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Feasibility Study of a Robot-Based Thermoplastic Mask for Conventional Linac-Based Cranial SRS Treatments Without Online Tracking

S Jang*, J Chang , D Heron , M Huq , UPMC Hillman Cancer Center and University of Pittsburgh School of Medicine, Pittsburgh, PA


(Wednesday, 7/17/2019) 9:30 AM - 10:00 AM

Room: Exhibit Hall | Forum 4

Purpose: We evaluated the online patient motion-data immobilized with the Orfit thermoplastic mask for cranial SRS to find the feasibility of the mask for linac-based SRS without online motion-tracking. The setup margin for cranial SRS treatments without online-tracking was also investigated to find the impact of the mask on the delivery accuracy of linac-based SRS.

Methods: Twenty-one challenging SRS cases immobilized with the Orfit thermoplastic mask and delivered with the CybeKnife-M6 were retrospectively selected. Translational (X, Y, Z) and rotational (yaw, pitch, roll) online tracking-data were analyzed in terms of patient motion over the entire treatment duration as well as inter-imaging duration, which were calculated with different motion data at two different imaging periods. The variation of motion-data with fractional delivered MU (0~0.25,0.25~0.5,0.5~0.75,0.75~1) was also calculated to find the impact of delivery time on the patient motion for the Orfit mask. With two patient data delivered with multi-fractions, inter-fractional motion variation were studied. Correlation of patient motion in terms of rotational versus translational motion and MU were analyzed.

Results: Average translational motion was 0.9±0.8mm, 0.7±0.6mm, 0.5±0.4mm for X, Y, and Z directions, and average rotational motion was 0.4±0.3°,0.5±0.3°,0.5±0.4° for roll, pitch, and yaw directions. As fractionated delivered MU varied from 0.25 to 1.0, patient motion was slightly increased with significant variance (translation: 0.7±0.5mm, 0.5±0.4mm, 0.4±0.3mm vs. 1.0±1.0mm, 0.8±0.7mm, 0.5±0.4mm; rotation: 0.3±0.2°,0.4±0.3°,0.3±0.2° vs. 0.5±0.4°, 0.6±0.3°, 0.5±0.4°), showing that setup margin of minimum 1mm should be considered for Orfit-based SRS treatments without online-tracking. However, average motion over the inter-imaging duration was not varied with delivered MU (translation:0.3±0.4mm,0.3±0.3mm,0.2±0.2mm; rotation: 0.1±0.2°,0.3±0.3°,0.3±0.3° at 0.75~1.0 fractional delivered-MU). Patient motion was not correlated with MU, inter-fractions, and magnitude of rotational-versus-translational motion.

Conclusion: Patient motion with the Orfit mask was evaluated and setup margin of minimum 1mm should be considered for Orfit-mask based SRS treatments without online tracking.


Stereotactic Radiosurgery, Immobilization


Not Applicable / None Entered.

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