Background: Transurethral ultrasound thermal therapies with planar, curvilinear, and sectored tubular devices are being developed as a minimally invasive alternative to surgery for treating prostate cancer or BPH. These devices must precisely heat prostate tissue and avoid unwanted heating beyond the prostate boundary. Preservation of the rectum and bladder is critical, but the pubic bones may also limit successful treatment. Temperature elevations resulting from energy absorption by high-attenuation bone can cause patient pain or bone necrosis. Bone position varies widely among patients, but can be less than 1 cm from portions of the prostate. This study was designed to assess worst-case pubic bone heating under typical treatment regimens and to suggest guidelines for controlling unwanted temperature rises.

Methods: Thermal treatment of prostate with these transurethral devices was simulated using an acoustic and biothermal prostate model modified to include rectum and pubic bones. The transient model accommodates sweeping of the planar and curvilinear applicators with boundary temperature control, and accounts for dynamic changes in perfusion and attenuation with thermal dose. A range of prostate size (3-5 cm) and bone distance from prostate (1-3 cm) was considered. The effects of power, applicator type, frequency (5-10 MHz), transducer size, and sweeping rate on bone heating were assessed. Full and 65% transmission models for bone were considered to bracket anticipated heating.

Results: With bone 2 or 3 cm from the prostate, heating is only problematic with the planar device at high power. With bone 1 cm away, the planar and curvilinear applicators can produce bone temperatures from 60 - 70 °C, but heating can be controlled by lowering power, increasing treatment time, and increasing frequency. A 4 cm gland can be treated at 10 MHz with bone temperature below 46 °C. Due to radial decay of energy from the tubular transducer, bone heating is only problematic at powers above 10 W in the 4 cm prostate with bone 1 cm away.

Conclusions: Pubic bone heating is an important problem in ultrasound thermal therapy of the prostate. Successful realization of this therapy will require patient specific treatment planning that takes bone into consideration.