A Guide for Water Bolus Temperature Selection for Semi-Deep Head and Neck Hyperthermia Treatments Using the HYPERcollar3D Applicator

SIMPLE SUMMARY: Hyperthermia cancer treatment is used as an adjuvant treatment modality to standard radiotherapy and/or chemotherapy treatments. The HYPERcollar3D allows focused microwave heating to 40–44 °C in the head and neck region. A flexible bolus is placed between patient and applicator, and deionized water is circulated, to improve power transfer efficiency and allow for surface cooling. The temperature at which this water is controlled influences the temperature distribution in this target region but its influence was unknown. To understand the impact of the water bolus temperature we performed a simulation study. We experimentally established the mean heat transfer coefficient for the water bolus as 292 W m(−2)K(−1) (range 59–520). Then, we studied the influence of the water bolus temperature on temperatures in the target region using 20 patient specific 3D models. We found that for targets located up to 20 mm from the surface (median depth), the water bolus HYPERcollar3D temperature 30 °C can be increased to 35 °C, which will increase the temperature in the target region and thus translates to overall improvements in the hyperthermia treatment quality. ABSTRACT: During hyperthermia cancer treatments, especially in semi-deep hyperthermia in the head and neck (H&N) region, the induced temperature pattern is the result of a complex interplay between energy delivery and tissue cooling. The purpose of this study was to establish a water bolus temperature guide for the HYPERcollar3D H&N applicator. First, we measured the HYPERcollar3D water bolus heat-transfer coefficient. Then, for 20 H&N patients and phase/amplitude settings of 93 treatments we predict the T50 for nine heat-transfer coefficients and ten water bolus temperatures ranging from 20–42.5 °C. Total power was always tuned to obtain a maximum of 44 °C in healthy tissue in all simulations. As a sensitivity study we used constant and temperature-dependent tissue cooling properties. We measured a mean heat-transfer coefficient of h = 292 W m(−2)K(−1) for the HYPERcollar3D water bolus. The predicted T50 shows that temperature coverage is more sensitive to the water bolus temperature than to the heat-transfer coefficient. We propose changing the water bolus temperature from 30 °C to 35 °C which leads to a predicted T50 increase of +0.17/+0.55 °C (constant/temperature-dependent) for targets with a median depth < 20 mm from the skin surface. For deeper targets, maintaining a water bolus temperature at 30 °C is proposed.