Ultrasound-induced Hyperthermia Research Articles

Dense speed-of-sound shift imaging for ultrasonic thermometry

Objective. Develop a dense algorithm for calculating the speed-of-sound shift between consecutive acoustic acquisitions as a noninvasive means to evaluating temperature change during thermal ablation. Methods. An algorithm for dense speed-of-sound shift imaging (DSI) was developed to simultaneously incorporate information from the entire field of view using a combination of dense optical flow and inverse problem regularization, thus speeding up the calculation and introducing spatial agreement between pixels natively. Thermal ablation monitoring consisted of two main steps: pixel shift tracking using Farneback optical flow, and mathematical modeling of the relationship between the pixel displacement and temperature change as an inverse problem to find the speed-of-sound shift. A calibration constant translates from speed-of-sound shift to temperature change. The method performance was tested in ex vivo samples and compared to standard thermal strain imaging (TSI) methods. Main results. Thermal ablation at a frequency of 2 MHz was applied to an agarose phantom that created a speed-of-sound shift measured by an L12-5 imaging transducer. A focal spot was reconstructed by solving the inverse problem. Next, a thermocouple measured the temperature rise during thermal ablation of ex vivo chicken breast to calibrate the setup. Temperature changes between 3 °C and 15 °C was measured with high thermometry precision of less than 2 °C error for temperature changes as low as 8 °C. The DSI method outperformed standard TSI in both spatial coherence and runtime in high-intensity focused ultrasound-induced hyperthermia. Significance. Dense ultrasonic speed-of-sound shift imaging can successfully monitor the speed-of-sound shift introduced by thermal ablation. This technique is faster and more robust than current methods, and therefore can be used as a noninvasive, real time and cost-effective thermometry method, with high clinical applicability.

Clinical trial protocol for PanDox: a phase I study of targeted chemotherapy delivery to non-resectable primary pancreatic tumours using thermosensitive liposomal doxorubicin (ThermoDox®) and focused ultrasound

BackgroundThe dense stroma of pancreatic ductal adenocarcinomas is a major barrier to drug delivery. To increase the local drug diffusion gradient, high doses of chemotherapeutic agent doxorubicin can be released from thermally-sensitive liposomes (ThermoDox®) using ultrasound-mediated hyperthermia at the tumour target. PanDox is designed as a Phase 1 single centre study to investigate enhancing drug delivery to adult patients with non-operable pancreatic ductal adenocarcinomas. The study compares a single cycle of either conventional doxorubicin alone or ThermoDox® with focused ultrasound-induced hyperthermia for targeted drug release.MethodsAdults with non-resectable pancreatic ductal adenocarcinoma are allocated to receive a single cycle of either doxorubicin alone (Arm A) or ThermoDox® with focused ultrasound-induced hyperthermia (Arm B), based on patient- and tumour-specific safety conditions. Participants in Arm B will undergo a general anaesthetic and pre-heating of the tumour by extra-corporal focused ultrasound (FUS). Rather than employing invasive thermometry, ultrasound parameters are derived from a patient-specific treatment planning model to reach the 41 °C target temperature for drug release. ThermoDox® is then concurrently infused with further ultrasound exposure. Tumour biopsies at the targeted site from all patients are analysed post-treatment using high performance liquid chromatography to quantify doxorubicin delivered to the tumour. The primary endpoint is defined as a statistically significant enhancement in concentration of total intra-tumoural doxorubicin, comparing samples from patients receiving liposomal drug with FUS to free drug alone. Participants are followed for 21 days post-treatment to assess secondary endpoints, including radiological assessment to measure changes in tumour activity by Positron Emission Tomography Response Criteria in Solid Tumours (PERCIST) criteria, adverse events and patient-reported symptoms.DiscussionThis early phase study builds on previous work targeting tumours in the liver to investigate whether enhancement of chemotherapy delivery using ultrasound-mediated hyperthermia can be translated to the stroma-dense environment of pancreatic ductal adenocarcinoma. If successful, it could herald a new approach towards managing these difficult-to-treat tumours.Trial registrationClinicalTrials.gov Identifier: NCT04852367. Registered 21st April 2022.EudraCT number: 2019–003950-10 (Registered 2019)Iras Project ID: 272253 (Registered 2019)Ethics Number: 20/EE/0284.

