Comparison of two-dimensional numerical approximation and measurement of SAR in a muscle equivalent phantom exposed to a 915 MHz slab-loaded waveguide

Hyperthermia treatment planning and temperature distribution reconstruction: a case study

While a great deal of effort has been applied toward solving the technical problems associated with modelling clinical hyperthermia treatments, much of that effort has focused on only estimating the power deposition. Little effort has been applied toward using the modelled power depositions (either electromagnetic (EM) or ultrasonic) as inputs to estimate the hyperthermia induced three-dimensional temperature distributions. This paper presents a case report of a patient treated with hyperthermia at the Duke University Medical Center where numerical modelling of the EM power deposition was used to prospectively plan the treatment. Additionally, the modelled power was used as input to retrospectively reconstruct the transient three-dimensional temperature distribution. The modelled power deposition indicated the existence of an undesirable region of high power in the normal tissue. Based upon this result, amplitudes and phases for driving the hyperthermia applicator were determined that eliminated the region of high power and subsequent measurements confirmed this. The steady-state and transient three-dimensional temperature distributions were reconstructed for four out of the seven treatments. The reconstructed steady-state temperatures agreed with the measured temperatures; root-mean-square error ranged from 0.45 to 1.21 degrees C. The transient three-dimensional tumour temperature was estimated assuming that the perfusion was constant throughout the treatment. Using the computed three-dimensional transient temperature distribution, the hyperthermia thermal dose was computed. The equivalent minutes at 43 degrees C achieved by 50% (T50Eq43) of the tumour volume was computed from the measured data and the three-dimensional reconstructed distribution yielding T50Eq43 = 40.6 and 19.8 min respectively.

Finite element simulation of Sigma 60 heating in the Utah phantom: computed and measured data compared

An initial series of comparisons are made between finite element computations and laboratory measurements obtained during heterogeneous phantom heating with the Sigma 60 applicator. The phantom is a relatively complex, though still idealized, rendering of the pelvic area which has been used to study the deep heating characteristics of the Sigma 60 in this anatomy. Direct electric field measurements as well as inferred SAR through transient temperature analysis are plotted against computed results along 11 one-dimensional tracks through the phantom. Quantitative comparisons provided through the track-by-track analysis show generally good agreement between computation and measurement. The finite element method is found to predict well the jumps in the electric field when polarized perpendicularly to a muscle/fat interface. Visualizations of the complete three-dimensional distributions are also highlighted and correlate well with physical reasoning about the expected behaviour of the fields produced. Some discrepancies in the data persist and are discussed and analysed in depth. They underscore the difficulties that can arise in performing comparisons between measured and computed results and stress the need for careful and thorough investigations when attempting these types of model validation studies.

Feasibility of estimating the temperature distribution in a tumor heated by a waveguide applicator

The feasibility of using a 2-dimensional (2D) modeling approach for retrospectively describing complete temperature distributions in the midplane of a tumor during a clinical hyperthermia treatment was tested. An experimental treatment, using a 915-MHz waveguide applicator to heat a large melanoma in a dog, was modeled. Detailed measurements of temperatures were made during the treatment. The steady-state blood flow distribution at the midplane was imaged by positron emission tomography (PET), and these data were used to prescribe the modeled perfusion pattern. A 2D finite element method (FEM) was used to approximate the solution to Maxwell's Equations to obtain the specific absorption rate (SAR) distribution. The blood-flow estimates, assumed material properties, SAR distribution, and temperature boundary conditions were then used with the same mesh in a second FEM program to obtain a solution to the bioheat transfer equation. This latter routine was embedded in a state-and-parameter-estimation program that systematically varied selected parameters until the differences between computed and measured temperatures were minimized. Optimizations were performed independently for three subsets of the measured temperature data to assess the sensitivity of the predicted temperature field to the number of measurements. The calculated temperature distributions that resulted were similar to each other, and the predicted temperatures at the sensor points excluded from these optimizations were in reasonable agreement with the measurements. However, lack of unique blood flow values following optimization indicates that the methods of estimating blood flow will need to be improved or that there are problems with model mismatch. This work is a clinical case study of an evolving 2D system of thermal dosimetry which relies on both empirical and theoretical concepts. The methodology is being evaluated for its ability to generate prognostically significant descriptors of the treatment temperature field.