Tumor Treating Fields (TTFields) is a non-invasive technique used in the treatment of glioblastoma. It consists in applying an electric field with a frequency of 200 kHz in two perpendicular directions alternately. This technique has an anti-mitotic effect if the induced field is at least 1 V/cm at the tumor bed. The Optune device, used to deliver these fields in real subjects, controls the injected current to keep scalp's temperature below potentially harmful values by controlling the temperature of the transducers. The thermal impact of this technique was recently predicted by our group. In this work we aim to investigate how the uncertainties regarding both the electric and thermal properties of tissues, transducers and gel affect the maximum temperature reached by each tissue and might affect the time that the device is applying the fields due to its thermal restrictions. We used a realistic head model created from MR images to perform our studies and run the simulations in COMSOL Multiphysics. After an extensive literature review, we selected a reasonable range of values for each property and performed a one-way sensitivity analysis. To obtain the temperature distribution we solved Pennes' equation in frequency domain studies. We observed that the most significant electric parameters are scalp, skull and gel's conductivity values. In general, an increase in the conductivity led to higher temperatures, as expected. In terms of changes in the temperature reached by the transducers it was seen that it can vary between +1.3°C (when scalp's conductivity is increased by a factor of 80% to 0.45 S/m) and -1.7°C (scalp's conductivity decreased to 0.14 S/m). The thermal parameters that are more critical are the ones that can directly change how quickly heat is transferred to the environment, namely scalp's properties. Transducer temperature varied between -0.6°C and +1.3°C when scalp's thermal conductivity was increased by 46% and decreased by 53%, respectively. Furthermore, the sweat rate and the convection factor and emissivity of the medical tape that covers the arrays also led to significant temperature variations. The latter analysis can be used to study new types of materials that can be more appropriate to apply these fields. The remaining electric (relative permittivity) and thermal (metabolic heat, blood perfusion, blood density and specific heat and thermal conductivity of skull, CSF and brain) parameters did not lead to significant temperature variations. These results show the importance of precisely knowing the most crucial physical properties in TTFields modelling studies. Due to the very strict current control injection mode of Optune based on the temperature of the transducers, it would be beneficial to experimentally measure the properties here discussed.
Citation Format: Nichal Gentilal, Pedro Cavaleiro Miranda. Influence of the thermal and electric properties of biological tissues on the maximum temperature during TTFields therapy [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5484.