Three-dimensional theoretical temperature distributions produced by 915 MHz dipole antenna arrays with varying insertion depths in muscle tissue

Application of High Amplitude Alternating Magnetic Fields for Heat Induction of Nanoparticles Localized in Cancer

OBJECTIVE

Magnetic nanoparticles conjugated to a monoclonal antibody can be i.v. injected to target cancer tissue and will rapidly heat when activated by an external alternating magnetic field (AMF). The result is necrosis of the microenvironment provided the concentration of particles and AMF amplitude are sufficient. High-amplitude AMF causes nonspecific heating in tissues through induced eddy currents, which must be minimized. In this study, application of high-amplitude, confined, pulsed AMF to a mouse model is explored with the goal to provide data for a concomitant efficacy study of heating i.v. injected magnetic nanoparticles.

METHODS

Thirty-seven female BALB/c athymic nude mice (5-8 weeks) were exposed to an AMF with frequency of 153 kHz, and amplitude (400-1,300 Oe), duration (1-20 minutes), duty (15-100%), and pulse ON time (2-1,200 seconds). Mice were placed in a water-cooled four-turn helical induction coil. Two additional mice, used as controls, were placed in the coil but received no AMF exposure. Tissue and core temperatures as the response were measured in situ and recorded at 1-second intervals.

RESULTS

No adverse effects were observed for AMF amplitudes of < or = 700 Oe, even at continuous power application (100% duty) for up to 20 minutes. Mice exposed to AMF amplitudes in excess of 950 Oe experienced morbidity and injury when the duty exceeded 50%.

CONCLUSION

High-amplitude AMF (up to 1,300 Oe) was well tolerated provided the duty was adjusted to dissipate heat. Results presented suggest that further tissue temperature regulation can be achieved with suitable variations of pulse width for a given amplitude and duty combination. These results suggest that it is possible to apply high-amplitude AMF (> 500 Oe) with pulsing for a time sufficient to treat cancer tissue in which magnetic nanoparticles have been embedded.

Fast and efficient computer modeling of ferromagnetic seed arrays of arbitrary orientation for hyperthermia treatment planning

PURPOSE

Effective hyperthermia treatment planning requires an ability to predict temperatures quickly and accurately from an arbitrary distribution of power. Our purpose was to design such a fast executing computer code, MGARRAY, to compute steady-state temperatures from ferromagnetic seed heating, allowing seeds to have arbitrary orientations and to be curved to permit more realistic modeling of clinical situations. We further required flexibility for the tissue domain, allowing inhomogeneity with respect to thermal conductivity and blood perfusion, as well as an arbitrary shaped boundary.

METHODS AND MATERIALS

MGARRAY uses multigrid methods and a finite volume discretization to solve the Pennes bioheat transfer equation in three dimensions. We used MGARRAY to compare temperature distributions that result from an array of straight, parallel seeds and from an array of seeds that were curved and tilted randomly by 13 degrees.

RESULTS

On a personal workstation the Central Processing Unit (CPU) time of MGARRAY was under 4 min. We found that the median temperature in a predetermined target volume was approximately 0.8 degrees C higher in the straight array than in the curved array. At specific locations within the target volume temperature differed by approximately 0.5-0.9 degrees C, but could differ by up to several degrees, depending on proximity to a seed and the level of blood perfusion.

CONCLUSION

These differences can impact on retrospective analyses whereby temperatures at a few locations are used to infer the overall temperature field and blood perfusion levels. The flexibility and computational speed of MGARRAY could potentially lead to a substantial improvement in both retrospective and prospective hyperthermia treatment planning.

Multivariate analysis of prognostic parameters using interstitial thermoradiotherapy (IHT-IRT): tumor and treatment variables predict outcome

PURPOSE

From January 1986 to October 1991, 90 patients with localized tumors were treated in a Phase I/II trial using low-dose 192Ir brachytherapy (IRT) plus interstitial 915 MHz microwave (MW) hyperthermia (IHT) and external beam radiotherapy (ERT). Tumors were classified as locally advanced primary (class 1: 27), recurrent (class 2: 40), metastatic (class 3: 10) and persistent (class 4: 13) lesions. The treatment sites included tumors of the head and neck (62), pelvis (26), and others (2). The mean cuboidal tumor volume was 63 cm3 (range: 8-288 cm3). Most recurrent and metastatic lesions (48) had received prior treatment including ERT.

