Session Item

Frameless stereotactic radiosurgery (SRS) on the Gamma Knife (GK) is facilitated by cone-beam CT (CBCT) imaging, online re-planning, and intra-fraction motion monitoring (IFMM). The purpose of this study is to evaluate the impact of target location on intra-fraction motion detected using frameless immobilization for GK-SRS through the CBCT and IFMM systems. 

Patients treated with frameless GK-SRS were included for analysis. At simulation, a patient specific thermoplastic mask and headrest is manufactured, followed by acquisition of a reference CBCT scan. For both single and multi-fraction GK-SRS, the patient is immobilized in the mask and headrest, with their treatment position verified using daily CBCTs and monitoring of a reflective nose marker using the IFMM system. Repeat localization CBCTs was required if IFMM thresholds are exceeded. Where possible, a post-treatment CBCT was acquired at the end of the treatment session. The intra-fraction motion measured between localization, repeat localization (where patient repositioning was not required) and post-treatment CBCTs were correlated to the intra-faction motion indicated by the IFMM log files through the Pearson coefficient (r). The impact of treatment target location (cerebellum, frontal, occipital, parietal, temporal, multiple, other) was correlated to IFMM and CBCT intra-fraction motion. 

302 plans were reviewed from 263 patients (114 male, 149 female; median age 65, range 20 - 97). The average treatment time for 218 single fraction plans was 61.7 minutes (range 12.1 – 383.1), and for 84 multi-fraction plans, 40.7 minutes (range 14.4 – 102.8). A total of 216 pairs of localization - repeat localization, and 260 localization – post-treatment CBCTs were analyzed against IFMM logs. For the localization – repeat localization pairs, the mean IFMM magnitude as detected by the nose was 1.85±0.95 mm and mean CBCT intra-fraction magnitude measured through the skull was 0.79±0.68 mm (r=0.06). For the localization – post-treatment pairs, the mean IFMM magnitude was 0.76±0.95 mm and mean CBCT intra-fraction magnitude was 0.68±0.68 mm (r=0.40). Separation by treatment target location did not improve the correlation between detected IFMM values and measured CBCT magnitudes (Table 1).  

Preliminary analysis suggests target location has minimal impact on intra-fraction motion during GK-SRS. The magnitude detected by the IFMM system overestimates patient intra-fraction motion measured through CBCTs.

Table 1: Intra-fraction motion monitoring (IFMM) magnitude and cone beam CT(CBCT) measurements triaged by target location