Feasibility of concurrent treatment with the scanning ultrasound reflector linear array system (SURLAS) and the helical tomotherapy system

Quality assurance guidelines for superficial hyperthermia clinical trials : II. Technical requirements for heating devices

Quality assurance (QA) guidelines are essential to provide uniform execution of clinical trials with uniform quality hyperthermia treatments. This document outlines the requirements for appropriate QA of all current superficial heating equipment including electromagnetic (radiative and capacitive), ultrasound, and infrared heating techniques. Detailed instructions are provided how to characterize and document the performance of these hyperthermia applicators in order to apply reproducible hyperthermia treatments of uniform high quality. Earlier documents used specific absorption rate (SAR) to define and characterize applicator performance. In these QA guidelines, temperature rise is the leading parameter for characterization of applicator performance. The intention of this approach is that characterization can be achieved with affordable equipment and easy-to-implement procedures. These characteristics are essential to establish for each individual applicator the specific maximum size and depth of tumors that can be heated adequately. The guidelines in this document are supplemented with a second set of guidelines focusing on the clinical application. Both sets of guidelines were developed by the European Society for Hyperthermic Oncology (ESHO) Technical Committee with participation of senior Society of Thermal Medicine (STM) members and members of the Atzelsberg Circle.

Development of a novel method to enhance the therapeutic effect on tumours by simultaneous action of radiation and heating

PURPOSE

This paper describes the development of a new type of electromagnetic hyperthermia applicator delivering dose control within large application fields and increased effectiveness by providing simultaneous action of radiation and heating (SRH) in malignant tumours, and development of a dosimetric feedback method to support SRH.

MATERIALS AND METHODS

Single and phased arrays of flexible applicators have been developed to allow simultaneous hyperthermia and external beam therapy. A frequency of 434 MHz is used to heat near-surface and moderately deep-seated tumours and 70 MHz for deep-seated tumours. Phase and amplitude control allows focusing of electromagnetic energy (EM) to deep-seated tumours. The specific absorption rate (SAR) dose distribution can be modified to achieve uniform heating of tumours with complex shapes and heterogeneous tissue properties. A lithium fluoride thermoluminescent dosimeter (TLD) in a flexible film cassette has been developed for real-time dose measurement.

RESULTS

Four types of 434 MHz applicators were manufactured with 3, 4, 9 or 12 independent applicators. Two types of 70 MHz applicators were made with 4 or 6 independent applicators. Phantom tests demonstrated the ability to control the SAR pattern by phase and amplitude control. Placement of the dosimeter between bolus and phantom increased the phantom surface temperature up to 3 °C and showed that the ratio of absorbed energy in TLD to dose in water approaches (0.83 ± 3%) for photon energies >60 keV.

CONCLUSIONS

Simultaneous and controlled radiation and local hyperthermia is technically feasible in a preclinical setting, a clinical feasibility test is the next step.

SonoKnife for ablation of neck tissue: in vivo verification of a computer layered medium model

PURPOSE

This study aimed to determine which treatment parameters of the SonoKnife device can be used to safely and effectively perform non-invasive thermal ablation of subcutaneous tissue.

METHODS

A three-dimensional computational layered medium model was constructed to simulate thermal ablation treatment of the SonoKnife device. The acoustic and thermal fields were calculated with the Fast Object-Oriented C++ Ultrasound-Simulator software and a finite difference code, respectively. Subcutaneous tissue was represented as layers of skin, fat and muscle. The simulations were conducted for ultrasound frequencies of 1 or 3.5 MHz. The thermal dose model was used to predict the size and location of the ablated regions. The computer simulations were verified by using the SonoKnife to perform subcutaneous ablations in the neck area of healthy pigs, in vivo. Triphenyltetrazolium chloride viability stain was used to differentiate viable tissue from ablated regions ex vivo.

RESULTS

The simulations for the layered medium model suggest that operating the SonoKnife at frequency of 1 MHz is more effective and safer than 3.5 MHz providing skin cooling is applied prior to ablation. These predictions were in agreement with the results observed in the animal studies. The required sonication time for ablation increased from 50 to 300 s by using 1 MHz.

CONCLUSION

Our modelling and animal studies suggest that 1 MHz with pretreatment skin cooling are the optimal settings to operate the SonoKnife to safely and effectively perform subcutaneous thermal ablation of porcine skin. More work is needed to optimise skin cooling and define the optimal sonication time.

SonoKnife: feasibility of a line-focused ultrasound device for thermal ablation therapy

PURPOSE

To evaluate the feasibility of line-focused ultrasound for thermal ablation of superficially located tumors.

METHODS

A SonoKnife is a cylindrical-section ultrasound transducer designed to radiate from its concave surface. This geometry generates a line-focus or acoustic edge. The motivation for this approach was the noninvasive thermal ablation of advanced head and neck tumors and positive neck nodes in reasonable treatment times. Line-focusing may offer advantages over the common point-focusing of spherically curved radiators such as faster coverage of a target volume by scanning of the acoustic edge. In this paper, The authors report studies using numerical models and phantom and ex vivo experiments using a SonoKnife prototype.

