BERKELEY, CA (UroToday.com) - Traditional gray-scale TRUS-guided systematic random biopsies have known shortcomings of under-sampling of clinically significant cancer and over-sampling of clinically insignificant cancer. With advanced technology of multi-parametric (mp)-MRI, in these 20 years, mp-MRI has become the most accurate imaging for prostate cancer detection and staging. Recent evidence suggests that MR/US image-fusion targeted biopsies would better diagnose clinically significant cancer compared to systematic random biopsies, while utilizing fewer cores.[1, 2] Yet, it must be remembered that all MR/US image-fusion biopsies are ultimately performed under real-time TRUS imaging.
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| Figure 1. Patient had previously 5 sets of negative biopsy sessions, and then, visited our institution to have pre-biopsy mp-MRI (ADC, T2w, Contrast-enhanced). The MR-suspicious lesion (circled) is clearly identified as a hypo-echoic lesion in right-anterior-horn peripheral zone in gray-scale TRUS. Although the gray-scale TRUS has other multiple hypo-echoic areas, the MR suggested the most suspicious one to be targeted. |
It therefore follows that whether or not the MR-suspected lesion is also visible on TRUS could impact the diagnostic yield of image-fusion guided biopsies. TRUS examination can often identify multiple lesions with hypo-echoic appearance (which likely result in significant false-positive findings, because they are not necessarily cancer but BPH, inflammation, etc.). However, image-fusion of mp-MR-visible suspicious lesion with the real-time TRUS image may indicate which specific hypo-echoic US-lesion best corresponds with the suspicious lesion on mp-MRI amongst the multiple hypo-echoic lesions (resulting in correction of the false-positive findings by TRUS alone).
Importantly, the operator with MR-data for biopsy should recognize that the pre-biopsy MR presents a ‘virtual’ target that neither shifts nor deforms during actual needle insertion; conversely, TRUS presents the ‘real’ target which does. Since real-time TRUS visualizes both the biopsy needle and the intra-prostatic target in real-time, TRUS-image is vital for precise MR/US image-fusion biopsy. Thus, when performing image-guided prostate biopsy, it is important to evaluate not only the ‘virtual’ fused MR-image, but also the ‘real’ TRUS-image; or, it is more important to look at the target in the ‘real’ TRUS-image.
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| Figure 2. MR/US image-fusion system (UroStation, Koelis) using 3D-TRUS organ tracking technology documented the 3D biopsy trajectory precisely going through the MR-lesion (located in the right-anterior-horn peripheral zone), which revealed Gleason 3+3=6, cancer-core-length of 11mm, 8mm in 2-cores of MR/US image-fusion biopsy. |
Our study showed that during MR/US image-fusion targeted biopsy, if a MR-suspicious lesion is also visible on TRUS, it increases biopsy accuracy leading to superior detection of clinically significant cancer. Our study also suggested that the higher the MRI-suspicion score, the more visible on TRUS; and biopsy becomes more accurate, increasing the detection rate of clinically significant cancer. Stated another way, TRUS-visibility of the MR-suspicious lesion allows the biopsy to be guided with greater accuracy, leading to superior detection of clinically significant cancer. Thus, expertise in performing TRUS, as well as in interpreting real-time TRUS and mp-MR is essential when using MR/US fusion technology.
Image-fusion between MR and TRUS involves multiple steps, including
- image acquisition of the 3D prostate volume data on both MRI and TRUS,
- segmentation of the prostate and target contours,
- image fusion,
- real-time TRUS-guidance of the biopsy, and
- recording of each biopsy trajectory.
An error in any one of these steps can lead to sampling error in the MR/US fusion biopsy. When the MR lesion is also visible on real-time TRUS, real-time TRUS can precisely guide the needle to the ‘real’ lesion. However, when the MR lesion is invisible on real-time TRUS, we must rely solely on the MR-derived virtual image of the target; as such, biopsy is dependent on the accuracy of image-fusion.
Every effort to enhance precision in all processes including mp-MR interpretation, TRUS interpretation, image-fusion, and targeted biopsy techniques is critical for developing a clinically meaningful MR/US fusion biopsy program.
References:
- Baco E, Rud E, Eri LM, et al. A Randomized Controlled Trial To Assess and Compare the Outcomes of Two-core Prostate Biopsy Guided by Fused Magnetic Resonance and Transrectal Ultrasound Images and Traditional 12-core Systematic Biopsy. Eur Urol. 2015 Apr 7. pii: S0302-2838(15)00272-9. doi: 10.1016/j.eururo.2015.03.041.
- Siddiqui MM, Rais-Bahrami S, Turkbey B, et al. Comparison of MR/Ultrasound Fusion–Guided Biopsy With Ultrasound-Guided Biopsy for the Diagnosis of Prostate Cancer. JAMA 2015;313:390.
Written by:
Osamu Ukimura, MD, PhD as part of Beyond the Abstract on UroToday.com. This initiative offers a method of publishing for the professional urology community. Authors are given an opportunity to expand on the circumstances, limitations etc... of their research by referencing the published abstract.
Professor of Clinical Urology
USC Institute of Urology, Keck School of Medicine
University of Southern California/Norris Cancer Center
Los Angeles, CA USA