Introduction: Contact flexible microstrip applicators (CFMA) are applied for superficial hyperthermia. In the clinic these flexible applicators are mostly applied bent along the body curvature. This paper investigates the specific absorption rate (SAR) patterns of CFMA applicators, when bent around an elliptical tissue-equivalent phantom.
Methods: The 2H (aperture size 14.8 x 14.3 cm(2)), 3H (28.7 x 20.7 cm(2)), 4H (19.6 x 19.6 cm(2)) and 5H (19.7 x 28.5 cm(2)) applicators were examined. Measurements were performed for the 5H applicator; existing measurement data were analysed for the 3H applicator. Finite difference time domain (FDTD) simulations with a resolution of 2 x 2 x 1 mm(3) were performed for all applicators. Applicators were bent around the top and the side of the elliptical phantom to examine different curvatures. The SAR deposition, effective field size (EFS) and effective heating depth (EHD) were evaluated and compared to results for straight applicators.
Results: Bending the applicators generally yielded a focusing effect of the SAR, which was most pronounced with a strong curvature, but especially the 5H applicator showed a stronger power absorption at the sides of the applicator, compared to the centre region. The EFS became smaller when bending the applicators; this effect was also more pronounced for a strong curvature. The EHD increased for bent applicators, but the degree depended strongly on the location.
Conclusion: The behaviour of bent CFMA applicators is not trivial and the SAR deposition is not similar for all applicators. The EFS decreases and the EHD increases, but very locally. Therefore, it is generally advisable to analyse the SAR distribution of flexible applicators in both straight and bent state.