Interleaved Mapping of Temperature and Longitudinal Relaxation Rate to Monitor Drug Delivery During Magnetic Resonance-Guided High-Intensity Focused Ultrasound-Induced Hyperthermia.

Magnetic resonance-guided high-intensity focused ultrasound (MR-HIFU) is a method to heat lesions noninvasively to a stable, elevated temperature and a well-suited method to induce local hyperthermia (41°C-43°C) in deep-seated tissues. Magnetic Resonance (MR) imaging provides therapy planning on anatomical images and offers temperature feedback based on near-real-time MR thermometry. Although constant acquisition of MR thermometry data is crucial to ensure prolonged hyperthermia, it limits the freedom to perform measurements of other MR parameters, which are of interest during hyperthermia treatments. In image-guided drug delivery applications, co-encapsulation of paramagnetic MR contrast agents with a drug inside temperature-sensitive liposomes (TSLs) allows to visualize hyperthermia-triggered drug delivery through changes of the longitudinal relaxation rate R1. While the drug accumulates in the heated tumor tissue, R1 changes can be used for an estimate of the tumor drug concentration. The main objective of this study was to demonstrate that interleaved MR sequences are able to monitor temperature with an adequate temporal resolution and could give a reasonable estimate of the achieved tumor drug concentration through R1 changes. To this aim, in vitro validation tests and an in vivo proof-of-concept study were performed. All experiments were performed on a clinical 3-T MR-HIFU system adapted with a preclinical setup. The validity of the R1 values and the temperature maps stability were evaluated in phantom experiments and in ex vivo porcine muscle tissue. In vivo experiments were performed on rats bearing a 9L glioma tumor on their hind limb. All animals (n = 4 HIFU-treated, n = 4 no HIFU) were injected intravenously with TSLs co-encapsulating doxorubicin and gadoteridol as contrast agent. The TSL injection was followed by either 2 times 15 minutes of MR-HIFU-induced hyperthermia or a sham treatment. R1 maps were acquired before, during, and after sonication, using a single slice Inversion Recovery Look-Locker (IR-LL) sequence (field of view [FOV], 50 × 69 mm; in-plane resolution, 0.52 × 0.71 mm; slice thickness, 3 mm; 23 phases of 130 milliseconds; 1 full R1 map every 2 minutes). The R1 maps acquired during treatment were interleaved with 2 perpendicular proton resonance frequency shift (PRFS) MR thermometry slices (dynamic repetition time, 8.6 seconds; FOV, 250 × 250 mm; 1.4 × 1.4 mm in-plane resolution; 4 mm slice thickness). Tumor doxorubicin concentrations were determined fluorometrically. In vitro results showed a slight but consistent overestimation of the measured R1 values compared with calibrated R1 values, regardless whether the R1 was acquired with noninterleaved IR-LL or interleaved. The average treatment cell temperature had a slightly higher temporal standard deviation for the interleaved PRFS sequence compared with the noninterleaved PRFS sequence (0.186°C vs 0.101°C, respectively). The prolonged time in between temperature maps due to the interleaved IR-LL sequence did not degrade the temperature stability during MR-HIFU treatment (Taverage = 40.9°C ± 0.3°C). Upon heat treatment, some tumors showed an R1 increase in a large part of the tumor while other tumors hardly showed any ΔR1. The tumor doxorubicin concentration showed a linear correlation with the average ΔR1 during both sonications (n = 8, Radj = 0.933), which was higher than for the ΔR1 measured after tumor cooldown (averaged for both sonications, n = 8, Radj = 0.877). The new approach of interleaving different MR sequences was applied to simultaneously acquire R1 maps and PRFS thermometry scans during a feedback-controlled MR-HIFU-induced hyperthermia treatment. Interleaved acquisition did not compromise speed or accuracy of each scan. The ΔR1 acquired during treatment was used to visualize and quantify hyperthermia-triggered release of gadoteridol from TSLs and better reflected the intratumoral doxorubicin concentrations than the ΔR1 measured after cooldown of the tumor, exemplifying the benefit of interleaving R1 maps with temperature maps during drug delivery. Our study serves as an example for interleaved MR acquisition schemes, which introduce a higher flexibility in speed, sequence optimization, and timing.

