the gland; software generates a 3-D image of the patient’s gland, and depicts the reconstructed location, shape and size of the suspicious area. Planning needle placement is accomplished by the software, with physician ability to override it. Benefit: It can be done in the urology office setting. Disadvantages: Requires a separate visit to a radiology center for the MRI; requires investing in a fusion device; the prostate seen by the doctor on real-time ultrasound will never exactly match the MRI scan, since different angles and patient movement can confound accuracy, and thus may require additional systematic biopsy to compensate.
- In-bore MRI guidance – mpMRI identifies suspicious lesion(s) and guides needle pathways in real time for optimum accuracy. Benefits: Single session biopsy; low number of needles reduces risks associated with TRUS biopsy; immediate confirmation of needle placement; studies suggest highest diagnostic accuracy rate. Disadvantages: Only done in a radiology setting; some patients may be claustrophobic in the MRI bore.
Is one of these methods better? Acar et al (2015) designed a study comparing the diagnostic accuracy of standard TRUS systematic biopsy vs. cognitive fusion targeted biopsy, and real-time in-bore mpMRI targeted biopsy in patients with positive screening indications for cancer but no previous biopsy. They retrospectively analyzed the clinical charts of 140 patients, all of whom had mpMRI prior to any biopsy. The imaging sequences included T2-weighted MRI, diffusion weighted MRI, and dynamic contrast-enhanced imaging. According to the authors, “Cognitive fusion biopsies were performed after a review of mp-MRI data, whereas TRUS-guided biopsies were performed blinded to MRI information. In-bore biopsies were conducted by means of real-time targeting under MRI guidance.” All men were treated by radical prostatectomy, and the pathology reports on the gland specimens were available for comparison with biopsy results.
Diagnosis rates of prostate cancer (PCa) were as follows:
| Biopsy type | TRUS (37 men) | Cognitive fusion (49 men) | In-bore (14 men) |
| PCa diagnosis rate | 51.3% PCa | 55.1% PCa | 71.4% PCa |
| Biopsy detection of csPCa | 69.1% | 70.3% | 90% |
| Prostatectomy detection csPCa | 85.7% | 93.3% | 100% |
Analysis of the data did not identify a statistically significant difference among the three biopsy types. However, the authors clarify: “Although the detection rate of clinically significant disease was not significantly different between the groups, it is evident that there is a trend towards improved diagnostic efficiency with the adoption of image-guided sampling techniques. Perhaps, with a larger patient population or prospective randomization into all available biopsy techniques, including cognitive fusion, it would have been possible to achieve more solid outcomes in favor of MRI-guided prostate biopsies.”
A more recent comparison of in-bore vs. fusion biopsy (cognitive-guided biopsy was not included) reported that in-bore biopsy was associated with “significantly greater likelihood of detection of any cancer” vs. fusion targeting. However, when fusion-guided biopsy was supplemented with systematic biopsy, there was no significant difference in cancer detection. Given the minimal number of needles used for in-bore biopsy without additional systematic sampling (reducing biopsy risks while enhancing accurate diagnosis), the authors conclude, “… in-bore MRI-targeted prostate biopsy had a higher target-specific cancer detection rate than did fusion biopsy. … Pending a larger prospective randomized multicenter comparison between in-bore and fusion biopsy, in-bore may be the preferred approach should performing only biopsy of a suspicious target, without concurrent systematic biopsy, be considered clinically appropriate.”
Several factors must be taken into account to determine which targeting guidance method to use when mpMRI prostate images are available and reveal area(s) suspicious for csPCa. Primary, of course, are the patient’s clinical indications as well as availability of device(s). In many healthcare systems, economy may also be a determinant. In any case, research evidence suggests the desirability of in-bore MRI targeted biopsy due to its diagnostic edge over fusion and cognitive guidance.
