Session Item

Alternating electric fields known as Tumor Treating Fields (TTFields) are used to treat glioblastoma multiforme (GBM). The intensity of the electric field has formerly been used to describe TTFields dose, but for this physical modality, the energy transferred by the treatment modality to the tissue is to consider to comprehensively capture the dose. This is because this measure quantifies the extent of alteration the modality can cause on the state of the objects on which it operates. The power (energy per time unit) the electric field transfers to the tissue is quantified as power loss density, which can be used as a measure of TTFields dose. In this study, we analyzed this new measure as a basis for further studies and its use in treatment planning.

With σ denoting tissue conductivity and E the magnitude of the electric field, the power loss density (L) of TTFields is defined as L[mW/cm³]=½σE². We numerically simulated TTFields delivery to realistic head models of GBM patients to analyze the power loss density distribution of TTFields released to the brain. Color maps showing the field intensity and power loss density distribution within the models were generated and compared in a qualitative manner. Calculation of the total power loss within the models yielded a measure of the power that TTFields released to the brain during treatment.

In regions of low conductivity, e.g. white matter, we detected an increase in electric field intensity, whereas in regions of high conductivity, e.g. the resection cavities or ventricles, field intensity was lowest. Power loss density, on the contrary, was found to be enhanced in regions of higher conductivity, even up to values as they are found in other tissue types within regions of high conductivity such as ventricles or resection cavity. Within the gross tumor volumes of all patients, the average power loss density was 5 mW/cm³ and the total power loss of TTFields in our simulated cases added up to 20-40 W. This corresponds to 412-825 kcal per day, an amount similar to the resting metabolic rate of the brain constituting about 20% of the whole body’s metabolic rate.

Our study demonstrated power loss density as a reasonable physical measure for quantitative determination of TTFields dose in treatment planning. In addition, we presented a comparison of the power delivered by TTFields to cells and the metabolic rate of the cells resulting in equivalent values. Our findings could lead to more insight into the mechanism of action of TTFields.