2236 - Retrospective Analysis of Intra-Fraction Motion and Treatment Time during Mask-Based Gamma Knife Radiosurgery

Purpose/Objective(s):

1. To report the incidence of CBCT frequency in a retrospective series of 426 frameless GKRS.

2. To assess the incremental increase in treatment duration due to positioning corrections during frameless GKRS and determinants of these corrections

Materials/Methods:

We retrospectively reviewed our frameless GKRS experience from December 2022 to March 2025 to identify treatment related factors affecting session duration and CBCT frequency. We analyzed 426 mask-based GKRS procedures utilizing HDMM. Demographic, clinical, and technical parameters were collected, including age, BMI, mask type (398 Klarity vs. 28 Elekta), and history of sleep apnea. Pearson correlation coefficients were calculated for continuous variables (height, weight, BMI, age), and group mean comparisons were performed for categorical variables (mask type, sleep apnea) to evaluate associations with the total number of CBCTs per treatment. Added treatment time (?), defined as the difference between actual and planned treatment durations, was calculated to assess the additional time burden. The number of CBCTs was also analyzed as a surrogate for positioning adjustments.

Results:

If patient movement exceeded 1.5 mm, HDMM paused treatment. Automatic resumption occurred if the patient returned within a set time; otherwise, the operator intervened and acquired a new CBCT. Mean planned treatment time was 40.5 minutes, mean observed time 50.8 minutes, yielding ? = 10.3 minutes, indicating most sessions required additional time. One hundred sixty-six patients had more than one CBCT. Sex, age, and history of sleep apnea were comparable across mask groups. Mean CBCTs per session were 1.72 (Klarity) and 2.20 (Elekta), with 21 sessions exceeding five CBCTs (p = 0.65). Longer planned sessions correlated with more CBCTs (mean = 3.02) and higher ? compared to shorter planned sessions (mean = 1.09). Multivariable regression showed CBCT frequency and planned treatment time independently predicted ? (both p < 0.001), whereas age, sleep apnea, and mask type were not significant.

CBCT frequency increased with planned time up to 45 minutes, plateaued between 45–75 minutes, and decreased thereafter.

Conclusion:

In this retrospective analysis of 426 frameless GKRS sessions, the main driver of treatment prolongation was longer planned session duration. Planning duration was also significantly associated with repeated CBCTs to maintain positioning. Patients requiring >5 CBCTs represented a subgroup with substantially longer treatment times and more frequent >10-minute delays. These findings highlight the importance of optimized planning with shorter durations to ensure precise positioning and efficient treatment delivery. Although therapist experience was not formally analyzed, it may influence mask quality and patient stability during treatment. Overall, most patients had a reasonable duration of treatment length with relative comfort and a low stress experience with the mask system (as opposed to a fixed frame).