SURLAS: A new clinical grade ultrasound system for sequential or concomitant thermoradiotherapy of superficial tumors: Applicator description

Volumetric hyperthermia delivery using the ExAblate Body MR-guided focused ultrasound system

OBJECTIVES

To investigate image-guided volumetric hyperthermia strategies using the ExAblate Body MR-guided focused ultrasound ablation system, involving mechanical transducer movement and sector-vortex beamforming.

MATERIALS AND METHODS

Acoustic and thermal simulations were performed to investigate volumetric hyperthermia using mechanical transducer movement combined with sector-vortex beamforming, specifically for the ExAblate Body transducer. The system control in the ExAblate Body system was modified to achieve fast transducer movement and MR thermometry-based hyperthermia control, mechanical transducer movements and electronic sector-vortex beamforming were combined to optimize hyperthermia delivery. The experimental validation was performed using a tissue-mimicking phantom.

RESULTS

The developed simulation framework allowed for a parametric study with varying numbers of heating spots, sonication durations, and transducer movement times to evaluate the hyperthermia characteristics for mechanical transducer movement and sector-vortex beamforming. Hyperthermic patterns involving 2-4 sequential focal spots were analyzed. To demonstrate the feasibility of volumetric hyperthermia in the system, a tissue-mimicking phantom was sonicated with two distinct spots through mechanical transducer movement and sector-vortex beamforming. During hyperthermia, the average values of Tmax, T10, Tavg, T90, and Tmin over 200 s were measured within a circular ROI with a diameter of 10 pixels. These values were found to be 8.6, 7.9, 6.6, 5.2, and 4.5 °C, respectively, compared to the baseline temperature.

CONCLUSIONS

This study demonstrated the volumetric hyperthermia capabilities of the ExAblate Body system. The simulation framework developed in this study allowed for the evaluation of hyperthermia characteristics that could be implemented with the ExAblate MRgFUS system.

Heating technology for malignant tumors: a review

The therapeutic application of heat is very effective in cancer treatment. Both hyperthermia, i.e., heating to 39-45 °C to induce sensitization to radiotherapy and chemotherapy, and thermal ablation, where temperatures beyond 50 °C destroy tumor cells directly are frequently applied in the clinic. Achievement of an effective treatment requires high quality heating equipment, precise thermal dosimetry, and adequate quality assurance. Several types of devices, antennas and heating or power delivery systems have been proposed and developed in recent decades. These vary considerably in technique, heating depth, ability to focus, and in the size of the heating focus. Clinically used heating techniques involve electromagnetic and ultrasonic heating, hyperthermic perfusion and conductive heating. Depending on clinical objectives and available technology, thermal therapies can be subdivided into three broad categories: local, locoregional, or whole body heating. Clinically used local heating techniques include interstitial hyperthermia and ablation, high intensity focused ultrasound (HIFU), scanned focused ultrasound (SFUS), electroporation, nanoparticle heating, intraluminal heating and superficial heating. Locoregional heating techniques include phased array systems, capacitive systems and isolated perfusion. Whole body techniques focus on prevention of heat loss supplemented with energy deposition in the body, e.g., by infrared radiation. This review presents an overview of clinical hyperthermia and ablation devices used for local, locoregional, and whole body therapy. Proven and experimental clinical applications of thermal ablation and hyperthermia are listed. Methods for temperature measurement and the role of treatment planning to control treatments are discussed briefly, as well as future perspectives for heating technology for the treatment of tumors.

Ultrasound Hyperthermia Technology for Radiosensitization

Hyperthermia therapy (HT) raises tissue temperature to 40-45°C for up to 60 min. Hyperthermia is one of the most potent sensitizers of radiation therapy (RT). Ultrasound-mediated HT for radiosensitization has been used clinically since the 1960s. Recently, magnetic resonance-guided high-intensity focused ultrasound (MRgHIFU), which has been approved by the United States Food and Drug Administration for thermal ablation therapy, has been adapted for HT. With emerging clinical trials using MRgHIFU HT for radiosensitization, there is a pressing need to review the ultrasound HT technology. The objective of this review is to overview existing HT technology, summarize available ultrasound HT devices, evaluate clinical studies combining ultrasound HT with RT and discuss challenges and future directions.

