RF coils for combined MR and hyperthermia studies: I. Hyperthermia applicator as an MR coil.

Combining hyperthermia, an experimental/adjuvant therapeutic modality for cancer, with the non-invasive metabolic studies using Magnetic Resonance (MR) is an interesting area of research. This two parts article discusses the development and testing of a conventional RF hyperthermia applicator for MR studies and vice versa. In this first part, an inductive type applicator known as 'Magnetrode' in RF hyperthermia has been used both as an MR volume resonator and a surface coil. Its concurrent performance as an hyperthermic applicator and an MR transmit/receive coil has been evaluated.

RF coils for combined MR and hyperthermia studies: II. MR coil as an hyperthermic applicator.

Single loop surface coil, often used in MR studies, was evaluated for its performance as an inductive hyperthermic applicator. The heat deposition pattern produced by the surface coil at 84 MHz and 34 MHz was mapped in muscle-mimicking agar phantoms. Temperatures were measured simultaneously at 64 points using multiple-junction thermocouples.

Simultaneous evaluation of the effects of RF hyperthermia on the intra- and extracellular tumor pH.

31P-magnetic resonance spectroscopy (MRS) and a fiberoptic pH meter were used simultaneously to follow the changes in intra- (pHi) and extracellular pH (pHe), respectively, of murine RIF-1 tumors with hyperthermia. Hyperthermia was induced at 34 MHz using the same coil used for MR. The study was carried out until 3.5 hr after hyperthermia. In untreated tumors (n = 29), pHi was always higher than pHe. pHi was reduced after hyperthermia (30 min) at both 42 degrees C and 45 degrees C. pHe registered an increase after 42 degrees C and a decrease after 45 degrees C. The reduction in pHi was larger than the initial differential between pHi and pHe, and the change in pHe was relatively small. Hyperthermia changed the acidity of the intra- and extracellular compartments, such that pHe became more alkaline than pHi by 0.15 +/- 0.13 units after 42 degrees C [pHe (7.20 +/- 0.12) and pHi (7.03 +/- 0.05)], and by 0.12 +/- 0.14 units after 45 degrees C [pHe (6.84 +/- 0.24) and pHi (6.72 +/- 0.19)]. Simultaneous measurements of pH from the intra- and extracellular compartments demonstrated reversal in the pH gradient after hyperthermic treatment.

A magnetic resonance-compatible experimental set-up for hyperthermia studies.

This study presents the development and testing of an experimental set-up for simultaneous measurements of pH (intracellular by MR and extracellular by fibre-optic pH meter), tumour bio-energetics (by MR), and core/tumour temperatures (by thermocouples) in tumour-bearing mice. Potential sources of measurement errors when using these techniques concurrently in an MR set-up are discussed. Emphasis is placed on simple practical solutions to these problems.

Response of 145 spontaneous canine head and neck tumours to radiation versus radiation plus microwave hyperthermia: results of a randomized phase III clinical study.

The results of a phase III, clinical trial of local microwave hyperthermia (target = 2 x 44 degrees C for 30 min) and megavoltage radiation (4 x 9 Gy fractions) in the treatment of 145 naturally occurring canine head and neck cancers are reported. Patients were re-examined at regular intervals following treatment until death. The median follow up time was 90 weeks. Tumour response, patient survival and normal tissue toxicity were analysed by treatment allocation. There was no significant difference in best tumour response nor patient survival between the two treatment groups. There was no difference in acute normal tissue toxicity but there was a suggestion that patients receiving RT and HT may suffer a higher incidence of late skin reactions. Histological type and tumour volume were of prognostic significance with smaller tumours and carcinomas showing higher response rates. There were also positive associations between minimum tumour dose and best tumour response and percentage of tumour heated and best tumour response. The results of this study must be interpreted in the knowledge of limitations on the dose and fractionation schedule for radiation therapy, the small number of hyperthermia treatments applied and the variation in tumour type and size that is inevitable in a clinical study. It is concluded that the quality of hyperthermia in terms of intra-tumour temperatures and the uniformity of heating is of paramount importance in governing response to adjuvant hyperthermia.

Radiotherapy, hyperthermia, and photodynamic therapy for central nervous system tumors.

This review concentrates on malignant gliomas, with brief mention of cerebral lymphomas, other primary intracranial tumors, and metastases. A recurring theme in the current literature on malignant glioma is the importance of prognostic factors other than treatment in determining outcome. Interesting results are emerging from newer strategies of stereotactic radiotherapy, brachytherapy, and photodynamic therapy, although authors who report these results acknowledge that they have often reviewed patients with intrinsically good prognoses. The year's literature has provided a consolidation of information on prognostic factors, relapse patterns, treatment volume, and dose, providing a basis for future progress.

Comparison of pH measurements made using 31P NMR and a fibreoptic pH meter.

