RTOG quality assurance guidelines for clinical trials using hyperthermia administered by ultrasound

Hyperthermia, radiation and chemotherapy: the role of heat in multidisciplinary cancer care

The compelling biologic basis for combining hyperthermia with modern cancer therapies including radiation and chemotherapy was first appreciated nearly half a century ago. Hyperthermia complements radiation as conditions contributing to radio-resistance generally enhance sensitivity to heat and sensitizing effects occur through increased perfusion/tumor oxygenation and alteration of cellular death pathways. Chemosensitization with hyperthermia is dependent on the particular mechanism of effect for each agent with synergistic effects noted for several commonly used agents. Clinically, randomized trials have demonstrated benefit including survival with the addition of hyperthermia to radiation or chemotherapy in treatment of a wide range of malignancies. Improvements in treatment delivery techniques, streamlined logistics, and greater understanding of the relationship of thermal dosimetry to treatment outcomes continue to facilitate wider clinical implementation. Evolving applications include thermal enhancement of immunotherapy, targeted drug delivery and application of principals of thermal biology towards integration of thermal ablation into multimodality oncologic care.

The role of hyperthermia in the battle against cancer

Aims and background

Hyperthermia, the heating of tumors to 41.5–43 °C, could be today considered the fourth pillar of the treatment of cancer. Employed for 20 years in Europe, the USA and Asia, hyperthermia, used in addition to radiotherapy, chemotherapy and surgery, increases both local control and overall survival, restores the chance of the surgery for inoperable tumors and allows a new low-dosage treatment of relapsed cancers previously treated with high radiotherapy dosage without increasing toxicity.

Methods

Hyperthermia can be either superficial, produced by a microwave generator, or regional, produced by a radiofrequency applicator with multiple antennas, which emanate a deep focalized or interstitial heating.

Results

The results are confirmed by phase III randomized trials, with level 1 evidence. A review of the international literature on hyperthermia, the experience of the University Hospital of Verona Radiotherapy Department (Italy) and a summary of the Symposium regarding the Evolution of Clinical Hyperthermia plus Radiotherapy during the Twentieth Congress of the French Society of Radiation Oncology (SFRO) are presented.

Conclusions

Hyperthermia is an important treatment modality in cancer treatment and its results are strongly supported by criteria of evidence-based medicine. Fifteen years of experience of the Radiation Oncology Department in Verona confirms the positive results obtained with international prospective trials, with level 1 evidence. Hyperthermia appears to be the fourth pillar beside surgery, radiotherapy and chemotherapy. Free full text available at www.tumorionline.it

Temperature measurement by thermal strain imaging with diagnostic power ultrasound, with potential for thermal index determination

Over the years, there has been a substantial increase in acoustic exposure in diagnostic ultrasound as new imaging modalities with higher intensities and frame rates have been introduced; and more electronic components have been packed into the probe head, so that there is a tendency for it to become hotter. With respect to potential thermal effects, including those which may be hazardous occurring during ultrasound scanning, there is a correspondingly growing need for in vivo techniques to guide the operator as to the actual temperature rise occurring in the examined tissues. Therefore, an in vivo temperature estimator would be of considerable practical value. The commonly-used method of tissue thermal index (TI) measurement with a hydrophone in water could underestimate the actual value of TI (in one report by as much as 2.9 times). To obtain meaningful results, it is necessary to map the temperature elevation in 2-D (or 3-D) space. We present methodology, results and validation of a 2-D spatial and temporal thermal strain ultrasound temperature estimation technique in phantoms, and its apparently novel application in tracking the evolution of heat deposition at diagnostic exposure levels. The same ultrasound probe is used for both transmission and reception. The displacement and thermal strain estimation methods are similar to those used in high-intensity focused ultrasound thermal monitoring. The use of radiofrequency signals permits the application of cross correlation as a similarity measurement for tracking feature displacement. The displacement is used to calculate the thermal strain directly related to the temperature rise. Good agreement was observed between the temperature rise and the ultrasound power and scan duration. Thermal strain up to 1.4% was observed during 4000-s scan. Based on the results obtained for the temperature range studied in this work, the technique demonstrates potential for applicability in phantom (and possibly in vivo tissue) temperature measurement for the determination of TI.