Focused ultrasound induced hyperthermia accelerates and increases the uptake of anti-HER-2 antibodies in a xenograft model

Image guided drug delivery has gained significant attention during the last few years. Labelling nanoparticles or macromolecules and monitoring their fate in the body provides information that can be used to modulate their biodistribution and improve their pharmacokinetics. In this study we label antibodies and monitor their distribution in the tumours post intravenous injection. Using Focused Ultrasound (FUS, a non-invasive method of hyperthermia) we increase the tumour temperature to 42°C for a short period of time (3–5min) and we observe an increased accumulation of labelled antibody. Repetition of focused ultrasound induced hyperthermic treatment increased still further the accumulation of the antibodies in the tumour. This treatment also augmented the accumulation of other macromolecules non-specific to the tumour, such as IgG and albumin. These effects may be used to enhance the therapeutic efficiency of antibodies and/or targeted nanoparticles.

In Vitro Localized Release of Thermosensitive Liposomes with Ultrasound-Induced Hyperthermia

Localized drug delivery with ultrasound-induced hyperthermia can enhance the therapeutic index of chemotherapeutic drugs by improving efficacy and reducing systemic toxicity. A novel invitro method for the activation of drug-loaded thermosensitive liposomes is described. In particular, a dual-compartment, acoustically transparent container is used in which thermosensitive liposomes suspended in cell culture medium are immersed in a thermally absorptive medium, glycerol. Hyperthermia is induced with ultrasound in the glycerol, which in turn heats the culture medium by thermal conduction. The method approximately mimics the invivo scenario of thermosensitive liposomes collected in the interstitial spaces of tumors, where ultrasound induces hyperthermia in the tumor tissue, which in turn heats the thermosensitive liposomes by conduction and induces release of the encapsulated drug. The acoustic conditions for the desired hyperthermia are derived theoretically and validated experimentally. Eighty percent release of doxorubicin from thermosensitive liposomes is achieved.

A non-exothermic cell-embedding tissue-mimicking material for studies of ultrasound-induced hyperthermia and drug release

Purpose: The present study aims to create and characterise a cell-embedding tissue-mimicking material (TMM) that has thermal and acoustic properties similar to liver tissue, in order to enable study and optimisation of protocols for ultrasound-induced hyperthermia and drug delivery.Materials and methods: An agarose-based, cell-embedding TMM was iteratively developed and characterised. The acoustic properties (attenuation coefficient, speed of sound and cavitation threshold) and thermal response of the material were compared with those of fresh degassed liver tissue over a range of acoustic pressures and frequencies. A luminescence intensity assay was used to evaluate viability of HuH-7 cells in the material. The efficacy of ultrasound-mediated chemotherapeutic treatment in the material was tested by localised activation of low temperature thermally sensitive liposomes. Drug activation was measured by fluorescence microscopy.Results: Similar acoustic properties (attenuation coefficient, speed of sound) to liver tissue were achieved over the therapeutically relevant frequency range of 1–4 MHz and similar thermal response was achieved for acoustic pressures up to 4.8 MPa peak to peak (ppk) at 1.1 MHz. Above 4.8 MPa ppk cavitation enhanced heating occurred in the TMM. Drug release from low-temperature-sensitive liposomes was achieved with 4.4 MPa ppk 6-s exposures at 1.1 MHz and cell compatibility of the material was confirmed.Conclusions: A platform for in vitro work for activation of thermally sensitive liposomes using high intensity focused ultrasound (HIFU)-induced hyperthermia was established. The TMM presents similar acoustic properties and thermal response to liver tissue over a broad range of ultrasound exposure conditions.