The objective of this study was to compare pH measurements made in biological samples using 31P NMR (pHNMR) with those made with a novel, dye-based fibreoptic pH measurement system (pHF), which is compatible with use in electromagnetic fields without field perturbation. Using protein-free model solutions, pHNMR was calibrated against pHF, giving a correlation coefficient of 0.969 and a mean difference (+/- SD) between pHNMR and pHF of 0.037 +/- 0.054 over the pH range 6.8-7.7. Further calibration of pHNMR with pHF was carried out for human red blood lysates and then pHNMR was compared with pHF for whole, packed red blood cells over the pH range 7.0-7.8. Values for pHNMR, the intracellular pH, were consistently lower than for pHF, the extracellular pH, by a mean (+/- SD) of 0.15 +/- 0.02 units. A close correlation of extracellular pHNMR with pHF was demonstrated for a blood sample exhibiting two P(i) peaks, over the pH range 7.03-7.71. We conclude that concurrent use of NMR and the fibreoptic pH meter provides a reliable method of simultaneous measurement of intracellular and extracellular pH in biological systems.

BW12C: effects on tumour hypoxia, tumour thermosensitivity and relative tumour and normal tissue perfusion in C3H mice.

BW12C (5-[2-formyl-3-hydroxypenoxyl] pentanoic acid) is an agent which stabilises oxyhaemoglobin and thus reduces oxygen delivery to tissues. It is of interest as a possible potentiator of bioreductive agents and/or hyperthermia. The increases in radiobiological hypoxic fraction of RIF-1 and KHT tumours 30 min after 70 mg kg-1 BW12C i.v. were measured and shown to be similar; factors (+/- 2 s.e.) ranged from 3.87 (2.84-5.29) to 5.92 (1.92-18.2) despite the large variation in initial hypoxic fraction, from 0.30 (0.18-0.50) % for RIF-1 intramuscularly in the leg to 16.3 (14.7-18.1) % for subcutaneous KHT flank tumours. Thermosensitivity of intramuscular KHT leg tumours was not enhanced by 70 mg kg-1 BW12C 30 min before heating at 43 degrees C, 43.5 degrees C or 44 degrees C, assayed by regrowth delay. The effect of 70 mg kg-1 BW12C on relative tissue perfusion (RTP), assayed by 86Rb extraction, was measured from 0.5 h to 6 h after treatment. After 1 h RTP (+/- 2 s.e.) in RIF-1 tumours was reduced to 84 +/- 5.7% and 68 +/- 9.6% of control in leg and flank tumours respectively, and to 86 +/- 6.4% in leg muscle while flank skin RTP was unaltered at 109 +/- 8.6%. There were substantial increases in kidney (149 +/- 10.7%) spleen (173 +/- 22.1%) and lung (128 +/- 10.4%) at 1 h but in liver there was a decrease at 2 h to 85 +/- 8.4%. Dose response studies showed that the threshold dose for reduction of tumour RTP is between 55 and 70 mg kg-1, but perturbations in normal tissue RTP occur at lower doses, e.g. 40 mg kg-1 for spleen. BW12C had minimal effects on renal function measured by 51CrEDTA clearance. The data as a whole indicate that reduction in tumour perfusion is likely to be an important determinant in the increase in tumour hypoxia induced by BW12C.

A study of the mechanism of hydralazine enhancement of thermal damage in the KHT tumour.

The mechanism of the potentiation of thermal damage by hydralazine (HDZ) has been investigated. Using the KHT sarcoma in the leg of C3H mice, it was shown that 5 mg/kg of HDZ given i.v. 15-20 min before irradiation or heat exposure: (i) increased the radiobiological hypoxic fraction from 1 to 32%; (ii) produced a greater than additive growth delay when combined with heating for 30 min at either 43 or 43.5 degrees C, or 60 min at 43 degrees C; (iii) produced only additive cell killing when combined with 30 min heating at 43, 43.5, or 44 degrees C, assayed by clonogenic cell survival immediately or 24 h after treatment; and (iv) produced a prolonged (greater than 72 h) reduction in relative tissue perfusion (RTP) in the tumour when combined with heating for 30 min at 43.5 degrees C. The effects of HDZ or heat alone lasted for less than 24 and 48 h, respectively. The RTP in skin was unaffected by either agent or combination of agents at the times assayed. The results show that this 30-fold increase in hypoxia does not increase the intrinsic thermosensitivity of KHT tumour cells, and that the prolonged reduction in RTP caused by the combined treatment is probably responsible, at least in part, for the greater than additive component of the measured growth delay in this system. The data suggest that non-perfused tumour vessels are more heat sensitive than perfused vessels.

Effect of the manipulation of oxyhemoglobin status by BW12C on tumor thermosensitivity and on blood flow in tumor and normal tissues in mice.

