Techniques and Clinical Experience of Interstitial Thermoradiotherapy. In: Sauer R, editor. Interventional Radiation Therapy. Berlin: Springer Berlin Heidelberg; 1991. pp. 343-57. [DOI: 10.1007/978-3-642-84163-7_44]

Ultrasound applicators for interstitial thermal coagulation

Direct-coupled (DC) and catheter-cooled (CC) ultrasound applicator configurations were evaluated for high-temperature ultrasound interstitial thermal therapy (USITT) using computer simulations, acoustic beam measurements, and in vivo temperature measurements. The DC devices consist of 2.2-mm diameter tubular ultrasound transducers encapsulated within a thin biocompatible plastic coating, which can be inserted directly into the tissue. The CC devices incorporate 1.5-mm diameter tubular transducers, which are inserted within 2.2to 2.4-mm diameter plastic implant catheters and require an integrated water-cooling scheme. Simulated transient temperature profiles and cumulative thermal dose distributions indicate that each of these applicator configurations can produce target temperatures greater than 50 degrees C and corresponding thermal doses greater than 300 to 600 equivalent minutes at 43 degrees C (EM(43 degrees C)) within 5 min at a radial depth of 1 to 1.5 cm in moderately perfused tissues. Theoretical investigations of air-cooling implemented within DC applicators demonstrated a significant enhancement of thermal penetration compared with non-cooled DC applicators, thus approaching performance attainable with CC devices. Temperature distributions achieved with DC and CC applicators in vivo were in agreement with theoretical calculations and further demonstrate that the devices are practical, sufficient power output levels can be obtained, and the angular heating profiles can be shaped or directed to protect non-targeted critical normal tissues. This preliminary study demonstrates that these interstitial ultrasound applicators have potential to provide controlled thermal coagulation and necrosis of small target regions and deserve further investigation and development for possible implementation in the treatment of benign and cancerous lesions in sites such as prostate, liver, and brain.

Direct-coupled interstitial ultrasound applicators for simultaneous thermobrachytherapy: a feasibility study

This study presents the design and performance evaluation of interstitial ultrasound applicators designed specifically for thermal therapy with simultaneous brachytherapy. The applicator consists of a multielement array of piezoceramic tubular radiators, each with separate power control, surrounded by thin layers of electrically-insulating and biocompatible coatings (< 2.6 mm OD). A catheter which is compatible with remote afterloaders and standard brachytherapy technology forms the inner lumen. These 'direct-coupled interstitial ultrasound applicators' (DCIUA's) are placed within the tumour or target region, with the coated transducer surface forming the outer wall of the implant catheter. Thermocouple sensors embedded in the coating over each transducer can be used for continuous monitoring of the tissue/applicator interface temperatures for feedback control of power to each transducer segment. Theoretical acoustic power deposition and corresponding temperature distributions from thermal simulations have demonstrated that the radius of effective heating is highly dependent upon the acoustic efficiency of the piezoceramic transducers, with effective heating extending > 1 - 1.5 cm radially for typical DCIUA applicators that are 60-65% efficient. This exceeds the effective heating radius of both thermal conduction and RF heating technologies. Measurements with prototype multielement ultrasound applicators have demonstrated acoustic efficiencies between 60 and 65% and beam distributions which are fairly uniform and collimated to the transducer axial length. Thermal dosimetry measurements within in vivo tissues have demonstrated controllable therapeutic temperature rises at 1 - 1.5 cm radial depth from the applicators, which were in agreement with the simulations. This study demonstrates that direct-coupled ultrasound applicators, designed without an active cooling mechanism in order to accommodate the insertion of radiation sources, are practicable for simultaneous thermobrachytherapy and promises to give more adjustable heating patterns than current alternative techniques.

Ultrasound applicators with integrated catheter-cooling for interstitial hyperthermia: theory and preliminary experiments

Theoretical and experimental methods were used to evaluate a design of ultrasound applicators for interstitial hyperthermia. The basic schema consists of a multielement array of tubular piezoceramic radiators (1.5-1.6 mm diameter), each 5-10 mm long with separate power control, designed to be inserted within a 13-14 gauge closed-end brachytherapy implant catheter. Channels for circulating temperature-regulated water are integrated within the applicator to provide control of the catheter/tissue interface temperature. A quantitative theoretical analysis was undertaken to determine heating performance as a function of applicator spacing, blood perfusion, catheter material, frequency and required acoustic power output. Prototype multielement applicators were constructed and characterized in terms of acoustic pressure-squared distributions and acoustic power output capabilities. This study demonstrated distinct advantages of these catheter-cooled multielement ultrasound applicators within an implant, including higher T90's achievable within highly perfused tissues, dynamic control of the longitudinal power deposition, and no interaction between adjacent applicators or elements or sensitivity to applicator alignment. Preliminary measurements of prototype devices have indicated that implementation of these interstitial ultrasound applicators with integrated catheter-cooling is practicable, yet further development and in vivo verification of performance is warranted.