Refining histotripsy: defining the parameter space for the creation of nonthermal lesions with high intensity, pulsed focused ultrasound of the in vitro kidney

PURPOSE

Focused ultrasound therapy is a promising modality for noninvasive tissue ablation. However, the relative contributions of thermal and cavitational effects are poorly defined. We characterized the ultrasound parameters within which tissue ablation occurs by cavitational mechanisms without significant thermal effect.

MATERIALS AND METHODS

In vitro porcine kidneys were submerged in degassed water. Tissue ablation was performed by delivering ultrasound (750 kHz and 20 microsecond pulses) of constant spatial peak energy dose (100 J/cm(2)) to adjacent foci in a 3 x 3 grid configuration. For each ablation different intensity (0.11 to 211 kW/cm(2)) and duty cycle (0.04% to 100%) parameters were selected. A thermocouple co-localized with the center of each grid continuously measured temperature. Following ablation each kidney was examined grossly and histologically.

RESULTS

Ablated tissue lesions were classified into 4 discrete morphological categories, including blanched--firm, pale, desiccated tissue, disrupted--a cavity containing thin, isochromatic liquid, mixed--a cavity containing pale, thick liquid with minimal blanching and no grossly visible effect. Morphologically similar lesions clustered at separable regions of the ultrasound parameter space. The maximal temperature attained in disrupted lesions was similar to that attained when there was no effect (44.2C and 47.2C, respectively, p = 0.31), although it was significantly lower than the maximal temperatures for desiccated or mixed lesions (67.5C and 59.4C, each p <0.0001).

CONCLUSIONS

In an in vitro model we defined the ultrasound parameter region within which purely cavitational ablation of tissue is possible with a negligible thermal component. Additional research is needed to optimize the parameters for in vivo cavitational tissue ablation, incorporating the influence of tissue perfusion.

Re-setting the biologic rationale for thermal therapy

This review takes a retrospective look at how hyperthermia biology, as defined from studies emerging from the late 1970s and into the 1980s, mis-directed the clinical field of hyperthermia, by placing too much emphasis on the necessity of killing cells with hyperthermia in order to define success. The requirement that cell killing be achieved led to sub-optimal hyperthermia fractionation goals for combinations with radiotherapy, inappropriate sequencing between radiation and hyperthermia and goals for hyperthermia equipment performance that were neither achievable nor necessary. The review then considers the importance of the biologic effects of hyperthermia that occur in the temperature range that lies between that necessary to kill substantial proportions of cells and normothermia (e.g. 39-42 degrees C for 1 h). The effects that occur in this temperature range are compelling-including inhibition of radiation-induced damage repair, changes in perfusion, re-oxygenation, effects on macromolecular and nanoparticle delivery, induction of the heat shock response and immunological stimulation, all of which can be exploited to improve tumour response to radiation and chemotherapy. This new knowledge about the biology of hyperthermia compels one to continue to move the field forward, but with thermal goals that are eminently achievable and tolerable by patients. The fact that lower temperatures are incorporated into thermal goals does not lessen the need for non-invasive thermometry or more sophisticated hyperthermia delivery systems, however. If anything, it further compels one to move the field forward on an integrated biological, engineering and clinical level.

