Author: Guang-Pei Chen, Nitish Chopra, Juan A. Garcia-Alvarez, Mi Huang, Slade J. Klawikowski, Haidy G. Nasief, George A. Noid, Abdul Parchur, Eric S. Paulson, Christina M. Sarosiek, Christopher Schultz 👨🔬
Affiliation: Department of Radiation Oncology, Medical College of Wisconsin 🌍
Purpose: During routine delivery of mask-based Gamma Knife (GK) SRS, motion is tracked via the position of a reflective sticker placed on the patient’s nose relative to two stationary reflectors on the mask adapter with a default gating threshold of 1.5 mm. However, the magnitude of motion of the patient’s nose may not directly reflect the magnitude of SRS target motion. This study quantifies the effect of detected motion on clinically relevant dosimetric criteria as function of simulated target position in an anthropomorphic phantom.
Methods:Six lesions of different sizes 3-, 5- and 10-mm were contoured anteriorly and posteriorly on images of an Alderson head phantom (AHP). A GK plan was generated to cover 20Gy at 57-67% IDLs for all lesions. The AHP was placed on a head rest in a GK Icon, a tracking sticker was placed with high-definition motion management (HDMM) gating threshold of 1.5mm, and a cone beam CT was acquired to establish stereotactic reference. Translational and rotational motions were introduced to the AHP to reach HDMM <1.5mm, 1.5mm, and above 1.5mm-4.7mm. For each perturbation, a new-CBCT was acquired, co-registered to planning-CBCT, and dose evaluation performed. Percent differences between planned and measured dosimetric parameters were evaluated to assess the effect of motion on each lesion.
Results:Dosimetric parameters min, mean, and max dose were < 3% for HDMM shifts >2mm. Paddick conformity and gradient index variations were larger for the 3mm and 5mm targets with changes up to 4.6% for 5mm posterior lesion at 4.7mm perturbation. For HDMM <2mm, no significant differences were measured. The largest dosimetric changes were correlated to the largest shifts with larger rotations.
Conclusion:Increasing HDMM thresholds to 2mm does not introduce significant dosimetric differences on anterior and posterior lesions in the AHP. A larger verification study with more sophisticated phantoms and patient data is warranted.