METHODS AND MATERIALS

The treatment protocol prescribed two heating sessions (each 60 min) at 41-44 degrees C before and after IRT. One hundred sixty-one IHT sessions were evaluated. Invasive thermal data were recorded at an average of 18 sites throughout the implant volume. Several thermal variables were analyzed (e.g., averaged parameters: Tmaxav, Tmean, Tminav; index parameters: T10, T50, T90). The study was evaluated with a minimum follow up (FU) of one year. Median follow-up was 19 months.

RESULTS

At 3 months FU, a complete response (CR) was observed in 59 of 90 (66%) patients. At 12 months FU, local control (LC) was achieved in 54 of 84 (64%) evaluable patients. Ten patients developed a local and 14 a regional recurrence (REC) after achieving a CR and/or LC. At last FU, a total of 31 (34%) patients were still alive and 28 (31%) patients had relapse-free survival. For all 90 patients, the median overall survival was 20 months and the median relapse-free survival was 17 months. Overall and relapse-free survival was significantly longer for primary and persistent lesions as compared to recurrent and metastatic lesions (p = 0.002; p < 0.001). Totally 22 (24%) patients experienced acute or subacute side-effects (Grade 1: 12 patients; Grade 2: eight patients; Grade 3: two patients).

CONCLUSION

Univariate logistic regression analysis revealed significant dependencies of CR, LC and REC upon tumor parameters as well as radiation and thermal parameters. The overall and relapse free survival was associated with tumor and radiation parameters. The multivariate analysis revealed two independent predictors of CR: tumor volume and minimum tumor temperature variables. We conclude, that IHT-IRT is a safe and effective treatment. The results provide important implications for planning HT-RT studies and for defining quality assurance (QA) criteria and thermal performance standards in HT studies.

A theoretical model for input impedance of interstitial microwave antennas with choke

PURPOSE

Two important characteristics for interstitial microwave antennas used in clinical hyperthermia are: (1) a good impedance match to minimize reflected power; and (2) a good power deposition pattern which is independent of insertion depth. A major problem of the miniature coaxial dipole antennas used for interstitial hyperthermia is the fact that the impedance and power deposition patterns of these antennas change with insertion depth. One possible solution is the addition of a coaxial choke. A theoretical model for calculating the input impedance of interstitial microwave antennas having a coaxial choke is presented, which may serve as the first step in the design of such antennas.

METHODS AND MATERIALS

A theoretical model for calculating the input impedance of coaxial microwave antennas with and without a choke is presented using insulated antenna theory. The theoretical model was used to calculate the input impedance of several prototype antennas having various choke and feedline dimensions, and comparison was made with experimentally measured impedance measurements in tissue-equivalent phantom.

RESULTS

The choke section of the antenna is not ideal if conventional plastic insulation is used as the choke dielectric, because the desired radiating length of the antenna is significantly shorter than the quarter-wavelength in the choke dielectric. Impedance calculations based on the theoretical model correlate reasonably well with experimentally measured impedance. Based on these calculations, the effect of parameters such as choke layer thickness and choke dielectric constant are discussed for a 915 MHz antenna with choke.

CONCLUSION

The theoretical model can serve as a design aid for optimizing choked microwave antenna designs, as well as predicting the impedance match of a given antenna design at a given insertion depth. The model allows the effect of some variables not accessible experimentally such as termination impedance to be studied, which may also be useful in the understanding of these antennas. Calculations are easily performed on a desktop computer.

SAR distributions in interstitial microwave antenna arrays with a single dipole displacement

The use of interstitial microwave antenna array hyperthermia (IMAAH) as a treatment for cancer, in conjunction with radiation therapy and chemotherapy, has been investigated widely. The heating pattern produced by a coherently phased 915 MHz asymmetric antenna array displays the maximum power deposition in the array center. This paper investigates the effect of variable insertion depth between antennas of an array on the heating patterns produced. The "study" of this heating behavior demonstrates a similar effect to that of the variably phased arrays, showing a shift of the heating peak towards the periphery of the tumor, offering a more simple method for the clinical treatment of such tumors.