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.

Practical considerations for maximizing heat production in a novel thermobrachytherapy seed prototype

PURPOSE

A combination of hyperthermia and radiation in the treatment of cancer has been proven to provide better tumor control than radiation administered as a monomodality, without an increase in complications or serious toxicities. Moreover, concurrent administration of hyperthermia and radiation displays synergistic enhancement, resulting in greater tumor cell killing than hyperthermia and radiation delivered separately. The authors have designed a new thermobrachytherapy (TB) seed, which serves as a source of both radiation and heat for concurrent brachytherapy and hyperthermia treatments when implanted in solid tumors. This innovative seed, similar in size and geometry to conventional seeds, will have self-regulating thermal properties.

METHODS

The new seed's geometry is based on the standard BEST Model 2301(125)I seed, resulting in very similar dosimetric properties. The TB seed generates heat when placed in an oscillating magnetic field via induction heating of a ferromagnetic Ni-Cu alloy core that replaces the tungsten radiographic marker of the standard Model 2301. The alloy composition is selected to undergo a Curie transition near 50 °C, drastically decreasing power production at higher temperatures and providing for temperature self-regulation. Here, the authors present experimental studies of the magnetic properties of Ni-Cu alloy material, the visibility of TB seeds in radiographic imaging, and the ability of seed prototypes to uniformly heat tissue to a desirable temperature. Moreover, analyses are presented of magnetic shielding and thermal expansion of the TB seed, as well as matching of radiation dose to temperature distributions for a short interseed distance in a given treatment volume.

RESULTS

Annealing the Ni-Cu alloy has a significant effect on its magnetization properties, increasing the sharpness of the Curie transition. The TB seed preserves the radiographic properties of the BEST 2301 seed in both plain x rays and CT images, and a preliminary experiment demonstrates thermal self-regulation and adequate heating of a tissue-mimicking phantom by seed prototypes. The effect of self-shielding of the seed against the external magnetic field is small, and only minor thermal stress is induced in heating of the seeds from room temperature to well above the seed operating temperature. With proper selection of magnetic field parameters, the thermal dose distribution of an arrangement of TB and hyperthermia-only seeds may be made to match with its radiation dose distribution.

CONCLUSIONS

The presented analyses address several practical considerations for manufacturing of the proposed TB seeds and identify critical issues for the prototype implementation. The authors' preliminary experiments demonstrate close agreement with the modeling results, confirming the feasibility of combining sources of heat and radiation into a single thermobrachytherapy seed.

Preclinical assessment of comfort and secure fit of thermobrachytherapy surface applicator (TBSA) on volunteer subjects

A thermobrachytherapy surface applicator (TBSA) was developed for simultaneous heat and brachytherapy treatment of chest wall (CW) recurrence of breast cancer. The ability to comfortably secure the applicator over the upper torso relative to the CW target throughout treatment is assessed on volunteers. Male and postmastectomy female volunteers were enrolled to evaluate applicator secure fit to CW. Female subjects with intact breast were also enrolled to assess the ability to treat challenging cases. Magnetic resonance (MR) images of volunteers wearing a TBSA over the upper torso were acquired once every 15 minutes for 90 minutes. Applicator displacement over this time period required for treatment preplanning and delivery was assessed using MR visible markers. Applicator comfort and tolerability were assessed using a questionnaire. Probability estimates of applicator displacements were used to investigate dosimetric impact for the worst‐case variation in radiation source‐to‐skin distance for 5 and 10 mm deep targets spread 17×13 cm on a torso phantom. Average and median displacements along lateral and radial directions were less than 1.2 mm over 90 minutes for all volunteers. Maximum lateral and radial displacements were measured to be less than 1 and 1.5 mm, respectively, for all CW volunteers and less than 2 mm for intact breast volunteers, excluding outliers. No complaint of pain or discomfort was reported. Phantom treatment planning for the maximum displacement of 2 mm indicated <10% increase in skin dose with <5% loss of homogeneity index (HI) for ‐2 mm uniform HDR source displacement. For +2 mm uniform displacement, skin dose decreased and HI increased by 20%. The volunteer study demonstrated that such large and uniform displacements should be rare for CW subjects, and the measured variation is expected to be low for multifraction conformal brachytherapy treatment.

