Multiparametric fit method in reconstruction of Fletcher-Suit-Delclos applicator.

In our institution we have introduced the variable angle reconstruction method with a nonisocentric C-arm as brachytherapy localizer in gynecological treatments. The main advantage of this method is that on the posterior oblique images the ovoids of the Fletcher-Suit-Delclos (FSD) applicator are clearly separated. The main problem with the nonisocentric machine is that the magnification factor is not known exactly and varies with the C-arm angle. For generation of treatment plans with a commercial brachytherapy planning system the magnification factors of the reconstruction images have to be determined previously. We present a multiparametric fit method (MPF) that is suitable for reconstruction of FSD applicators and that utilizes pairs of posterior-anterior and lateral or preferably posterior oblique reconstruction images obtained by a C-arm. The MPF reconstruction relies on the known geometry of the tandem and the ovoid. The actual FSD insertion is reconstructed by adaptation of the geometry of tandem and ovoid by translations and rotations. The fixation mechanism limits the possible excursions of the FSD applicator's parts. The limited freedom of excursions and the determination of the magnification factors are also incorporated.