Computational evaluation of changes in ionizing radiation dose distribution in tissues caused by EM applicators when external radiation and hyperthermia act simultaneously.

PURPOSE: The presented theoretical study investigates the influence of CFMA-like electromagnetic microstrip applicators (MAs) on ionizing radiation intensity and depth radiation dose distribution (DRDD) in irradiated tissues which are tightly covered with a MA. METHODS: It is shown that at relatively low photon energy (<200 keV) the MA does not affect noticeable the profile of the DRDD curve and does not lead to skin overdosing. Nevertheless, it significantly (up to 20-35%) decreases the low energy ionizing radiation intensity. For high energy photons (>1 MeV), on the contrary, the decrease of radiation intensity, caused by the MA, is small (3-10%), but the profile of the DRDD curve, calculated by means of the Monte-Carlo method, is significantly affected. RESULTS: The radiation dose maximum is shifted to the skin, resulting in possible skin overdosing. Radiation absorption characteristics of MA are calculated and compared with published parameters of EM horn and US applicators now in use for external simultaneous radiation and hyperthermia (ESRH) procedures. The MA provides the minor ionizing radiation absorption. Due to it and owing to their conformability with the tissue surface the MAs would not require any additional means or devices to be used for ESRH treatment procedures with any common ionizing radiation equipment. CONCLUSIONS: The necessity of development means for decreasing the time of radiation equipment occupation during ESRH procedures is pointed out.

Assessment of the performance characteristics of a prototype 12-element capacitive contact flexible microstrip applicator (CFMA-12) for superficial hyperthermia.

The electrical performance of the CFMA-12 operating at 433 MHz is assessed under laboratory conditions using a RF network analyser. From measurements of the scattering parameters of the CFMA-12 on both a multi-layered muscle- and fat/muscle-equivalent phantom, the optimal water bolus thickness, at which the transfer of the energy to the phantom configuration is maximal, is determined to be approximately 1 cm. The SAR distribution of the CFMA-12 in a multi-layered muscle-equivalent phantom is characterized using Schottky diode sheets and a TVS-600 IR camera. From the SAR measurements using the Schottky diode sheets it is shown that the contribution of the E(x) component to the SAR (SAR(x)) is maximal 7% of the contribution of the E(y)component to the SAR (SAR(y)) at different layers in both phantom configurations. The complete SAR distribution (SAR(tot)) at different depths is measured using the power pulse technique. From these measurements, it can be seen that SAR(y)at a depth of 0 cm in the muscle-equivalent phantom represents up to 80% of SAR(tot). At 1 and 2 cm depth, SAR(y) is up to 95% of SAR(tot). Therefore, in homogeneous muscle-equivalent phantoms, E(y) is the largest E-field component and measurement of SAR(y) distribution is sufficient to characterize SAR-steering performance of the CFMA-12. SAR steering measurements at 1 cm depth in the muscle-equivalent phantom show that the SAR maximum varies by 40% (1 SD) around the average value of 38.8 W kg(-1) (range 10-65 W kg(-1)) between single antenna elements. The effective fieldsize (E(50)) varies by 14% (1 SD) around the average value of 19.1 cm(2).

Resonance effects in applicator water boluses and their influence on SAR distribution patterns.

Spurious electromagnetic oscillations in the water layer of applicator water boluses are investigated. Two oscillation types--volume oscillations (VO) and surface standing wave oscillations (SWO)--are considered. Conditions leading to their excitation in water boluses, commonly used in clinical practice for local hyperthermia, are examined. It was found that the thickness d of the water layer is the main critical parameter which determines the possibility of spurious oscillation modes excitation. Analytical equations are derived, allowing calculations of critical d(c) and resonance d(r) values for a given water bolus, and determine the approximate structure of electrical field strength distribution at its radiating plane, taking account of spurious oscillations. Calculated d(c) and d(r) values are in satisfactory quantitative agreement with experimental data, whereas experimentally observed complicated SAR distribution perturbations correlate qualitatively with the calculations in terms of volume and/or surface standing wave oscillations excited within the water bolus.

A HF EM installation allowing simultaneous whole body and deep local EM hyperthermia.

The structure and main features of a HF EM installation based upon a new approach for creating electromagnetic fields destined for whole body (WBH) and deep local (DLH) hyperthermia are discussed. The HF EM field, at a frequency of 13.56 MHz, is created by a coplanar capacity type applicator positioned under a distilled water filled bolus that the patient is lying on. The EM energy being released directly in the deep tissues ensures effective whole body heating to required therapeutic temperatures of up to 43.5 degrees C, whereas the skin temperature can be maintained as low as 39-40.5 degrees C. For DLH, the installation is equipped with additional applicators and a generator operating at a frequency of 40.68 MHz. High efficiency of the WBH applicator makes it possible to carry out the WBH procedure without any air-conditioning cabin. Due to this, a free access to the patient's body during the WBH treatment is provided and a simultaneous WBH/DLH or WBH/LH procedure by means of additional applicators is possible. Controllable power output in the range of 100-800 W at a frequency of 13.56 MHz and 50-350 W at a frequency of 40.68 MHz allows accurate temperature control during WBH, DLH and WBH/DLH procedures. SAR patterns created by the WBH and DLH applicators in a liquid muscle phantom and measured by means of a non-perturbing E-dipole are investigated. The scattered EM field strength measured in the vicinity of the operating installation during the WBH, DLH and WBH/DLH procedures does not exceed security standards. Examples of temperature versus time graphs in the course of WBH, DLH and WBH/DLH procedures in clinics are presented. The installation is successfully used in leading oncological institutions of Russia and Belarus, though combined WBH/DLH procedures are evidently more complicated and demand thorough planning and temperature measurements to avoid overheating.

An attempt at quantitative specification of SAR distribution homogeneity.

A homogeneity coefficient (HC) is suggested allowing quantitative evaluation of SAR distribution homogeneity over the heated area. Its efficacy in appreciation of SAR patterns quality is demonstrated on idealized and real SAR distributions. The HC parameter of a series of applicators widely used in clinics are calculated. HC could be assumed as a useful parameter additional to the qualified effective field size (EFS) in characterizing the applicator's properties.

SAR characteristics of three types of Contact Flexible Microstrip Applicators for superficial hyperthermia.

The Effective Field Sizes (EFS) and the Effective Heating Depths (EDH) of Contact Flexible Microstrip Applicators (CFMA), tuned at a frequency of 434 MHz, were determined on two fat/muscle phantoms. One phantom was box-shaped with a flat top layer of 1 cm thick artificial fat and the other one a tube of which the cross-section was elliptically shaped (25 x 36 cm) having a fat equivalent shell of 1 cm thick. For the muscle material a 6 g/l saline (NaCl) solution was used. On the flat rectangular phantom, the effective field size at 1 cm depth in saline was measured to be 4.7 x 13.5 cm, 17.5 x 17.7 cm and 12.5 x 14.0 cm for the 1H-applicator, the 3H-applicator and the 5H-applicator, respectively. For the 3H-and the 5H-applicator, the Specific absorption rate (SAR) distribution at 1 cm depth showed a single maximum of SAR for a thin bolus, which split into two separate "hot spots' for a thicker bolus. The Effective Heating Depths in the phantom with a flat surface were of the order of that of a plane wave (approximately 1.4 cm), whereas a larger EHD of 2.4 cm was achieved below the 3H-applicator bent to fit the elliptical phantom. Due to the large effective field size and the "flatness' of the SAR distribution, the applicators 3H and 5H are suitable to adequately treat large superficial tumours.