A microcomputer controlled, local hyperthermia system for uniform tumour heating in unanaesthetized mice

Radiofrequency capacitive heaters: the effect of coupling medium resistivity on power absorption along a mouse leg

A capacitive radiofrequency source in conjunction with a temperature-controlled electromagnetic coupling medium has the potential of delivering uniform heating distributions in a mouse leg for experimental studies to investigate the use of hyperthermia as a treatment for cancer. The system has been adopted by a number of groups who have confirmed that uniform temperatures can be achieved in the presence of blood flow along a one-dimensional line which extends between the plates across the leg. In this paper, a simple mathematical model is presented and verified experimentally to demonstrate that parallel-plate capacitive radiofrequency heaters produce an inherent absorbed power distribution along the leg which is determined by the impedances across the loads. Hence, the thicker or thinner regions of the leg can be preferentially heated by using a coupling medium with lower or higher salinity, respectively. Uniform power absorption along the mouse leg required that the coupling medium had equivalent electrical properties to those of tissue.

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.