Assessment Of Intra-Fraction Motion During Frameless Stereotactic Radiosurgery

Frameless stereotactic radiosurgery (SRS) is facilitated by cone-beam CT (CBCT) imaging, online re-planning, and intra-fraction motion monitoring (IFMM). The purpose of this study was to evaluate the intra-fraction motion detected using frameless immobilization for SRS through the CBCT and IFMM systems. Patients treated with frameless SRS over a 2-month study period were included for analysis. For SRS, patients are immobilized in a personalized thermoplastic mask and headrest followed by attachment of a reflective marker on their nose. The patient’s treatment position is verified using a localization CBCT against the reference CBCT acquired at simulation, while the IFMM system continuously monitors their nose marker. Repeat localization CBCTs were acquired during treatment if IFMM thresholds (1.5mm) were exceeded or for voluntary patient breaks. Where feasible, a post-treatment CBCT was acquired after treatment delivery. Using bony anatomy for image registration, the difference between localization and post CBCTs, and localization and repeat localization CBCTs (in situations where IFMM thresholds were exceeded) were used to quantify intra-fraction motion. The IFMM data associated with the time points of the CBCTs were also extracted to assess intra-fraction motion. Thirty plans were reviewed from 26 patients (19 single fraction, 11 multi-fraction, 4 patients treated concurrently with single and multi-fractionation). Over 52 treatment fractions, 113 localization, repeat localization and post-treatment CBCTs were acquired. 28 sets of localization-post CBCTs and 25 sets of localization-repeat localization CBCTs were included for analysis. The average ± standard deviation intra-fraction motion quantified on CBCTs was -0.11±0.37mm, -0.13±0.20mm, 0.18±0.74mm in the left/right (L/R), anterior/posterior (A/P) and superior/inferior (S/I) axes. The average ± standard deviation rotational displacement measured on CBCTs was 0.07±0.56⁰, -0.14±0.64⁰, 0.02±0.54⁰ in pitch, yaw and roll. The average ± standard deviation intra-fraction motion quantified on IFMM was -0.09±0.68mm, -0.11±0.5mm, 0.25±0.76mm in the L/R, A/P and S/I axes. There was a positive correlation between the measured vector CBCT and IFMM displacements (r = 0.49). Preliminary analysis indicate the largest intra-fraction motion, as measured by both CBCT and IFMM systems for frameless GK-SRS, is in the S/I axis. Positional deviations noted by the IFMM system during SRS provides a good indication of intra-fraction patient movement.