After building the bipolar radio frequency hyperthermia model, proceed to set up the electrical problem. Right click on the electric currents node. For the electrical boundary conditions in the displayed figure, select terminal and ground as boundaries.
For terminal, manually select the proximal end of the top of one of the two needles. The identified needle will provide the input voltage. Then under terminal, select power, and specify the appropriate value.
For this protocol, select 0.5 watt for mild hyperthermia, based on preliminary ex vivo experiments. Next, select ground and manually select the proximal surface of the second needle, acting as a return electrode for the returning electrical current path. Apply electrical insulation to the remaining external surface of the model.
To set up for the thermal problem, select heat transfer in solids node and specify 33 degrees Celsius as the initial value of the temperature. To model the heat sink effect due to the blood flow, right click on heat transfer in solids, add the heat source domain, and select the geometry where the heat sink effects should be considered, which are tumor and normal tissue. Select general source, followed by user-defined, and type the expression for the heat sink.
For the thermal boundary conditions in the displayed figure, right click on heat transfer, add heat flux as a boundary condition, and specify the external surfaces to which the heat flux is applied. Select convective heat flux as the flux type. For the heat transfer coefficient, use H equals 215 watt per meter squared per kelvin to model the mechanism of natural heat exchange between the skin and the air.
Specify the external temperature. Use T equals 20 degrees Celsius to model the ambient temperature in the laboratory environment. To set up the fluid dynamics problem, select coefficient form PDE node and specify pressure as the dependent variable.
At this stage, the unit Pascal is automatically assigned. Specify fluid conductance unit one per second as source term quantity. Define the name to identify the variable PI, or interstitial fluid pressure in this study.
Next, right click on coefficient form PDE node and select the coefficient form domain. Specify the geometrical entity to which the equation refers as tumor. Repeat the same steps and select the remaining tissue as normal tissue, to which a different PDE needs to be applied.
For the tumor model, specify the displayed coefficients in terms to obtain the conservation of mass equation. For the tumor model, neglect the contribution of the lymphatic system. Set all other coefficients equal to zero.
Similarly, for the normal tissue model, specify the displayed coefficients in terms to obtain the conservation of mass equation. For normal tissue, consider the contribution of the lymphatic system. Set all other coefficients equal to zero.
Then right click on coefficient form PDE and select initial values. Select the geometrical domain as tumor, and repeat the same step for the normal tissue model. Specify PI zero for tumor and normal tissue, according to the values listed in the displayed table.
For the boundary conditions related to the fluid dynamic study shown in the displayed figure, right click on coefficient form PDE and select Dirichlet boundary conditions. Select the external surface of the normal tissue domain and assign the initial value of interstitial pressure, indicated as PI zero, corresponding to the normal tissue. Finally, to run the simulations, select frequency transient from the study node.
Specify the time unit as seconds, and set the frequency as 500 kilohertz. After 15 minutes of the simulated heating with 0.5 watt applied power, more than 50%of the tumor volume reached a state of mild hyperthermia, with the temperature in the region of the tumor closest to the needle exceeding 45 degrees Celsius. Compared to the initial conditions, the interstitial fluid pressure gradually decreased from nine millimeters of mercury in the center of the tumor to zero at the edge.
Fluid velocity did not exceed 0.2 micron per second within the entire tumor domain, including the periphery. The interstitial fluid pressure over time changed differently at different radial distances from the heat source. Within three millimeters from the needles, the fluid pressure responded to the rapid increase in the temperature, but finally showed no change at the end of the heating.
However, the pressure in the remaining part of the tumor continuously decreased.