The effect of 70 mgkg-1 BW12C 30 min before heating on the thermosensitivity of RIF-1 leg tumors was studied. This schedule is known to increase the hypoxic fraction by a factor of 5. Heating, using a combined radio-frequency and saline bath technique, was for 30 min at 43, 43.5, and 44 degrees C and response was assayed by clonogenic cell survival immediately and 24 hr after treatment. BW12C did not alter RIF-1 thermosensitivity. The effects of heat up to 44 degrees C on the oxygen saturation curves of normal and BW12C-modified blood were compared and P50s were shown to rise from 36 to 52 mm Hg and 6.5 to 8.0 mm Hg respectively, showing the latter to be relatively resistant to right-shifting by heat. 86Rb extraction studies on BW12C-treated unheated animals showed that blood flow in leg and flank tumours 60 min after BW12C was reduced to 64% and 34% of control values respectively, indicating a further mechanism for induction of tumour hypoxia by BW12C. Blood flow in leg muscle, liver, and spleen was unchanged but in kidney and lung was increased to 127% and 119% of control respectively 60 min after BW12C.

Stimulation by localized tumor hyperthermia of reductive bioactivation of 2-nitroimidazole benznidazole in mice.

We have investigated the effects of localized tumor hyperthermia (LTH; 43.5 degrees C x 30 min) on the reductive bioactivation of the 2-nitroimidazole benznidazole in C3H mouse normal tissues and KHT tumors. Mice were allocated to one of three treatment groups: (a) unrestrained controls, (b) sham tumor treatment, and (c) LTH. Concentrations of benznidazole and its amine metabolite were determined by high-performance liquid chromatography. Conscious mice were given LTH or sham treatment 2.5 h after 2.5 mmol/kg benznidazole i.p. This gave steady-state plasma benznidazole concentrations of 120-170 micrograms/ml at 2-5 h in all three groups. Plasma amine concentrations were very low at 0.1-1 micrograms/ml in all cases. Liver benznidazole concentrations were similar to plasma but amine concentrations were 30-40-fold greater at 20-40 micrograms/g in all three groups, implicating the liver as a major site of reductive metabolism. Benznidazole concentrations in tumors from unrestrained mice were comparable to those in plasma and liver, with tumor/plasma ratios of 85-113%. Tumor amine concentrations were intermediate at about 2-3 micrograms/g, indicating reductive bioactivation had occurred. Sham treatment decreased tumor benznidazole concentrations by 25-50%, particularly at later times, and amine concentrations were correspondingly increased. This may be a result of sham tumor treatment at 37 degrees C, a temperature 3-4 degrees C higher than in unrestrained controls. More importantly, LTH further decreased tumor benznidazole concentrations over sham treatment, e.g., by 59% from 114 to 47 micrograms/g (P less than 0.01) immediately after heating. Amine concentrations were correspondingly elevated, e.g., by 40% from 5.1 to 8.4 micrograms/g (P less than 0.01). These results clearly show that LTH can selectively enhance the reductive bioactivation of benznidazole in KHT tumors in mice, and support a particular role for the use of bioreductive agents with heat.

Effects of localised tumour hyperthermia on pimonidazole (Ro 03-8799) pharmacokinetics in mice.

We have investigated the effects of localised tumour hyperthermia (LTH; 43.5 degrees C x 30 min) on the acute toxicity and pharmacokinetics of the hypoxic cell sensitizer pimonidazole (Ro 03-8799) in mice. There were three treatment groups: unrestrained controls, sham-treated and LTH treated mice. LTH had minimal effects on the acute toxicity (LD50/7d) of pimonidazole with no significant difference between the three treatment groups. Pharmacokinetic studies were carried out at the maximum tolerated dose (MTD; approximately 60% LD50) of 437 micrograms g-1 i.v. in plasma, brain and tumour. Sham tumour treatment consistently increased plasma drug concentrations compared to unrestrained controls but had minimal effects on the elimination t1/2. The AUC0-infinitive was increased by 35% and the plasma clearance decreased by 26%. By contrast, LTH had minimal effects on these parameters compared to sham treatment. Brain pimonidazole concentrations were increased in restrained mice (sham and LTH treatments) compared to unrestrained controls, but average brain/plasma ratios were similar in all three groups at between 400 and 500%. Sham tumour treatment markedly reduced peak tumour pimonidazole concentrations compared to unrestrained controls giving a 29% lower AUC0-180min. Average tumour/plasma ratios were reduced from 236 to 129%. The most important finding was that LTH further reduced pimonidazole tumour concentrations, giving a 31% lower AUC0-180 min compared to sham treated tumours. Tumour/plasma ratios for pimonidazole were reduced by 41%. Plasma exposure to the pimonidazole N-oxide metabolite, Ro 31-0313, was unaltered by LTH. The markedly reduced drug concentrations in heated tumours may be a result of hyperthermia-stimulated bioreductive drug activation.

