Miniature microwave applicator for murine bladder hyperthermia studies.

Purpose: Novel combinations of heat with chemotherapeutic agents are often studied in murine tumour models. Currently, no device exists to selectively heat small tumours at depth in mice. In this project we modelled, built and tested a miniature microwave heat applicator, the physical dimensions of which can be scaled to adjust the volume and depth of heating to focus on the tumour volume. Of particular interest is a device that can selectively heat murine bladder.

Materials And Methods: Using Avizo(®) segmentation software, we created a numerical mouse model based on micro-MRI scan data. The model was imported into HFSS™ (Ansys) simulation software and parametric studies were performed to optimise the dimensions of a water-loaded circular waveguide for selective power deposition inside a 0.15 mL bladder. A working prototype was constructed operating at 2.45 GHz. Heating performance was characterised by mapping fibre-optic temperature sensors along catheters inserted at depths of 0-1 mm (subcutaneous), 2-3 mm (vaginal), and 4-5 mm (rectal) below the abdominal wall, with the mid depth catheter adjacent to the bladder. Core temperature was monitored orally.

Results: Thermal measurements confirm the simulations which demonstrate that this applicator can provide local heating at depth in small animals. Measured temperatures in murine pelvis show well-localised bladder heating to 42-43°C while maintaining normothermic skin and core temperatures.

Conclusions: Simulation techniques facilitate the design optimisation of microwave antennas for use in pre-clinical applications such as localised tumour heating in small animals. Laboratory measurements demonstrate the effectiveness of a new miniature water-coupled microwave applicator for localised heating of murine bladder.