The effects of residual temperature rise on ultrasound heating

In recent theoretical studies, the temperature rise produced by diagnostic ultrasound was estimated by solving the Bioheat Transfer Equation (BHTE) but ignoring the initial temperature rise. The temperature rise was determined in our study by the BHTE including an initial temperature rise. We discuss how the initial temperature rise occurs during an ultrasound examination, and how the initial temperature rise affects subsequent ultrasound heating. We theoretically show that the temperature rise produced by the ultrasound examination (exposure time of 500 s) in a tissue sample having an initial temperature rise was higher than that in a tissue sample with no initial temperature rise that was exposed to ultrasound (exposure time of 1200 s). The theoretical results for these two cases were 5.64 degrees C and 3.58 degrees C, respectively. In our experimental study, the highest temperature rise was measured in the presence of an initial temperature rise as in the theoretical study under the same exposure conditions. Mean temperature rises for tissue without an initial temperature rise and for tissue with an initial temperature rise were 2.42 +/- 0.13 degrees C and 3.62 +/- 0.17 degrees C, respectively. Both theoretical and experimental studies show that unless the initial temperature rise produced by the first ultrasound examination decreases to 0 degrees C, the next ultrasound examination on the same tissue sample may cause the temperature rise to be higher than expected.

The cooling effect of liquid flow on the focussed ultrasound-induced heating in a simulated foetal brain

There is a need to investigate the thermal effects of diagnostic ultrasound (US) to assist the development of appropriate safety guidelines for obstetric use. The cooling effect of a single liquid flow channel was measured in a model of human foetal brain and skull bone heated by a focussed beam of simulated pulsed spectral Doppler US. Insonation conditions were 5.7 micros pulses, repeated at 8 kHz from a focussed transducer operating with a centre frequency of 3.5 MHz, producing a beam of -6 dB diameter of 3.1 mm at the focus and power outputs of up to 255 +/- 5 mW. Brain perfusion was simulated by allowing distilled water to flow at various rates in a 2 mm diameter wall-less channel in the brain soft tissue phantom material. This study established that the cooling effect of the flowing water; 1. was independent of the acoustic source power, 2. was more effective close to the flow channel, for example, there was a marked cooling at a distance of 1 mm and negligible cooling at a distance of 3 mm from the channel; and 3. initially increased at low flow rates, but further increase above normal perfusion had very little effect.

A multi-user networked database for analysis of clinical and temperature data from patients treated with simultaneous radiation and ultrasound hyperthermia

A database was developed using commercially available development software that allows the entry of clinical data and automatically analyses temperature and power data from a commercial ultrasound hyperthermia system. The database can be accessed via network connections by more than one authorized user, thus facilitating the entry, management, and analysis of clinical data. The software automatically estimates ultrasound induced temperature artifacts and calculates thermal dose parameters such as T90s, equivalent minutes at 43 degrees, and time at or above index temperatures using the corrected temperatures. These parameters also become part of the database. Digital photographs of treatment setup, probe placement, and tumour or normal tissue response can be included in the database for documentation and reference. Ultrasound diagnostic images that document the depth and reproducibility of probe placement can be scanned into the PC and included in the database as well. This short communication documents experiences developing this tool that may be useful to other investigators.

A two-parameter method for the estimation of ultrasound-induced temperature artifacts

A two-parameter method for the estimation of ultrasound-induced temperature artifacts was evaluated and compared with other commonly applied methods using analytical solutions to the bioheat equation. The two parameters are the exponent of the assumed temperature decay curve after power is turned off and the baseline temperature. These parameters are found by optimizing the fit of the temperature data from 30 to 60s after power is turned off. The artifact is modelled as a point source at the centre of a Gaussian temperature distribution. The blood flow, baseline temperature, and variance of the Gaussian temperature distribution were varied to simulate different clinical situations. Noise was added to the model to investigate the effects of thermometry resolution and sampling intervals. It was found that for artifacts of < 2 degrees C the two-parameter method had errors of less than 0.25 degrees C, whereas other methods generally had greater errors depending on the conduction rate and blood flow rate. The effects of the temperature sampling interval and resolution on the ability of the methods to estimate the artifact were also investigated, and it was found that the two-parameter method was much more sensitive to these parameters than other commonly applied methods.

