First experiences with computer assisted frameless stereotactic interstitial brachytherapy (CASIB)

Purpose: To reach an optimal treatment result and to avoid damage to critical structures a homogenous dose distribution in the tumor volume with a rapid decreasing dose to the surrounding structures is necessary. Fractionated interstitial brachytherapy of tumors in the ENT region employing needles depends on exact localization of the target volume during all fractions. Therefore reproducibility of positioning of the needle(s) plays an important role. Material and Methods: We used the ISG Viewing Wand system in combination with the Vogele-Bale-Hohner (VBH) head holder and a new targeting device. Point of entrance, pathway, and target point of the needle were planned and insertion of the needle simulated in advance. To date we have treated 7 patients with inoperable tumors in the ENT region. The actual position of the needle in the control CT was compared to the planned position. Results: The accuracy of positioning of the needle depended on the location of the tumor. In a patient with a recurrent retroorbital adenocarcinoma the mean accuracy was 1 mm. Due to soft tissue displacement in the neck region and the resulting necessity to readjust the targeting device the needle was placed with a mean deviation of 1 mm between the planned and the actual position. Conclusions: Computer-assisted frameless stereotactic interstitial brachytherapy allows for precise, reproducible and preplanned insertion of hollow needles into target structures closely adherent to the surrounding tissue: thus avoiding damage of neighbouring structures. This technique is of great advantage in treating deeply seated tumors which are fixed to bony structures. especially at the skull base. Inaccuracy in the neck region caused by soft tissue shift requires improvement of the immobilization in this region.