PACS numbers: 41.20.Jb, 41.75.‐i, 44.

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.

Improving conformal tumour heating by adaptively removing control points from waveform diversity beamforming calculations: a simulation study

Waveform diversity is a phased array beamforming strategy that determines an optimal sequence of excitation signals to maximise power at specified tumour control points while simultaneously minimising power delivered to sensitive normal tissues. Waveform diversity is combined with mode scanning, a deterministic excitation signal synthesis algorithm, and an adaptive control point removal algorithm in an effort to achieve higher, more uniform tumour temperatures. Simulations were evaluated for a 1444 element spherical section ultrasound phased array that delivers therapeutic heat to a 3 cm spherical tumour model located 12 cm from the array. By selectively deleting tumour control points, the tumour volume heated above 42°C increased from 2.28 cm3 to 11.22 cm3. At the expense of a slight increase in the normal tissue volume heated above the target temperature of 42°C, the size of the tumour volume heated above 42°C after tumour points were deleted was almost five times larger than the size of the original heated tumour volume. Several other configurations were also simulated, and the largest heated tumour volumes, subject to a 43°C peak temperature constraint, were achieved when the tumour control points were located along the back edge of the tumour and laterally around the tumour periphery. The simulated power depositions obtained from the results of the adaptive control point removal algorithm, when optimised for waveform diversity combined with mode scanning, consistently increased the penetration depth and the size of the heated tumour volume while increasing the heated normal tissue volume by a small amount.

Feasibility of noncontact piezoelectric detection of photoacoustic signals in tissue-mimicking phantoms

The feasibility of air-coupled ultrasound transducers to detect laser-induced ultrasound from artificial blood vessels embedded in an optically scattering phantom is demonstrated. These air-coupled transducers allow new applications in biomedical photoacoustic imaging where contact with tissue is not preferred. One promising application of such transducers is the addition of photoacoustic imaging to the regular x-ray mammographic screening procedure.

Thermal characteristics of thermobrachytherapy surface applicators for treating chest wall recurrence

The aim of this study was to investigate temperature and thermal dose distributions of thermobrachytherapy surface applicators (TBSAs) developed for concurrent or sequential high dose rate (HDR) brachytherapy and microwave hyperthermia treatment of chest wall recurrence and other superficial diseases. A steady-state thermodynamics model coupled with the fluid dynamics of a water bolus and electromagnetic radiation of the hyperthermia applicator is used to characterize the temperature distributions achievable with TBSAs in an elliptical phantom model of the human torso. Power deposited by 915 MHz conformal microwave array (CMA) applicators is used to assess the specific absorption rate (SAR) distributions of rectangular (500 cm(2)) and L-shaped (875 cm(2)) TBSAs. The SAR distribution in tissue and fluid flow distribution inside the dual-input dual-output (DIDO) water bolus are coupled to solve the steady-state temperature and thermal dose distributions of the rectangular TBSA (R-TBSA) for superficial tumor targets extending 10-15 mm beneath the skin surface. Thermal simulations are carried out for a range of bolus inlet temperature (T(b) = 38-43 degrees C), water flow rate (Q(b) = 2-4 L min(-1)) and tumor blood perfusion (omega(b) = 2-5 kg m(-3) s(-1)) to characterize their influence on thermal dosimetry. Steady-state SAR patterns of the R- and L-TBSA demonstrate the ability to produce conformal and localized power deposition inside the tumor target sparing surrounding normal tissues and nearby critical organs. Acceptably low variation in tissue surface cooling and surface temperature homogeneity was observed for the new DIDO bolus at a 2 L min(-1) water flow rate. Temperature depth profiles and thermal dose volume histograms indicate bolus inlet temperature (T(b)) to be the most influential factor on thermal dosimetry. A 42 degrees C water bolus was observed to be the optimal choice for superficial tumors extending 10-15 mm from the surface even under significant blood perfusion. Lower bolus temperature may be chosen to reduce the thermal enhancement ratio (TER) in the most sensitive skin where maximum radiation dose is delivered and to extend the thermal enhancement of radiation dose deeper. This computational study indicates that well-localized elevation of tumor target temperature to 40-44 degrees C can be accomplished by large surface-conforming TBSAs using appropriate selection of coupling bolus temperature.