In vivo heating of the guinea-pig fetal brain by pulsed ultrasound and estimates of thermal index

Temperature was measured in the brain in live near-term fetal guinea pigs (62-66 d gestational age), during in utero exposure to a fixed beam of pulsed ultrasound at intensity ISPTA 2.82 W/cm2. Mean temperature increases of 4.3 degrees C close to parietal bone and 1.1 degrees C in the mid-brain were recorded after 2-min exposures. These values were lower (12%) than those obtained for ultrasound-induced heating near the bone in dead fetuses insonated in utero. A significant cooling effect of vascular perfusion was observed only when guinea pig fetuses reached late gestation, near term, when the cerebral vessels were well developed. The estimated value for the thermal index (TIB), as used in AIUM/NEMA output display standard, underestimated the measured temperature increase at the bone-brain interface. The ratio of measured temperature to the TIB is 1.3. A modification of the cranial thermal index provided a more reasonable, conservative, estimate of the temperature increase at a biologically significant point of interest at the brain-bone interface.

Increased heating efficiency of hyperthermia using an ultrasound contrast agent: a phantom study

It is known that there are large temperature elevations in proximity to air bubbles during US (ultrasound) heating. The existence of tiny air bubbles in the target tissue may enhance the temperature elevation in US hyperthermia. To examine this hypothesis, phantom tissue experiments using an US contrast agent consisting of tiny air bubbles surrounded by a 5% (w/v) human albumin shell (Alb) were performed. As a phantom tissue, a 2 cm cube of beef was used. The phantom tissue was heated with or without the US contrast agent by an US hyperthermia device for 3 min. The heating device was operated at 1.5 MHz with the US intensity of 0.9 W/cm2. Physiological saline solution, iodized oil, and ethanol were used for control experiments. The effect of multiple needle punctures to the beef phantom was also examined. The temperature elevation rate (TER) was defined as the ratio of temperature elevation by heating with Alb or control materials to the temperature elevation by US heating alone. The TER of Alb was 1.7, whereas the TERs of the control materials and of the multiple needle punctures were approximately 1. The administration of Alb significantly increased the temperature in US hyperthermia. In addition, the heating efficiency of Alb was compared to the effect of an increase in the US intensity. Phantom tissue was heated at various US intensities. When the US intensity was increased from 0.9 to 1.8 W/cm2, the temperature elevated by approximately 1.7-fold. Thus, the effect of the administration of Alb was almost equivalent to the effect of increase in US power intensities from 0.9 to 1.8 W/cm2 in the present experimental settings. The results suggest that the US contrast agent can be a potential enhancer in US hyperthermia.

Measurements of the thermal focus of an experimental focused ultrasound thermotherapy system

The thermal focus of an experimental thermotherapy equipment based on high energy focused ultrasound has been investigated. The in-house built equipment has a treatment head of seven separate focused transducers arranged in a semispherical fixture with a common focus at 100 mm from the transducer surfaces. Each transducer has a diameter of 50 mm and operates at 0.5 MHz. The ultrasound field of this seven-transducer arrangement, as well as of a single transducer of the same type as the ones in the arrangement, has been investigated in terms of temperature distribution evolved by absorption in castor oil. The results of the single transducer measurements show an ellipsoid-shaped focus displaced from the geometrical focus towards the transducer, whereas the measurements of the treatment head show a thermal focus which practically coincides with the geometrical one. Differences in location and shape of the thermal focuses depend on differences in the focusing action and the absorption in the media. Pilot investigations on tissue lesioning in vitro on pig muscle samples and in vivo on pig thigh were also carried out. Well discriminated local lesions with ellipsoid shape were obtained in the experiments in vitro and in vivo. Temperatures of up to 80 degrees C were measured in tissue at the focus in vitro. The results show that the equipment is well suited for thermotherapy applications.