Enhanced in vivo bioluminescence imaging using liposomal luciferin delivery system

To provide a continuous and prolonged delivery of the substrate D-luciferin for bioluminescence imaging in vivo, luciferin was encapsulated into liposomes using either the pH gradient or acetate gradient method. Under optimum loading conditions, 0.17 mg luciferin was loaded per mg of lipid with 90–95% encapsulation efficiency, where active loading was 6 to 18-fold higher than that obtained with passive loading. Liposomal luciferin in a long-circulating formulation had good shelf stability, with 10% release over 3-month storage at 4 °C. Pharmacokinetic profiles of free and liposomal luciferin were then evaluated in transgenic mice expressing luciferase. In contrast to rapid in vivo clearance of free luciferin ( t 1/2 = 3.54 min), luciferin encapsulated into long-circulating liposomes showed a prolonged release over 24 h. The first-order release rate constant of luciferin from long-circulating liposomes, as estimated from the best fit of the analytical model to the experimental data, was 0.01 h − 1 . Insonation of luciferin-loaded temperature-sensitive liposomes directly injected into one tumor of Met1- luc tumor-bearing mice resulted in immediate emission of light. Systemic injection of luciferin-loaded long-circulating liposomes into Met1- luc tumor-bearing mice, followed by unilateral ultrasound-induced hyperthermia, produced a gradual increase in radiance over time, reaching a peak at 4–7 h post-ultrasound.

Effect of Ultrasound-Induced Hyperthermia on Cellular Uptake of P-gp Substrate and Non-P-gp Substrate in MDR Cells

A previous report recently demonstrated that ultrasound-induced hyperthermia (USHT:0.4 watts (W)/ at ) could increase cellular uptake of P-glycoprotein (P-gp) substrates in P-gp expressing cancer cell lines. Since P-gp plays a major role in limiting drug permeability in the multi-drug resistant (MDR) cells, studies were conducted to elucidate the mechanism of USHT on cellular accumulation of P-gp and non-P-gp substrate in MDR cells. To accomplish this aim, we studied the effects of USHT on the accumulation of P-gp substrate, R123 and non-P-gp substrate, antipyrine in MDR cells. We demonstrated that USHT increased permeability of hydrophobic molecules (R123 and -antipyrine). The enhanced permeability is reversible and size-dependent as USHT produces a much larger effect on cellular accumulation of -antipyrine (MW 188) than that of R123 (MW 380.8). These results suggest that USHT could affect MDR cells more sensitive than BBMECs. Also, the present results point to the potential use of USHT to increase cellular uptake of P-gp recognized substrates, mainly anti-cancer agents into cancer cells.

Overcoming MDR by Ultrasound-Induced Hyperthermia and P-Glycoprotein Modulation

We assessed the effects of combining ultrasound-induced hyperthermia (USHT) with the P-glycoprotein modulator PSC 833 on cellular retention and cytotoxicity of rhodamine 123 (R123) and doxorubicin in the parent and multidrug resistance (MDR) variants of two human cancer lines. USHT significantly increased cellular uptake of R123 and doxorubicin. Without PSC 833, release of R123 and doxorubicin from both USHT-treated and untreated cells was rapid. As expected, PSC 833 (0.5 μM) only slowed their release into the MDR lines. Interestingly, despite the differences in their starting amounts, PSC 833 was effective in prolonging R123 and doxorubicin release from both USHT-treated and untreated MDR cells. PSC 833 did not augment the cytotoxicity of doxorubicin in parent lines but did cause a significant increase in cytotoxicity of doxorubicin in the MDR lines. However, the combined effect of USHT and PSC 833 on cytotoxicity of doxorubicin far exceeded that produced by USHT or PSC 833 alone.

