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Millimeter-wave heating in vitro: local microscale temperature measurements correlated to heat shock cellular response

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Millimeter (MMW)-induced heating can be exploited in the biomedical field for non-invasive and spatially localized thermal treatment of superficial skin cancers, such as spreading melanoma. Cellular response to heat may largely vary depending on the cell type, temperature, and exposure duration. For this reason, the success of a thermal treatment strongly depends on 1) the specific sensitivity to heat of the tissue of interest, 2) the precise control of the temperature elevation during the treatment, and 3) focusing of thermal energy in the target area avoiding the damage of the non-cancerous tissue in proximity of the tumor.

In this paper, we study the sensitivity of the A375 melanoma cell line to continuous wave-heating at 58.4 GHz between 37 and 47◦C. A metallic micro thermocouple (µTC) was used to record the temperature elevation during the exposure. Numerical and experimental microdosimetry was performed to yield electromagnetic and thermal field distributions as well as specific absorption rate (SAR) and temperature elevation.

To evaluate the heat-induced cellular stress, phosphorylation of a small heat shock protein, e.g., HSP27, was quantified as a function of the distance from the MMW radiation source using an experimental approach based on fluorescence microscopy.

Cells were exposed for 90 minutes between 37 and 47◦C induced by varying the input power between 120 and 430 mW (SAR values between 4.6 and 17 kW/kg).

Our results demonstrated that the HSP27 response is triggered at temperatures ≥ 41 ◦C, and it increases exponentially with temperature and thermal dose. In addition, we showed that the thermal stress is mainly focused within a small area with a radius of 1.8 mm, suggesting that MMW heating can be efficiently and selectively focused within a small region of interest. Results of this study contribute to preliminary planning and optimization of clinical treatment of superficial spreading melanoma.

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