Numerical and in vitro evaluation of temperature fluctuations during reflected-scanned planar ultrasound hyperthermia

Temperature fluctuations inside a target volume during reflected-scanned planar ultrasound hyperthermia were investigated numerically and in vitro. The numerical approach consisted of integrating an ultrasonic power deposition model for a scanning ultrasound reflector linear array system (SURLAS) designed for simultaneous thermoradiotherapy, and a three-dimensional transient version of Pennes' bioheat transfer equation. The in vitro approach consisted of delivering hyperthermia to a fixed-perfused canine kidney phantom using a SURLAS prototype. Both approaches allowed the study of temperature fluctuations for several important clinically relevant parameters: scan time, scan distance, perfusion rate and skin cooling. The simulation results showed that the largest temperature fluctuations were located at the opposite ends of the scan window where the scanning reflector comes to a sudden and complete stop and reverses direction. The smallest fluctuations were located at the centre of the scan window. For a given scan distance, the magnitude of the temperature fluctuations increased linearly with increasing scan time, and increased almost linearly as a function of blood perfusion rate. For a scan window of 10 cm x 10 cm and a blood perfusion rate of 5 kg/m3 s, the simulated temperature fluctuations were within +/- 0.5 degree C from the average temperature for scan times less than or equal to 20 s. The in vitro results agreed well with the numerical findings. The measured temperature fluctuations were less than 1.0 degree C for flow rates into the renal artery of less than 200 ml/min and scan times less than 20 s.

Ultrasound-induced temperature increase in the guinea-pig fetal brain in vitro

The temperature of the brain of fetal guinea pigs was measured in vitro during exposure to an unscanned beam of pulsed ultrasound at intensity ISPTA 2.8 W/cm2. A mean temperature increase of 5.1 degrees C recorded after 2 min of insonation confirms results of an earlier similar study. The water-bath exposure system provided enhanced cooling of superficial tissue by acoustic streaming. When the scalp was removed, the ultrasound-induced temperature increase was substantially reduced (by 35%) due to cooling through radiation force-induced bulk fluid streaming along the direction of propagation in the water bath. The measured temperature increase in guinea pig fetal brain correlated with a modified cranial thermal index.

Biological rationale and clinical experience with hyperthermia

Hyperthermia (HT) as an adjunct to radiation therapy (RT) has been a focus of interest in cancer management in recent years there have been numerous randomized and nonrandomized studies conducted to assess the efficacy of HT combined with either RT or chemotherapy especially in the treatment of superficially seated malignant tumors. The major impact of HT is currently on locoregional control of tumor. Heat may be directly cytotoxic to tumor cells or inhibit repair of both sublethal and potentially lethal damage after radiation. These effects are augmented by the physiological conditions in tumor that lead to states of acidosis and hypoxia. Blood flow is often impaired in tumor relative to normal tissues, and HT may lead to a further decrease in blood flow and augment heat sensitivity. Three major areas of clinical investigation have borne the greatest fruit for HT as adjunctive therapy to RT. These include recurrent and primary breast lesions, melanoma, and head and neck neoplasms. Thermal enhancement ratio was increased in all cases and is approximately 1.4 for neck nodes, 1.5 for breast, and 2 for malignant melanoma. In general, the most important prognostic factors for complete response (CR) are RT dose, tumor size and minimal thermal parameters minimal thermal dose (t43min), mean minimal temperature (Tmin) or T90, i.e., temperature exceeded by 90% of thermal sensors]. The number of HT fractions administered per week appears to have no bearing on the overall response, which may be indicative of the effects of thermotolerance. The total number of HT fractions delivered also appears irrelevant provided adequate HT is delivered in one or two sessions. The major prognostic factors for the duration of local control were tumor histology, concurrent RT dose, tumor depth and Tmin. Although numerous single institution studies showed increased CR rates and improved local control, the efficacy of HT as an adjunct to RT should be assessed with well-designed multi-institutional randomized clinical trials. Such clinical trials are underway.