Ultrasound-Induced hyperthermia increases cellular uptake and cytotoxicity of P-glycoprotein substrates in multi-drug resistant cells.

Localized hyperthermia has been shown previously to augment the cytotoxicity of some lipophilic anticancer drugs. Because many of the substrates for the multi-drug resistance (MDR) transporter P-glycoprotein (P-gp) are lipophilic in nature, studies were conducted to test the hypothesis that hyperthermia induced by ultrasound could also increase cellular uptake and cytotoxicity of P-gp substrates by P-gp-expressing cells. To test this hypothesis, we studied the effects of hyperthermia and ultrasound on cellular accumulation of putative P-gp substrates, rhodamine 123 (R123) and doxorubicin (DOX), and cytotoxicity of DOX in the parent and MDR variants of two human cancer cell lines. Treatment of cells with hyperthermia or ultrasound (20 min at 41 degrees C) both caused a significant increase over controls (no ultrasound treatment) in R123 and DOX accumulation in the parent and MDR lines of MV522 and KB cells. Ultrasound also substantially increased the antiproliferative effects of DOX in both the parent and MDR variants of MV522 and KB cell lines when compared with controls. Our results also indicated that ultrasound exerted a much greater effect on cellular accumulation of R123 and DOX and cytotoxicity enhancement of DOX in the MDR variants than putative P-gp antagonist such as verapamil. The present results point to the potential use of ultrasound-induced hyperthermia as a much safer alternative to P-gp antagonist for reversal of MDR.

Simultaneous measurements of temperature and pH in vivo using NMR in conjunction with TmDOTP5-.

NMR techniques for temperature and pH measurements have attracted increasing interest in recent years, motivated in part by the growing importance of medical hyperthermia for the treatment of cancer. The chemical shifts of thulium 1, 4, 7, 10-tetraazacyclododecane-1, 4, 7, 10-tetrakis(methylene phosphonate) (TmDOTP5-) have been studied as a function of temperature and pH. The results demonstrate that TmDOTP5- resonance shifts are highly sensitive to temperature (approximately 1.0 ppm/degrees C) and pH (approximately 3.2 ppm/pH unit) at clinically relevant field strengths. A new method is presented which utilizes two magnetically non-equivalent protons in TmDOTP5- for simultaneous NMR measurements of both temperature and pH. The difference in the chemical shift values of pairs of 1H resonances provides a temperature sensitivity of about 1.6 ppm/ degrees C. The technique is demonstrated in live rats undergoing ultrasound-induced hyperthermia therapy.

The effects of artery occlusion on temperature homogeneity during hyperthermia in rabbit kidneys in vivo

To investigate the role of arterial occlusion on temperature homogeneity during hyperthermia for deep seated tissue, a renal hyperthermia animal model has been established using New Zealand white rabbits. The effects of ultrasound-induced renal hyperthermia, with or without continuous and intermittent renal artery occlusion, were compared and analysed. Both continuous and intermittent occlusion showed certain protection of surrounding tissue and demonstrated improved temperature homogeneity and heating efficiency. The benefits of continuous vs. intermittent occlusion are compared and discussed as well.

Ultrasound-enhanced effects of adriamycin against murine tumors.

Earlier studies from our and other laboratories have demonstrated that ultrasound (US) enhances the cytotoxicity in vitro of the antitumor agent Adriamycin (Adr) (Harrison et al. 1991; Loverock et al. 1990; Saad and Hahn 1987, 1989; Yang et al. 1991; Yumita et al. 1987, 1989). We have now tested the possibility that this additional cytotoxicity can be translated into antitumor activity in vivo. Mice, bearing either a fibrosarcoma (RIF-1) or a melanoma (B-16) on their thighs, were injected with a single dose of Adr (10-20 mg/kg). The tumors were then heated locally to 41 degrees -43 degrees C for 30 min, either by insonation with US or by immersion of the animals' limbs into hot water baths. Antitumor efficacy was scored two ways: by serial measurements of tumor volume to determine the time for the tumor to double in size, or by determining the X-ray dose required to sterilize 50% of the tumors (TCD50) after the Adr-hyperthermia treatment. Both assays gave similar results. Ultrasound-induced hyperthermia was substantially more effective in enhancing Adr activity than was hyperthermia induced by the water bath. The mean-doubling time was 13 days for tumors treated with the combination of Adr and US and 6 days for tumors heated with a water bath immediately after injection of Adr. The TCD50 was 21.2 +/- 0.8 Gy for the combination of US and Adr and 36.1 +/- 0.9 Gy for the water bath heating and Adr.(ABSTRACT TRUNCATED AT 250 WORDS)

