Optimizing MRI-targeted fusion prostate biopsy: the effect of systematic error and anisotropy on tumor sampling

Abstract

Magnetic resonance imaging (MRI)-targeted, 3D transrectal ultrasound (TRUS)-guided "fusion" prostate biopsy aims to reduce the 21-47% false negative rate of clinical 2D TRUS-guided sextant biopsy. Although it has been reported to double the positive yield, MRI-targeted biopsy still has a substantial false negative rate. Therefore, we propose optimization of biopsy targeting to meet the clinician's desired tumor sampling probability, optimizing needle targets within each tumor and accounting for uncertainties due to guidance system errors, image registration errors, and irregular tumor shapes. As a step toward this optimization, we obtained multiparametric MRI (mpMRI) and 3D TRUS images from 49 patients. A radiologist and radiology resident contoured 81 suspicious regions, yielding 3D surfaces that were registered to 3D TRUS. We estimated the probability, , of obtaining a tumor sample with a single biopsy, and investigated the effects of systematic errors and anisotropy on . Our experiments indicated that a biopsy system's lateral and elevational errors have a much greater effect on sampling probabilities, relative to its axial error. We have also determined that for a system with RMS error of 3.5 mm, tumors of volume 1.9 cm 3 and smaller may require more than one biopsy core to ensure 95% probability of a sample with 50% core involvement, and tumors 1.0 cm 3 and smaller may require more than two cores.Keywords: Prostate cancer diagnosis, 3D guided biopsy planning, 3D ultrasound, 3D tumor shape, needle guidance error, anisotropic guidance error, systematic guidance error. DESCRIPTION OF PURPOSEThe current clinical standard for prostate cancer (PCa) diagnosis is 2D transrectal ultrasound (TRUS)-guided biopsy. As PCa is seldom detectable on ultrasound, a systematic template commonly containing 6 or 12 cores is used to guide the biopsy needle. However, this approach has been shown to have up to a 21-47% false negative rate and therefore many patients return to the clinic for repeat biopsies 1,2 . Additionally, this biopsy method may underestimate the true Gleason score of a patient's cancer 3 .It has been shown that mpMRI is effective for PCa detection and localisation 4 . To avoid the cost of in-bore MRIguided biopsy and enable widespread implementation in an office setting, 3D TRUS-guided biopsy systems were developed to improve upon the spatial information of 2D TRUS and allow for MRI targeting via image registration. A 3D TRUS biopsy system with MRI-3D TRUS fusion ("fusion biopsy") has been shown to produce significantly higher (p < 0.01) positive biopsy core rates, mean Gleason scores and tumor volumes sampled, compared to 2D-TRUS guided 12-core systematic biopsy 5 . Although fusion biopsy increased positive core rates, there is clinically significant room for further improvement. This approach may be further improved by investigating the overall uncertainty in delivering a biopsy needle to a point target within a 3D contour.There have been two recent studies that provide insight into potential avenues for impro...

A comparison of prostate tumor targeting strategies using magnetic resonance imaging‐targeted, transrectal ultrasound‐guided fusion biopsy

“…Dr. Cesare Romagnoli (deceased) made significant contributions to this manuscript, including performing tumor contouring on MRI described in Section 2.B and highly valuable input into matters of clinical importance for this research. This manuscript is an extension of preliminary work by the authors submitted to the SPIE Medical Imaging Conference, wherein some Introduction and Materials text is drawn from that work …”