RESULTS

Acoustic edges were generated by cylindrical-section single-element ultrasound transducers numerically, and by the prototype experimentally. Numerically, simulations were performed to characterize the acoustic edge for basic design parameters: transducer dimensions, line-focus depth, frequency, and coupling thickness. The dimensions of the acoustic edge as a function of these parameters were determined. In addition, a step-scanning simulation produced a large thermal lesion in a reasonable treatment time. Experimentally, pressure distributions measured in degassed water agreed well with acoustic simulations, and sonication experiments in gel phantoms and ex vivo porcine liver samples produced lesions similar to those predicted with acoustic and thermal models.

CONCLUSIONS

Results support the feasibility of noninvasive thermal ablation with a SonoKnife.

Conformal microwave array (CMA) applicators for hyperthermia of diffuse chest wall recurrence

PURPOSE

This article summarises the evolution of microwave array applicators for heating large area chest wall disease as an adjuvant to external beam radiation, systemic chemotherapy, and potentially simultaneous brachytherapy.

METHODS

Current devices used for thermotherapy of chest wall recurrence are reviewed. The largest conformal array applicator to date is evaluated in four studies: (1) ability to conform to the torso is demonstrated with a CT scan of a torso phantom and MR scan of the conformal water bolus component on a mastectomy patient; (2) specific absorption rate (SAR) and temperature distributions are calculated with electromagnetic and thermal simulation software for a mastectomy patient; (3) SAR patterns are measured with a scanning SAR probe in liquid muscle phantom for a buried coplanar waveguide CMA; and (4) heating patterns and patient tolerance of CMA applicators are characterised in a clinical pilot study with 13 patients.

RESULTS

CT and MR scans demonstrate excellent conformity of CMA applicators to contoured anatomy. Simulations demonstrate effective control of heating over contoured anatomy. Measurements confirm effective coverage of large treatment areas with no gaps. In 42 hyperthermia treatments, CMA applicators provided well-tolerated effective heating of up to 500 cm(2) regions, achieving target temperatures of T(min) = 41.4 ± 0.7°C, T(90) = 42.1 ± 0.6°C, T(ave) = 42.8 ± 0.6°C, and T(max) = 44.3 ± 0.8°C as measured in an average of 90 points per treatment.

CONCLUSION

The CMA applicator is an effective thermal therapy device for heating large-area superficial disease such as diffuse chest wall recurrence. It is able to cover over three times the treatment area of conventional hyperthermia devices while conforming to typical body contours.

Present and future technology for simultaneous superficial thermoradiotherapy of breast cancer

This paper reviews systems and techniques to deliver simultaneous thermoradiotherapy of breast cancer. It first covers the clinical implementation of simultaneous delivery of superficial (microwave or ultrasound) hyperthermia and external photon beam radiotherapy, first using a Cobalt-60 teletherapy unit and later medical linear accelerators. The parallel development and related studies of the Scanning Ultrasound Reflector Linear Arrays System (SURLAS), an advanced system specifically designed and developed for simultaneous thermoradiotherapy, follows. The performance characteristics of the SURLAS are reviewed and power limitation problems at high acoustic frequencies (>3 MHz) are discussed along with potential solutions. Next, the feasibility of simultaneous SURLAS hyperthermia and intensity modulated radiation therapy/image-guided radiotherapy (IMRT/IGRT) is established based on published and newly presented studies. Finally, based on the encouraging clinical results thus far, it is concluded that new trials employing the latest technologies are warranted along with further developments in treatment planning.

Influence of the SURLAS applicator on radiation dose distributions during simultaneous thermoradiotherapy with helical tomotherapy

Simultaneous thermoradiotherapy has been shown to maximize the effect of hyperthermia as a radiation sensitizer in cancer treatment. Here we follow our previous work on feasibility of thermoradiotherapy with the scanning ultrasound reflector linear array system (SURLAS) and TomoTherapy HiArt treatment system, and investigate the influence of the SURLAS hyperthermia applicator on delivered radiation dose with the TomoTherapy. A radiation treatment plan was calculated and the treatment was delivered to a phantom with SURLAS on top simulating the likely clinical setup. Proper positioning of the SURLAS was assisted with a magnetic position-and-orientation tracking device (POTD) and was verified with megavoltage-computed tomography. The delivered dose was measured with an ionization chamber (point measurement) and a radiographic film (2D dose distributions). The planned and delivered point dose data agreed within 0.61% +/- 0.63%. Planar dose data agreed within a dose difference of < or =3% of the maximum dose, and a distance-to-dose-agreement of < or =1 mm. The susceptibility of the delivered radiation dose on correct SURLAS positioning was studied as well. The largest dose discrepancy was measured for a position for which a maximum number of radiation beams intersected the incorrectly positioned SURLAS within one TomoTherapy gantry rotation. The point dose disagreed by 6.14% +/- 0.52%, and 2.25% of pixels of the 2D dose distribution did not pass the 3% dose difference/1 mm distance-to-dose-agreement criteria. Our study showed that correct positioning of the SURLAS applicator had an influence on the delivered radiation dose. Delivered and planned dose distributions were in an excellent agreement when SURLAS was positioned according to the treatment plan. Moving the applicator from its planned position was found to cause a modification of delivered dose distributions. A precise and reproducible positioning of the applicator was assured with a POTD.