The effect of hyperthermia and radiation on small bowel permeability using 51Cr EDTA and 14C mannitol in man.

A prospective study was conducted to study the effect of radiotherapy (RT) and regional pelvic hyperthermia (HT) on intestinal permeability in three groups of patients. Fifteen acted as cancer controls, receiving RT away from the peritoneal cavity, 21 patients received radical pelvic or abdominopelvic RT and 13 patients pelvic RT followed by pelvic HT using a BSD 1000 phased array applicator. Small bowel permeability was measured by oral administration of a mixture of [51Cr]EDTA, [14C]mannitol and lactulose before and after a course of treatment. The absorption of each marker was calculated by measuring the urinary excretion over 0-6 and 0-12 hours. The 6 hour collection gave results similar to the 12 hour collection, but had logistical advantages. The EDTA absorption rose and the mannitol absorption fell during a course of treatment, but the best index of permeability change was the EDTA/mannitol ratio (E/M). The E/M ratio rose by a factor of 2.4 (P less than 0.001) and 1.82 (P = 0.05) following RT and RT/HT respectively. There was no significant difference between the RT and RT/HT groups but the thermal dose to the RT/HT group was low (23 min./equiv 43 degrees C over three or four fractions in 4 weeks). There was no correlation between small bowel permeability and bowel frequency. The E/M permeability test is a useful simple functional assay for assessing small bowel damage after RT and RT/HT.

Microwave hyperthermia in the treatment of spontaneous canine tumours: an analysis of treatment parameters and tumour response.

Fifty-one spontaneous canine tumours were treated with combined radiation and hyperthermia in a pilot study designed to investigate the feasibility and efficacy of the combined modality treatment in the dog. The tumours varied in site and histological type: 35 were in the oral cavity and included squamous cell carcinoma, melanoma and various 'sarcomas'. All animals received radiation (36-40 Gy in four weekly fractions) and post-irradiation hyperthermia (prescription -44 degrees C for 30 min) on one or two occasions. The microwave hyperthermia system was technically satisfactory in elevating tumour temperature to 44 degrees C or above in 95 per cent of treatments. However, thermal gradients of the order of 3-5 degrees C were frequently measured across the tumour, and rarely did all peripheral points achieve the target of 44 degrees C for 30 min. The overall tumour response rate (CR + PR) was 87.7 per cent with 60.7 per cent of tumours achieving complete regression. Smaller lesions showed a significantly greater response rate (P = 0.004) and those lesions which received two thermal treatments show an increased response (P = 0.0095). Fifty-one per cent of tumours showed significant necrosis following hyperthermia. Normal tissue necrosis was seen in three patients; in two cases this was attributed to hyperthermia. Local tumour control rate and necrosis was not correlated with measured minimum, mean or maximum thermal doses.

The effects of whole body hyperthermia on the pharmacokinetics and toxicity of the basic 2-nitroimidazole radiosensitizer Ro 03-8799 in mice.

We have investigated the effects of 50 min whole-body hyperthermia (WBH; 15 min equilibration followed by 41 degrees C for 35 min) on the toxicity and pharmacokinetics of the radiosensitizer Ro 03-8799 in mice. WBH markedly reduced Ro 03-8799 LD50/7d from 779 to 259 micrograms g-1 (P less than 0.001). Pharmacokinetics were studied at 175 micrograms g-1 (approximately 0.6 WBH LD50/7d) with and without heat and 437 micrograms g-1 (approximately 0.6 control LD50/7d) without heat. WBH increased Ro 03-8799 plasma concentrations and prolonged its elimination t1/2 by 26% (P less than 0.01). Total plasma area under the curve (AUC0-infinity) was increased by 22%, but was still less than 50% of the unheated high-dose value. Ro 03-8799 concentrated 300-400% in tumour and brain relative to plasma. Absolute tumour and brain levels were unaltered by WBH, giving reduced tissue/plasma ratios. WBH greatly inhibited glomerular filtration (51Cr EDTA clearance) during heating, contributing to the increased plasma Ro 03-8799 concentrations. WBH increased peak plasma concentrations of the Ro 03-8799 N-oxide metabolite Ro 31-0313 by 61% and the beta-phase AUC of i.v. administered Ro 31-0313 by 36%. Since Ro 31-0313 levels were increased to a greater extent after Ro 03-8799 and WBH than Ro 31-0313 and WBH, WBH must both increase metabolite production and decrease its plasma clearance. WBH had no effect on Ro 31-0313 tumour concentrations or its exclusion from brain. These complex effects of WBH on Ro 03-8799 pharmacokinetics may contribute to the enhanced toxicity, possibly through hyperthermia-stimulated bioreductive drug activation, but do not wholly explain it.