Robotic assistance for ultrasound-guided prostate brachytherapy

Abstract

We present a robotically assisted prostate brachytherapy system and test results in training phantoms and Phase-I clinical trials. The system consists of a transrectal ultrasound (TRUS) and a spatially co-registered robot, fully integrated with an FDA-approved commercial treatment planning system. The salient feature of the system is a small parallel robot affixed to the mounting posts of the template. The robot replaces the template interchangeably, using the same coordinate system. Established clinical hardware, workflow and calibration remain intact. In all phantom experiments, we recorded the first insertion attempt without adjustment. All clinically relevant locations in the prostate were reached. Non-parallel needle trajectories were achieved. The pre-insertion transverse and rotational errors (measured with a Polaris optical tracker relative to the template's coordinate frame) were 0.25 mm (STD=0.17 mm) and 0.75 degrees (STD=0.37 degrees). In phantoms, needle tip placement errors measured in TRUS were 1.04 mm (STD=0.50mm). A Phase-I clinical feasibility and safety trial has been successfully completed with the system. We encountered needle tip positioning errors of a magnitude greater than 4mm in only 2 of 179 robotically guided needles, in contrast to manual template guidance where errors of this magnitude are much more common. Further clinical trials are necessary to determine whether the apparent benefits of the robotic assistant will lead to improvements in clinical efficacy and outcomes.

Fig. 1

System setup with an anthropomorphic phantom in the operating room (left) and a closer view of the robot from the hysician’s perspective (right). Note how the standard clinical hardware and setup are preserved.

Fig. 2

CAD model of the parallel robot mounted over the TRUS probe on the mounting posts of the template.

Fig. 3

The robot control architecture

Fig. 4

Calibration with the Interplant kit. Note that the robot replaces the template in an otherwise standard calibration process.

Fig. 5

Insertion of angulated needles. The needle is slanted upward to reach behind the pubic arch (left). Laterally slanted therapy needle in the presence of two stabilizer needles (right).

Fig. 6

Comparison of needle guidance between template and robot with Polaris tracker (left). Error bars for the translation (middle) and rotation (right) differences.

Fig. 7

Accuracy of robotically guided needle insertion relative to target marked in TRUS

Fig. 8

Dynamic dosimetry screen from Interplant. The needle and seeds are captured in TRUS images as they are being inserted, while the resulting dose display is updated.

Fig. 9

Left: clinical case during setup. The TRUS probe and robot are in place before the robot fingers and needle guide sleeve are attached. Right: seed implantation with a Mick applicator.

Fig. 10

Direction and magnitude of corrective adjustments (≥ 3 mm) made to the needle position for all 5 patients. Note the pattern of needles requiring correction toward the center of the prostate, consistent with a tendency for tissue deflection toward the edges of the prostate.