Modified thermal clearance technique for determination of blood flow during local hyperthermia

A moderate thermal dose is sufficient for effective free and TSL based thermochemotherapy

Hyperthermia, i.e. heating the tumor to a temperature of 40-43 °C is considered by many a valuable treatment to sensitize tumor cells to radiotherapy and chemotherapy. In recent randomized trials the great potential of adding hyperthermia to chemotherapy was demonstrated for treatment of high risk soft tissue sarcoma: +11.4% 5 yrs. overall survival (OS) and for ovarian cancer with peritoneal involvement nearly +12 months OS gain. As a result interest in combining chemotherapy with hyperthermia, i.e. thermochemotherapy, is growing. Extensive biological research has revealed that hyperthermia causes multiple effects, from direct cell kill to improved oxygenation, whereby each effect has a specific temperature range. Thermal sensitization of the tumor cell for chemotherapy occurs for many drugs at temperatures ranging from 40 to 42 °C with little additional increase of sensitization at higher temperatures. Increasing perfusion/oxygenation and increased extravasation are two other important hyperthermia induced mechanisms. The combination of free drug and hyperthermia has not been found to increase tumor drug concentration. Hence, enhanced effectiveness of free drug will depend on the thermal sensitization of the tumor cells for the applied drug. In contrast to free drugs, experimental animal studies combining hyperthermia and thermo-sensitive liposomal (TSL) drugs delivery have demonstrated to result in a substantial increase of the drug concentration in the tumor. For TSL based chemotherapy, hyperthermia is critical to both increase perfusion and extravasation as well as to trigger TSL drug release, whereby the temperature controlled induction of a local high drug concentration in a highly permeable vessel is driving the enhanced drug uptake in the tumor. Increased drug concentrations up to 26 times have been reported in rodents. Good control of the tissue temperature is required to keep temperatures below 43 °C to prevent vascular stasis. Further, careful timing of the drug application relative to the start of heating is required to benefit optimal from the combined treatment. From the available experimental data it follows that irrespective whether chemotherapy is applied as free drug or using a thermal sensitive liposomal carrier, the optimal thermal dose for thermochemotherapy should be 40-42 °C for 30-60 min, i.e. equivalent to a CEM43 of 1-15 min. Timing is critical: most free drug should be applied simultaneous with heating, whereas TSL drugs should be applied 20-30 min after the start of hyperthermia.

The response of human tumour blood flow to a fractionated course of thermoradiotherapy

The response of human tumour blood flow to a fractionated course of thermoradiotherapy was documented in four superficial but bulky tumours (three adenocarcinomas, one melanoma). Blood flow was measured 15, 30, 45, and 60 min after the onset of heating. These measurements were made at the same intra-tumour point during each heat fraction by use of a modified thermal clearance technique in which a correction was made for the heat dissipated by thermal conduction. This point was at least 2 cm beneath the surface in the central portion of the tumour. Extracellular pH was measured within 1 cm of this point prior to the first heat fraction and 2-3 weeks later. Hyperthermia was administered for 60 min, twice a week for 4 weeks by use of a 16-channel 915 MHz microwave applicator. Each patient also received a radiation dose of 40 Gy fractionated at 2 Gy/fx, five times a week (adenocarcinomas) or 4 Gy/fx, twice a week (melanoma). Blood flow remained relatively constant during heating after steady state conditions were attained. However, an overall decrease in tumour blood flow was observed in each patient over the course of thermoradiotherapy. In each case, a relatively small decrease in blood flow occurred between most heat fractions which resulted in an overall decrease which ranged from 50-100%. However, there was a tendency for blood flow to increase following the initial heat fraction at points where the steady state temperature was approximately 41 degrees C or less. Extracellular pH increased in two of three patients and decreased in the other.

Application of the time-dependent Green's function and Fourier transforms to the solution of the bioheat equation

A theory for solving the bioheat equation is developed using a time-dependent Green's function and Fourier transform techniques. The description of both steady-state and time-dependent data are placed into a single framework which can also describe the effects of inhomogeneous blood perfusion. The theory is illustrated by examples including the modelling of a thermal conduction hyperthermia system and a new RF interstitial system. A possibility for measuring the blood perfusion parameter and thermal conductivity from the steady state temperature distribution of a point source is also proposed.