A recommended revision in the RTOG thermometry guidelines for hyperthermia administered by ultrasound

RTOG thermometry guidelines for clinical trials of hyperthermia using planar ultrasound recommended that temperatures be mapped in polyurethane catheters by use of single-junction copper-constantan thermocouples. These guidelines were based on an assumption that the error in temperature measurement due to thermal conduction would generally not exceed +/- 0.3 degrees C. The validity of this assumption was tested with a commercially available single-junction copper-constantan thermocouple. The width of the point spread function, an indicator of the relative magnitude of the conduction error, was five times greater than expected. As a result, the conduction error is projected to exceed 0.3 degrees C in a temperature gradient of only 1.5 degrees C/cm. This projection was confirmed by mapping a thermal peak which simulates a typical clinical temperature profile. This peak had an amplitude of 6 degrees C, a full-width at half-maximum of 3.5 cm, and a maximum gradient of approximately 3 degrees C/cm. Temperatures measured at 0.5-cm intervals over the span of this peak were in error by a mean of +/- 0.6 degrees C. It is strongly recommended that the RTOG guidelines be revised to replace copper-constantan thermocouples with manganin-constantan single- or multi-junction thermocouples which will assure that the conduction error will be < +/- 0.3 degrees C.

Direct clinical comparison of ultrasound and radiative electromagnetic hyperthermia applicators in the same tumours

Hyperthermia in conjunction with radiation therapy is a promising method for the treatment of superficially or eccentrically located recurrent or advanced primary malignant tumours. The external hyperthermia applicators most commonly used are radiative electromagnetic (including microwave) or ultrasound devices. Each type of device has its own limitations. The aim was to evaluate the temperature distributions obtained as well as the acute and subacute toxicities in patients that were treated with both radiative radiative electromagnetic and ultrasound applicators to the same tumours. Thirty-nine patients treated to 41 hyperthermia fields for a total of 197 hyperthermia treatments were analysed. Thermal parameter include mean, Tmax, mean Tave, mean Tmin, T50, T90, %T > 43.5 degrees C and %T < 41 degrees C. Acute toxicities including pain in field, referred pain, blister/ulceration, positional discomfort and subacute toxicities (occurring with 24 h of treatment) were determined for each type of hyperthermia applicator. Although there were increased acute toxicities (in-field or referred pain) associated with the ultrasound treatments no significant differences between the two methods of heating were observed in temperature distributions or subacute toxicities. We conclude that there is no generally preferred method of heating superficially or eccentrically located tumours and the type of applicator should be selected on a tumour-size and site-specific basis.

Temperature field estimation using electrical impedance profiling methods. I. Reconstruction algorithm and simulated results

Algorithmic methods for estimating complete temperature fields during hyperthermia treatments based on surface and internal electrical measurements are presented. The techniques utilized draw upon impedance imaging concepts, but rather than limit the measurements to positions on the body surface, internal impedance recording sites are allowed. Theoretical simulations show that this strategy improves the reconstructed image in the target region when either internal measurement locations are added to a given number of external recording sites or some external measurement locations are replaced by internal recording positions. The algorithms developed are tested on a set of problems with increasing levels of complexity. The culmination of these investigations is a complete simulation of a hyperthermia treatment and reconstruction of a thermal image for a body cross-section of an actual cancer patient. The results of this work suggest that the surface plus internal measurement approach holds some promise as a method for estimating temperature distributions during hyperthermia treatments. However, the simulations while promising are idealizations in that they are two-dimensional with modest levels of additive noise. In a companion paper, we explore the viability of this approach in several laboratory phantom experiments which include both static and heat-induced transient electrical property profiles.

Temperature field estimation using electrical impedance profiling methods. II. Experimental system description and phantom results

An electrical impedance tomography system has been developed and tested for the purpose of thermal imaging. Since impedance changes with temperature, images of impedance subtracted from normothermic baselines will provide a map of temperature data. A system was designed to be operational at 10-50 kHz and to utilize 16 external electrodes around the periphery of a tissue-equivalent phantom encompassing the region of interest. These electrodes serve as current sources for the 5 mA constant-current inputs and are also used for reading differential voltages. Hyperthermia treatments for cancer require that internal thermometry probes be inserted into the tumour volume. Linear arrays of electrodes with thermometry tracks for micro-dimension thermometry serve this function, as well as providing localized voltage measurements in the region of interest. The embedded temperature sensors provide a quality assurance and calibration standard for the linear arrays in reconstruction of impedance profiles. Results of transient heating experiments with conductive and ultrasound heating are shown where image reconstruction is performed using a finite element model. Temperature predictions in these studies were accurate to better than 1 degree C on average when using information from surface electrodes combined with internal linear arrays. Maximum temperature errors, however, was found to be > 5 degrees C which suggests that further noise reduction during data acquisition and improvements in the reconstructions algorithms are needed.

