Optimal power deposition patterns for ideal high temperature therapy/hyperthermia treatments

Hyperthermia: How Can It Be Used?

Hyperthermia (HT) is a method used to treat tumors by increasing the temperature of the cells. The treatment can be applied in combination with other verified cancer treatments using several different procedures. We sought to present an overview of the different HT tumor treatment, recent advances in the field, and combinational treatment sequences and outcomes. We used a computer-aided search to identify articles that contained the keywords hyperthermia, cancer treatment, chemotherapy, radiotherapy, nanoparticle, and cisplatin. There are three types of HT treatment, which each need the use of applicators that are in contact with or in the proximity of the patient for the purpose of heating. Heating can be achieved using different types of energy (including microwaves, radio waves, and ultrasound). However, the source of energy will depend on the cancer type and location. The temperature used will also vary. HT is rarely used alone, and can be combined with other cancer treatments. When used in combination with other treatments, improved survival rates have been observed. However, despite in vitro and in vivo studies that support the use of concurrent hypothermia treatments, contradictory results suggest there is a need for more studies to identify other hidden effects of HT.

Higher temperature improves the efficacy of magnetic fluid hyperthermia for Lewis lung cancer in a mouse model

PURPOSE

The aim of this study was to investigate the effect of a higher temperature on the efficacy of magnetic fluid hyperthermia for Lewis lung cancer in a mouse model.

MATERIALS AND METHODS

Magnetic fluids were prepared in vitro and directly injected into tumors. Twenty-four hours later, the mice were subjected to an alternating magnetic field. The temperature in the tumor was increased to 46.0°C, higher than the usual temperature used in hyperthermia therapy. The higher temperature was maintained for 30 min with a stable strength of magnetic field.

RESULTS

Magnetic fluid hyperthermia with a higher temperature significantly inhibited the growth of the tumors (P < 0.05). The tumors completely regressed in four out of 12 mice. Histological analysis demonstrated that the tumor cells underwent apoptosis and necrosis, and the cells were arrested at the G1/G0 phase of the cell cycle. The lifespan of the treated animals also increased significantly (P < 0.05).

CONCLUSIONS

Magnetic fluid hyperthermia with a higher temperature could improve the efficacy of this therapy on lung cancer.

Online feedback focusing algorithm for hyperthermia cancer treatment

PURPOSE

Magnetic resonance (MR) imaging is increasingly being utilized to visualize the 3D temperature distribution in patients during treatment with hyperthermia or thermal ablation therapy. The goal of this work is to lay the foundation for improving the localization of heat in tumors with an online focusing algorithm that uses MR images as feedback to iteratively steer and focus heat into the target.

METHODS

The algorithm iteratively updates the model that quantifies the relationship between the source (antenna) settings and resulting tissue temperature distribution. At each step in the iterative process, optimal settings of power and relative phase of each antenna are computed to maximize averaged tumor temperature in the model. The MR-measured thermal distribution is then used to update/correct the model. This iterative procedure is repeated until convergence, i.e. until the model prediction and MR thermal image are in agreement. A human thigh tumor model heated in a 140 MHz four-antenna cylindrical mini-annular phased array is used for numerical validation of the proposed algorithm. Numerically simulated temperatures are used during the iterative process as surrogates for MR thermal images. Gaussian white noise with a standard deviation of 0.3 degrees C and zero mean is added to simulate MRI measurement uncertainty. The algorithm is validated for cases where the source settings for the first iteration are based on erroneous models: (1) tissue property variability, (2) patient position mismatch, (3) a simple idealized patient model built from CT-based actual geometry, and (4) antenna excitation uncertainty due to load dependent impedance mismatch and antenna cross-coupling. Choices of starting heating vector are also validated.

RESULTS

The algorithm successfully steers and focuses a tumor when there is no antenna excitation uncertainty. Temperature is raised to > or = 43 degrees C for more than about 90% of tumor volume, accompanied by less than about 20% of normal tissue volume being raised to a temperature > or = 41 degrees C. However, when there is antenna excitation uncertainty, about 40% to 80% of normal tissue volume is raised to a temperature > or = 41 degrees C. No significant tumor heating improvement is observed in all simulations after about 25 iteration steps.