Design of a water coupling bolus with improved flow distribution for multi-element superficial hyperthermia applicators

A water bolus used in superficial hyperthermia couples the electromagnetic (EM) or acoustic energy into the target tissue and cools the tissue surface to minimise thermal hotspots and patient discomfort during treatment. Parametric analyses of the fluid pressure inside the bolus computed using 3D fluid dynamics simulations are used in this study to determine a bolus design with improved flow and surface temperature distributions for large area superficial heat applicators. The simulation results are used in the design and fabrication of a 19 x 32 cm prototype bolus with dual input-dual output (DIDO) flow channels. Sequential thermal images of the bolus surface temperature recorded for a step change in the circulating water temperature are used to assess steady state flow and surface temperature distributions across the bolus. Modelling and measurement data indicate substantial improvement in bolus flow and surface temperature distributions when changing from the previous single input-single output (SISO) to DIDO configuration. Temperature variation across the bolus at steady state was measured to be less than 0.8 degrees C for the DIDO bolus compared to 1.5 degrees C for the SISO water bolus. The new DIDO bolus configuration maintains a nearly uniform flow distribution and low variation in surface temperature over a large area typically treated in superficial hyperthermia.

Multilayer conformal applicator for microwave heating and brachytherapy treatment of superficial tissue disease

PURPOSE

The purpose of this study was to construct and perform preliminary functionality evaluations of a multilayer conformal applicator with provisions for thermal monitoring, tight conformity and simultaneous microwave heating and brachytherapy treatment of large-area contoured surfaces.

MATERIALS AND METHODS

The multilayer conformal applicator consists of thermal monitoring catheters for fibre-optic monitoring of skin temperatures, a waterbolus, a PCB microwave antenna array, a dielectric spacer for brachytherapy considerations, brachytherapy catheters for delivering HDR radiation and an inflatable air bladder for improving conformity to contoured surfaces. The applicator also includes an elastic attachment structure to hold the applicator securely in place on the patient. The conformity of the applicator to irregular surfaces was evaluated through CT imaging of the applicator fitted onto a life-sized human torso phantom. The fluid flow dynamics of the waterbolus, which impact the effectiveness of temperature control, were evaluated with thermometry during a 19 degrees C step change temperature of the circulating water.

RESULTS

CT imaging showed improved conformity to the torso phantom surface following the application of gentle inward pressure from inflating the outer air bladder. Only a small number of 1-5 mm sized air gaps separated the conformal applicator and tissue surface. Thermometry testing of the bolus fluid flow dynamics demonstrated temperature uniformity within +/-0.82 degrees C across a 19 x 34 x 0.6 cm area bolus and +/-0.85 degrees C across a large 42 x 32 x 0.6 cm area bolus.