Ultrasound-induced hyperthermia as adjuvant therapy for palmoplantar psoriasis

The use of ultrasound-induced hyperthermia as adjuvant therapy was evaluated in seven patients with chronic palmoplantar psoriasis in a bilateral paired comparison study. Treatments consisted of twice-weekly 30-min applications of an ultrasound transducer operating at 8 MHz. The intensity of the device was varied between 0.30 and 0.54 W/cm2 to achieve temperatures of 42–44C at a depth of 2–3 mm beneath the skin surface as monitored with a thermocouple probe. The mean clinical score for the treatment sites was 6.0 1.6 before treatment and 2.3 0.70 after hyperthermia treatment. The mean clinical score of the control site was 5.6 1.76 and remained unchanged during the treatment period. There were no sidesffects. At the 3 month follow-up three patients continued to show clinically less severe psoriasis on the hyperthermia treated area compared with the control area.

Flow dependence of 2-D temperature distributions induced in the perfused canine kidney by ultrasound

The influence of perfusate flow rate on the two-dimensional (2-D) temperature distributions induced by hyperthermia in the canine kidney was evaluated. Localized hyperthermia was induced by ultrasound (frequency 2.040 MHz, transducer diameter 6.7 cm) in the ex-vivo perfused kidney. Temperatures were mapped using shielded copper/constantan thermocouple sensors. The 2-D temperature distribution of the kidney was obtained by pulling these sensors through a planar array of eight stainless-steel trocars inserted along its longitudinal axis. The perfusion system allowed for the control of internal/external organ temperature and type of perfusate, and it simulated vasodilation and vasoconstriction by changing the arterial volume flow rate. The temperature data obtained at flow rates ranging from 0 to 570 ml/min and power levels from 0 to 150 W showed that acoustic power densities in excess of 1.5 W/cm2 were required to maintain therapeutic temperatures in this system. Contour maps for characterizing the 2-D temperature distributions induced in this system were analysed with the introduction of a quantitative measure based on the area within a given isotherm. They show the effects of tissue heterogeneity, flow rate, and non-uniform power deposition. Time constants computed from the exponentially decaying temperatures measured following power off ranged from 5 to 555 s. These findings provide a basis for comparison with temperature measurements being acquired in the in-vivo kidney system. Information acquired from this system may facilitate the process of developing tissue-equivalent dynamic phantoms for ultrasound-induced hyperthermia. The ex-vivo model presented in this paper might be used to study the performance of alternative heating applicators, the effect of haematocrit, blood viscosity, and the use of vasoactive drugs.

Applicators for generating ultrasound-induced hyperthermia in neoplastic tumours and for use in ultrasound physiotherapy

Ultrasound in the low megahertz frequency range is used by physiotherapists to treat a variety of conditions. The treatments reduce swelling, reduce pain and increase movement at joints. Ultrasound is now also beginning to be used to treat malignant tumours. The tumours are heated to temperatures between 42 degrees and 45 degrees C, producing cellular damage, the extent of which is determined by the duration and number of treatments and 'concurrent' treatments by chemo and X-ray therapy. In the light of the results of computer simulation, the authors discuss: (a) The properties of a five-element divergent transducer array to treat large superficial neoplastic tumours. The field distribution measured by the Sarvazyan method is described and also compared with that of computer simulation. The benefits of frequency wobbling are also discussed. (b) The possibility of employing a multiple field applicator for use by physiotherapists to obviate the need for continuous movement of a small transducer of area approximately 5 cm2 over the affected area is considered.