Techniques for intraoperative hyperthermia with ultrasound: the Dartmouth experience with 19 patients

Over the course of 3 years, tumours of 19 patients were heated with ultrasound in the operating room during surgical resection. Immediately following intraoperative radiation therapy, thermocouples were inserted into tumour and adjacent normal structures. Patients were then given a 60-min heat treatment with ultrasound after a 10-15-min heatup period. Temperatures were measured at a total of 133 fixed locations for the 19 patient series. Temperature mapping was done in the tumour volume when logistically feasible. Treatment sites included colorectal (n = 3), portahepatus (n = 1), pancreas (n = 7), liver (n = 1), pelvis (n = 3), sacrum (n = 2), and abdomen (n = 2). A sterile, constant-volume water circulating system was utilized to control surface temperatures. Three generations of completely immersible transducers were designed over the course of this study with a 4-cm height specification. Since the ultrasound transducer was assembled on the sterile field during surgery, a 1, 2 or 3 MHz ceramic element was placed in either a 6, 8 or 10 cm diameter aluminium housing to conform the acoustic field to the tumour size. Average of the maximum temperatures attained was 46.6 degrees C. Temperature with which 90% of all measured points equalled or exceeded (T90) was 39.2 degrees C. The T50 was 42.9 degrees C. This compared favourably with T90 and T50 of 38.8 and 41.9 degrees C, respectively, in our outpatient clinic series, in which superficial tumours were treated with a similar external applicator, and patient tolerance was often a treatment limitation.

Estimation of temperature artifact from a short interruption in ultrasonic power

An error in temperature measurement, commonly referred to as a temperature artifact, frequently occurs during ultrasound hyperthermia as a result of viscous and absorption heating of the thermometer probe. At the present time there is no convenient method for correcting the clinical data for this error, which can be 0.5 degrees C, or greater. A technique is described by which the artifact can be estimated from a 10-s interruption in ultrasonic power. This technique is based on the observation that thermal decay recorded by a probe can be represented by the summation of two exponential decay rates, one of which represents the decay of tissue temperature and other decay of the artifact. Temperature drop during the first 10 s arises primarily from decay of the artifact because its time constant is approximately 7 s whereas that of the tissue generally ranges from 100 to 1000 s. A relationship between artifact and temperature drop derived from clinical data shows that the artifact is directly proportional to temperature drop. This relationship can be used to estimate the artifact during therapy if power is interrupted for 10 s. Because the interruption in power is brief, it is feasible to sample the artifact periodically during therapy and to make an on-line correction for the temperature artifact.

RTOG quality assurance guidelines for clinical trials using hyperthermia for deep-seated malignancy

Quality assurance has been vague or lacking in many previous hyperthermia trials. Recent publications by the Hyperthermia Physics Center, the Center for Devices and Regulatory Health, and the Radiation Therapy Oncology Group have described general guidelines for quality assurance in equipment reliability and reproducibility, superficial applications, and microwave techniques. The present report details quality assurance factors that are believed to be important for hyperthermia of deep clinical sites, defined as extending at least 3 cm beyond the skin surface. This document will discuss patient and physician factors, as well as thermometric accuracy, assessment of specific absorption rates (SAR), assurance of adequate coverage of tumors by the energy deposition pattern of the treatment device, and recommended documentation of the location, quantity, and frequency of treatment, specifically oriented to deep hyperthermia. The recommendations are structured to facilitate compliance in multiinstitutional trials.