  Purpose: Transurethral ultrasound therapy is an investigational treatment
modality which could potentially be used for the localized treatment of
prostate cancer. One of the limiting factors of this therapy is prostatic
calcifications. These attenuate and reflect ultrasound and thus reduce the
efficacy of the heating. The aim of this study is to investigate how prostatic
calcifications affect therapeutic efficacy, and to identify the best sonication
strategy when calcifications are present. Methods: Realistic computational
models were used on clinical patient data in order to simulate different
therapeutic situations with naturally occurring calcifications as well as
artificial calcifications of different sizes (1-10 mm) and distances (5-15 mm).
Furthermore, different sonication strategies were tested in order to deliver
therapy to the untreated tissue regions behind the calcifications. Results: The
presence of calcifications in front of the ultrasound field was found to
increase the peak pressure by 100% on average while the maximum temperature
only rose by 9% during a 20-s sonication. Losses in ultrasound energy were due
to the relatively large acoustic impedance mismatch between the prostate tissue
and the calcifications (1.63 vs 3.20 MRayl) and high attenuation coefficient
(0.78 vs 2.64 dB/MHz^1.1/cm), which together left untreated tissue regions
behind the calcifications. In addition, elevated temperatures were seen in the
region between the transducer and the calcifications. Lower sonication
frequencies (1-4 MHz) were not able to penetrate through the calcifications
effectively, but longer sonication durations (20-60 s) with selective
transducer elements were effective in treating the tissue regions behind the
calcifications.
