Effect of applicator diameter on lesion size from high temperature interstitial ultrasound thermal therapy

High temperature ultrasound thermal therapy using interstitial and external approaches is becoming increasingly acceptable as a minimally invasive clinical treatment for cancerous and benign disease. The diameter of an interstitial applicator can influence its clinical practicality and effectiveness as well as application site. The purpose of this study was to determine whether the use of larger ultrasound transducers and the inherent increase in applicator size could be justified by potentially producing larger lesion diameters. Four applicator configurations and sizes were studied using ex vivo tissue experiments in liver and beef and using acoustic and biothermal simulations. Catheter-cooled and internally cooled applicators with outer diameters between 2.2 and 4.0 mm produced 3.5 to 5.0 cm diameter lesions in ex vivo liver and 3.0 to 3.5 cm lesions in ex vivo beef muscle with 20-40 W/cm applied for 10 min. Larger applicators produced lesions with radial penetration depths superior to their smaller counterparts at power levels in the 20-40 W/cm range. The higher cooling rates along the outer surface of the larger diameter applicators due to their greater surface area was a dominant factor in increasing lesion size. The higher cooling rates pushed the maximum temperature farther from the applicator surface and reduced the formation of high acoustic attenuation tissue zones. Applicator configuration and frequency (6.7-8.2 MHz) had less influence on lesion size than diameter in the ranges studied. Acoustic and biothermal simulations matched the experimental data well and were applied to model these applicators within sites of clinical interest such as prostate, uterine fibroid, brain, and normal liver. Lesions of 3.9 to 4.7 cm diameter were predicted for moderately perfused tissues such as prostate and fibroid and 2.8 to 3.2 cm for highly perfused tissues such as normal liver. In sites such as uterine fibroid where larger applicators placed using an endoscopic approach could be tolerated, treatment volume increases of 37% were predicted for an applicator diameter increase from 2.4 to 4.0 mm.