CONCLUSION

CT scans of the applicator confirmed that the applicator conforms well to complex body contours and should maintain good conformity and positional stability even when worn on a mobile patient. Thermometry testing of two different waterbolus geometries demonstrated that uniform circulation and temperature control can be maintained throughout large, complex bolus shapes.

Non-invasive estimation of hyperthermia temperatures with ultrasound

Ultrasound is an attractive modality for temperature monitoring because it is non-ionizing, convenient, inexpensive and has relatively simple signal processing requirements. This modality may be useful for temperature estimation if a temperature-dependent ultrasonic parameter can be identified, measured and calibrated. The most prominent methods for using ultrasound as a non-invasive thermometer exploit either (1) echo shifts due to changes in tissue thermal expansion and speed of sound (SOS), (2) variation in the attenuation coefficient or (3) change in backscattered energy from tissue inhomogeneities. The use of echo shifts has received the most attention in the last decade. By tracking scattering volumes and measuring the time shift of received echoes, investigators have been able to predict the temperature from a region of interest both theoretically and experimentally in phantoms, in isolated tissue regions in vitro and preliminary in vivo studies. A limitation of this method for general temperature monitoring is that prior knowledge of both SOS and thermal-expansion coefficients is necessary. Acoustic attenuation is dependent on temperature, but with significant changes occurring only at temperatures above 50 degrees C, which may lead to its use in thermal ablation therapies. Minimal change in attenuation, however, below this temperature range reduces its attractiveness for use in clinical hyperthermia. Models and measurements of the change in backscattered energy suggest that, over the clinical hyperthermia temperature range, changes in backscattered energy are dependent on the properties of individual scatterers or scattering regions. Calibration of the backscattered energy from different tissue regions is an important goal of this approach. All methods must be able to cope with motion of the image features on which temperature estimates are based. A crucial step in identifying a viable ultrasonic approach to temperature estimation is its performance during in vivo tests.

A thermal monitoring sheet with low influence from adjacent waterbolus for tissue surface thermometry during clinical hyperthermia

This paper presents a complete thermal analysis of a novel conformal surface thermometer design with directional sensitivity for real-time temperature monitoring during hyperthermia treatments of large superficial cancer. The thermal monitoring sheet (TMS) discussed in this paper consists of a 2-D array of fiberoptic sensors embedded between two layers of flexible, low-loss, and thermally conductive printed circuit board (PCB) film. Heat transfer across all interfaces from the tissue surface through multiple layers of insulating dielectrics surrounding the small buried temperature sensor and into an adjacent temperature-regulated water coupling bolus was studied using 3-D thermal simulation software. Theoretical analyses were carried out to identify the most effective differential TMS probe configuration possible with commercially available flexible PCB materials and to compare their thermal responses with omnidirectional probes commonly used in clinical hyperthermia. A TMS sensor design that employs 0.0508-mm Kapton MTB and 0.2032-mm Kapton HN flexible polyimide films is proposed for tissue surface thermometry with low influence from the adjacent waterbolus. Comparison of the thermal simulations with clinical probes indicates the new differential TMS probe design to outperform in terms of both transient response and steady-state accuracy in selectively reading the tissue surface temperature, while decreasing the overall thermal barrier of the probe between the coupling waterbolus and tissue surface.

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

PURPOSE

To evaluate the feasibility of concurrent treatment with the scanning ultrasound reflector linear array system (SURLAS) and helical tomotherapy (HT) intensity modulated radiation therapy (IMRT).

METHODS

The SURLAS was placed on a RANDO phantom simulating a patient with superficial or deep recurrent breast cancer. A megavoltage CT (MVCT) of the phantom with and without the SURLAS was obtained in the HT system. MVCT images with the SURLAS were obtained for two configurations: (1) with the SURLAS's long axis parallel and (2) perpendicular to the longitudinal axis of the phantom. The MVCT simulation data set was then transferred to a radiation therapy planning station. Organs at risk (OAR) were contoured including the lungs, heart, abdomen and spinal cord. The metallic parts of the SURLAS were contoured as well and constraints were assigned to completely or directionally block radiation through them. The MVCT simulation data set and regions of interest (ROI) files were subsequently transferred to the HT planning station. Several HT plans were obtained with optimization parameters that are usually used in the clinic. For comparison purposes, planning was also performed without the SURLAS on the phantom.

