Geometric evaluation of systematic transrectal ultrasound guided prostate biopsy

Abstract

Purpose: Transrectal ultrasound guided prostate biopsy results rely on physician ability to target the gland according to the biopsy schema. However, to our knowledge it is unknown how accurately the freehand, transrectal ultrasound guided biopsy cores are placed in the prostate and how the geometric distribution of biopsy cores may affect the prostate cancer detection rate.

Materials and methods: To determine the geometric distribution of cores, we developed a biopsy simulation system with pelvic mock-ups and an optical tracking system. Mock-ups were biopsied in a freehand manner by 5 urologists and by our transrectal ultrasound robot, which can support and move the transrectal ultrasound probe. We compared 1) targeting errors, 2) the accuracy and precision of repeat biopsies, and 3) the estimated significant prostate cancer (0.5 cm(3) or greater) detection rate using a probability based model.

Results: Urologists biopsied cores in clustered patterns and under sampled a significant portion of the prostate. The robot closely followed the predefined biopsy schema. The mean targeting error of the urologists and the robot was 9.0 and 1.0 mm, respectively. Robotic assistance significantly decreased repeat biopsy errors with improved accuracy and precision. The mean significant prostate cancer detection rate of the urologists and the robot was 36% and 43%, respectively (p <0.0001).

Conclusions: Systematic biopsy with freehand transrectal ultrasound guidance does not closely follow the sextant schema and may result in suboptimal sampling and cancer detection. Repeat freehand biopsy of the same target is challenging. Robotic assistance with optimized biopsy schemas can potentially improve targeting, precision and accuracy. A clinical trial is needed to confirm the additional benefits of robotic assistance.

Figure 1

A, biopsy simulation system with prostate mock-up and optical tracking system. B, TRUS robot supporting TRUS probe.

Figure 2

Capsule-shaped sampling volume. Capsule radius (R) is same as that of minimal significant PCa tumor (0.5 cm³) and length (L) is same as that of needle core.

Figure 3

Targeting error. Two views of prostate with gold standard points (green circles) and actual biopsy cores (red circles) by urologist.

Figure 4

A, core distribution by 1 urologist shows gold standard (green circles) and actual cores (red circles) of all 6 mock-ups, and minimum enclosing spheres for each gold standard point (blue circles). B, same core distribution with robotic assistance.

Figure 5

Intersection of green capsule shapes with prostate represents sampled regions. Significant tumor detection rate is estimated by ratio of total volume to prostate volume. A, urologist. B, robotic assistance.