Simulation study on the heating of the surrounding anatomy during transurethral ultrasound prostate therapy: a 3D theoretical analysis of patient safety.

Purpose: MRI-guided transurethral ultrasound therapy can generate highly accurate volumes of thermal coagulation conforming to 3D human prostate geometries. This work simulated, quantified, and evaluated the thermal impact of these treatments on the rectum, pelvic bone, neurovascular bundles (NVBs), and urinary sphincters because damage to these structures can lead to complications.

Methods: Twenty 3D anatomical models of prostate cancer patients were used with detailed bioacoustic simulations incorporating an active feedback algorithm which controlled a rotating, planar ultrasound transducer (17, 4 x 3 mm2 elements, 10 W(acoustic)/cm2). Heating of the adjacent surrounding anatomy was evaluated at 4.7, 9.7, and 14.2 MHz using thermal tolerances reported in literature.

Results: Heating of the rectum posed the most important safety concern, influenced largely by the water temperature of an endorectal cooling device (ECD); depending on anatomy, temperatures of 7-37 degrees C were required to limit potential damage to less than 10 mm3 on the outer 1 mm layer of the rectal wall. Heating of the pelvic bone could be important at 4.7 MHz. A smaller sized ECD or a higher ultrasound frequency in sectors where the bone was less than 10 mm from the prostate reduced heating in all cases below the threshold for irreversible damage. Heating of the NVB was significant in 75% of the patient models in the absence of treatment planning; this proportion was reduced to 5% by increasing treatment margins up to 4 mm. To avoid damaging the urinary sphincters, the transducer should be positioned at least 2-4 mm from the sphincters, depending on the transurethral cooling temperature.

Conclusions: Simulations show that MRI-guided transurethral therapy can treat the prostate accurately, but in the absence of treatment planning, some thermal impact can be predicted on the surrounding anatomy. Treatment planning strategies have been developed, which reduce thermal injury to the surrounding anatomy.