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

Abstract

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.

FDTD simulations to assess the performance of CFMA-434 applicators for superficial hyperthermia

“…The measured EFS was 4.7 Â 13.5 cm [19]; the simulation yielded a wider Table II. Overview of the simulated air layer configurations for the different types of CFMA applicators.…”

“…Simulated water bolus thicknesses are in correspondence with the bolus thicknesses during the measurements [19]. The SAR distribution in the phantom generated by the 1H and 3H applicator was simulated for a bolus thickness of 1.5 and 1.7 cm, respectively.…”

“…Figure 3 shows a schematic representation of the modelled geometry and the computational domain. Dielectric properties applied for the simulations were obtained from literature [19,27,28] and are shown in Table I.…”

“…Furthermore, in the absence of reliable 3D temperature monitoring in the tumour, it is very important that the EFS and EHD are stable under clinical conditions. Earlier research showed that the water bolus thickness can have a strong effect on the SAR distribution [19]. During SAR measurements in the past it was noticed that the reproducibility was moderate, even when the exact measurement set-up was reproduced very carefully.…”

Performance evaluation of a conformal thermal monitoring sheet sensor array for measurement of surface temperature distributions during superficial hyperthermia treatments

“…It should be noted that this paper does not address the ability of hyperthermia applicators to treat to any specific depth, but rather this thermometry approach will provide the critical information necessary to change power levels of multi-element applicators to deliver most uniform lateral distribution of heating across large superficial tissue disease [31]. Efforts thus far have been aimed at evaluating the prototype TMS for use in monitoring and control of surface temperature distributions under multi-element array microwave hyperthermia applicators like the 16-element Microtherm and 32-element CMA applicator developed for treatments of chest wall recurrence of breast cancer [1,5,32,33]. This thermometry approach should be equally useful for superficial hyperthermia induced with numerous other multielement array microwave applicators [9,13,14,16,34,35].…”