Potential for localized, adjustable deep heating in soft-tissue environments with a 30-beam ultrasonic hyperthermia system

Abstract

Initial heating rates (degrees C/min) along parallel tracks at depths of 1-14 cm in a static, muscle-like phantom were determined from time-temperature profiles obtained with 'Helios', a 30-beam ultrasonic hyperthermia system developed by Varian Associates. Data were taken at a single operating frequency of 556 kHz, for different sets of focal plane ring diameters of the four-ring array applicator, different levels of transducer driving power and two different focal plane depths, 6 cm and 9 cm. In each experiment, at each point of temperature measurement, analysis of temperature versus time data over a 2 min heating interval permitted separation of the desired phantom heating from artefactual heating resulting primarily from absorption of transverse (shear) waves produced at phantom-metal probe catheter interfaces. The results of the studies conducted suggest that in a non-translating carriage mode, Helios can produce axially and laterally localized deep heating in soft tissues for tissue volumes of lateral dimension up to a minimum of 4 cm and tissue depths of at least 11 cm. The results obtained also suggest that Helios can produce laterally localized heating to tissue depths of at least 11 cm without excessive heating of superficial soft-tissue layers, for tissue volumes of lateral dimension up to a minimum of 8 cm. The methodology used in the phantom studies was applied to the production of localized heating in the right lobe of the liver of adult pigs. Temperature versus time profiles obtained in the in vivo studies indicated that, for the set of system parameters employed, concentration of ultrasonic power at greater depths in the liver (e.g. 10.5 cm versus 5 cm) could be achieved, suggesting that Helios should be able to produce localized heating of targeted hepatic volumes when its operating parameters are selected in accordance with effective treatment planning techniques.

Re-setting the biologic rationale for thermal therapy

“…In the early and mid-1980s, a dozen or more companies were established to manufacture hyperthermia equipment for clinical use. Some larger companies, such as Varian, were briefly invested in the idea as well [45,46]. However, the perceived difficulties of achieving what was considered therapeutically effective temperatures, combined with technical difficulties of performing hyperthermia in general and lack of adequate reimbursement led to loss of commercial interest in the technology [7].…”

Status of hyperthermia in the treatment of advanced liver cancer

“…Ultrasound (US) waves heat of tissue by causing physical vibration and friction at a macroscopic level [96]. The larger wavelengths of ultrasound at the frequencies needed to penetrate tissues are conducive to improved tissue penetration compared to electromagnetic radiation [97], and also to improved beam focus [38]. As a result, US should theoretically be able to deposit high amounts of energy accurately in a well de®ned, predetermined volume without damage to overlying tissue.…”

Local ablation for unresectable liver tumors: is thermal best?

“…High‐intensity focussed ultrasound (HIFU) is a largely experimental technique. Ultrasound has a potential advantage when compared with other forms of thermal ablation such as RFA and MCT, as the wavelength of ultrasound allows far superior tissue penetration,90 while being delivered extracorporeally. The intensity of the ultrasound is up to 1000 times greater than that used for diagnostic ultrasound 91…”