An investigation into the combined effects of x-irradiation and 3MHz ultrasound-induced hyperthermia on pig skin

The thermal enhancement of radiation-induced damage in pig skin has been investigated. Heating at 43°C for 60 min was produced by a scanned 3MHz ultrasound transducer, immediately after single doses of X rays. The ED 50 values for the dermal reactions of dusky/mauve erythema and necrosis after irradiation alone were 18.6 ± 0.5 Gy and 20.5 ± 0.4 Gy, respectively. The reduction in the ED 50 values to 15.3 ± 0.4 Gy and 17.7 ± 0.5 Gy after irradiation plus heating was significant and suggested a thermal enhancement ratio (TER) of between 1.15 and 1.22. These TER values were within the range obtained in both pig and rat skin using other ‘dry’ heating methods. This would suggest that the non-thermal effects of ultrasound do not influence the thermal enhancement of x-irradiation damage.

The response of a solid tumor to X-irradiation as modified by dose rate, fractionation, and hyperthermia.

The KHT sarcoma in C3H mice was exposed to a total dose of 3000 cGy x-irradiation delivered in three equal fractions at intervals of either 48, 72, or 96 hr. The dose rate was high, 212 cGy/min or low, 19 cGy/min. The effect of hyperthermia, 42.5 degrees C for 30 minutes, was determined by inducing the hyperthermia using either water or ultrasound. For radiation alone at high dose rate, tumor control was maximum at a fraction interval of 96 hr. At low dose rate, local tumor control was not influenced by fractionation interval and was not different from the maximum tumor control achieved at high dose rate. Hyperthermia alone was ineffective for tumor control. When high dose rate irradiation and hyperthermia were combined, thermal enhancement was highest at fractionation intervals of less than 96 hr where the effects or irradiation alone were least. At low dose rate, the thermal enhancement was independent of fractionation interval. Only for irradiation at high dose rate was the ultrasound-induced hyperthermia more effective at potentiating the radiation than water bath-induced hyperthermia. A possible explanation for the interaction of the dose rate effect with fractionation and hyperthermia is proposed.

Tumour blood flow following local ultrasound heating computed from thermal clearance curves.

Thermal clearance curves following termination of ultrasound-induced hyperthermia in human mammary carcinomas implanted into the flanks of nude rats were studied. They were found to be monoexponential in form, both with and without blood flow. From the difference between the inverse time constants with and without flow, the tumour blood flow rate could be calculated. Blood flow was found to increase with very short exposure times at the therapeutic hyperthermia temperature and subsequently decrease as the exposure time increased. A higher therapeutic hyperthermia temperature augmented this effect.

The Response of Previously Irradiated Skin to Combinations of X Radiation and Ultrasound-Induced Hyperthermia

Areas of skin approximately 1.5 cm in diameter on the legs of mice were made hyperthermic (30 min at 42.7 degrees C) by exposure to an ultrasound beam (780 kHz), a single dose of X irradiation (2000 rad), or a combination of these treatments. After 35 days, when the acute reaction had reached a steady state, the same tissue was given a second treatment by either hyperthermia, irradiation, or a combination of hyperthermia and irradiation. When the first treatment was irradiation and the second treatment was either irradiation or a combination of hyperthermia and irradiation, the acute skin reactions were similar to those of skin not previously irradiated, indicating a large proportion of recovery from the first irradiation. When irradiation was the first treatment, a comparison of second treatments by hyperthermia plus irradiation with irradiation alone showed a thermal enhancement of 1.45. When the first treatment was hyperthermia plus irradiation, a comparison of second treatments by hyperthermia plus irradiation with irradiation also showed an enhancement factor of 1.45 for the combined treatment.