Positioning reproducibility with and without rotational corrections for 2 head and neck immobilization systems

Abstract

PurposeThe purpose of this study was to evaluate the impact of offline rotational corrections and assess intrafraction motion for head and neck (H&N) cancer patients immobilized with and without a custom neck cushion. Methods and materialsFifty H&N cancer patients were immobilized and imaged with pretreatment and posttreatment cone beam computed tomography (CBCT) for each treatment fraction. Of these patients, 25 had a custom neck cushion added to their immobilization. Each CBCT was registered to the simulation computed tomography offline. Registrations were performed with automatching tools and a matching volume of interest that consisted of a 5-mm expansion around the mandible, occipital bone, C1/C2, and C7/T1. To determine positioning accuracy, the registration was inspected to confirm these bony anatomy structures were contained within a 3- or 5-mm expansion of the simulation position. If not, the registration was repeated with rotational corrections included and re-evaluated. For each fraction, intrafraction motion was also quantified through the difference between the pretreatment and posttreatment CBCT registration coordinates. ResultsFor translational registrations, the bony anatomy in pretreatment imaging was outside the 3-mm or 5-mm expansion structure, respectively, for 49% and 15% of fractions on average for patients without a custom headrest and for 48% and 13% of fractions on average for patients with a custom headrest. The addition of rotational corrections reduced these numbers to 21% and 4% and to 28% and 6%, respectively. Intrafraction motion was significantly lower for patients immobilized with the addition of a custom neck cushion: 1.0 ± 0.5 mm compared with 1.8 ± 1.6 mm for patients with the standard headrest only (P = .02). This was reflected in posttreatment positioning accuracy, which was significantly reduced in the case of the standard headrest compared with pretreatment imaging (P values of < .001 to .048). ConclusionsRotational corrections significantly improved pretreatment patient positioning accuracy (P < .001). Intrafraction motion was reduced significantly through the addition of a custom neck cushion and resulted in an increase in posttreatment positioning accuracy for these patients.