CONCLUSIONS

A feedback control algorithm is presented and shown to be successful in iteratively improving the focus of tissue heating within a four-antenna cylindrical phased array hyperthermia applicator. This algorithm appears to be robust in the presence of errors in assumed tissue properties, including realistic deviations of tissue properties and patient position in applicator. Only moderate robustness was achieved in the presence of misaligned applicator/tumor positioning and antenna excitation errors resulting from load mismatch or antenna cross coupling.

An analytical study of ‘Poisson conduction shape factors’ for two thermally significant vessels in a finite, heated tissue

To conveniently and properly account for the vessel to vessel and vessel to tissue heat transfer rates to predict in vivo tissue temperature distributions, this paper analyses two different types of Poisson conduction shape factors (PCSFs) for unheated and/or uniformly heated, non-insulated, finite tissue domains. One is related to the heat transfer rate from one vessel to another (vessel-vessel PCSF (VVPCSF)) and the other is related to the vessel to tissue heat transfer rates (vessel-tissue PCSF (VTPCSF)). Two alternative formulations for the VTPCSFs are studied; one is based on the difference between the vessel wall and tissue boundary temperatures, and the other on the difference between the vessel wall and the average tissue temperatures. The effects of a uniform source term and of the diameters and locations of the two vessels on the PCSFs are studied for two different cases: one, when the vessel wall temperatures are lower than the tissue boundary temperature, i.e., the vessels cool the tissue, and vice versa. Results show that, first, the VVPCSFs are only geometry dependent and they do not depend on the applied source term and the vessel wall and tissue boundary temperatures. Conversely, the VTPCSFs are strong functions of the source term and of the temperatures of the vessel walls and tissue boundary. These results suggest that to account for the vessel to vessel heat transfer rates, the VVPCSFs can be evaluated solely based on the vessel network geometry. However, to account for the vessel to tissue heat transfer rates, the VTPCSFs should be used iteratively while solving for the tissue temperature distributions. Second, unlike the tissue boundary temperature-based VTPCSFs which may become singular only in heated tissues, the average tissue temperature-based VTPCSFs have the potential to become singular in both unheated and heated tissues. These results suggest that caution should be exercised in the use of the VTPCSFs since they may approach singularity by virtue of their definition and thus may introduce large errors in the evaluation of tissue temperature distribution. Presented results are new and complementary to the previous shape factor results since these include the effect of (1) source term and (2) unequal vessel-tissue heat transfer rates from the two vessels to the tissue.

Blood perfusion and thermal conduction effects in Gaussian beam, minimum time single-pulse thermal therapies

A previous analytical study has shown that the minimum obtainable treatment time for a single pulse that delivers a given thermal dose to a specified point at a specified time occurs when the temperature at that point is rapidly raised to its maximum allowable value. The present study extends that result by investigating the spatial distribution of thermal effects of a single Gaussian shaped focal zone pulse that reaches that maximum allowable temperature at the center point of the focal zone. Analytical solutions are obtained that separately include the effects of perfusion and conduction. This situation is analyzed for a conservative treatment strategy in which the desired thermal dose is delivered when the tumor cools down to basal conditions. The results show that for a specified thermal dose delivered by a spherical Gaussian beam with focal widths below approximately 4 mm, the maximum allowable temperature, the minimum obtainable treatment time, and the size of the treatment zone (as a percentage of the size of the Gaussian beam) are all independent of the tissue blood perfusion, and are only functions of the focal zone size. Conversely, for focal widths above approximately 20 cm, these results are independent of the focal width and are only functions of blood perfusion. Between these two sizes (where most practical treatments will occur, since single pulses with widths of <4 mm and >20 cm will be uncommon in practice) a transition zone exists in which both perfusion and conduction effects are important. Thus while it is possible to implement a truly perfusion-independent, single pulse thermal treatment by using focal widths of <4 mm, in practice many such pulses will be needed to treat most tumors. This is especially true since the nonlinear temperature/thermal dose relationship causes the width of the delivered dose distribution to be only approximately 25%-30% of the width of the focal zone. However, shorter overall treatment times can be obtained when multiple pulses are linked together by using larger focal zone sizes, but this gain in treatment time is accompanied by increased effects of perfusion, illustrating the conflict between attaining both perfusion-independence and minimal treatment time for multiple-pulse thermal treatments.