RESULTS

All plans with the SURLAS on the phantom showed adequate dose covering 95% of the planning target volume (PTV D95%), average dose and coefficient of variation of the planning target volume (PTV) dose distribution regardless of the SURLAS's orientation with respect to the RANDO phantom. Likewise, all OAR showed clinically acceptable dose values. Spatial dose distributions and dose-volume histogram (DVH) evaluation showed negligible plan degradation due to the presence of the SURLAS. Beam-on time varied depending on the selected optimization parameters.

CONCLUSION

From the perspective of the radiation dosage, concurrent treatment with the SURLAS and HT IMRT is feasible as demonstrated by the obtained clinically acceptable treatment plans. In addition, proper orientation of the SURLAS may be of benefit in reducing dose to organs at risk in some cases.

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.

Treatment delivery software for a new clinical grade ultrasound system for thermoradiotherapy

A detailed description of a clinical grade Scanning Ultrasound Reflector Linear Array System (SURLAS) applicator was given in a previous paper [Med. Phys. 32, 230-240 (2005)]. In this paper we concentrate on the design, development, and testing of the personal computer (PC) based treatment delivery software that runs the therapy system. The SURLAS requires the coordinated interaction between the therapy applicator and several peripheral devices for its proper and safe operation. One of the most important tasks was the coordination of the input power sequences for the elements of two parallel opposed ultrasound arrays (eight 1.5 cm x 2 cm elements/array, array 1 and 2 operate at 1.9 and 4.9 MHz, respectively) in coordination with the position of a dual-face scanning acoustic reflector. To achieve this, the treatment delivery software can divide the applicator's treatment window in up to 64 sectors (minimum size of 2 cm x 2 cm), and control the power to each sector independently by adjusting the power output levels from the channels of a 16-channel radio-frequency generator. The software coordinates the generator outputs with the position of the reflector as it scans back and forth between the arrays. Individual sector control and dual frequency operation allows the SURLAS to adjust power deposition in three dimensions to superficial targets coupled to its treatment window. The treatment delivery software also monitors and logs several parameters such as temperatures acquired using a 16-channel thermocouple thermometry unit. Safety (in particular to patients) was the paramount concern and design criterion. Failure mode and effects analysis (FMEA) was applied to the applicator as well as to the entire therapy system in order to identify safety issues and rank their relative importance. This analysis led to the implementation of several safety mechanisms and a software structure where each device communicates with the controlling PC independently of the others. In case of a malfunction in any part of the system or a violation of a user-defined safety criterion based on temperature readings, the software terminates treatment immediately and the user is notified. The software development process consisting of problem analysis, design, implementation, and testing is presented in this paper. Once the software was finished and integrated with the hardware, the therapy system was extensively tested. Results demonstrated that the software operates the SURLAS as intended with minimum risk to future patients.

Evolving technology for thermal therapy of cancer

This paper is intended as a succinct review of technology used for clinical hyperthermia therapy for cancer, as culled from a presentation at the special workshop on Thermal Medicine, Heat Shock Proteins, and Cancer at the Society for Thermal Medicine conference in Spring 2005. Following a brief overview of thermal therapy treatment options and available mechanisms for heating tissue, the paper focuses on the evolution of equipment from basic single element heating devices of the early 1980s to adjustable multi-element heating devices currently in use or in final stages of development. Representative devices from the past, present and near future are cited for further investigation by the interested reader. The paper concludes with a summary of general trends in the evolution of clinical hyperthermia techniques and a statement of current challenges remaining for the field.