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Evaluation of Dosimetric and Geometric Accuracy of Gated Lung SBRT with CIRS Lung Motion Phantom

S Yu

S Yu*, B Han, L Wang, E Mok, Stanford University Cancer Center, Stanford, CA

SU-C-141-2 Sunday 1:00PM - 1:55PM Room: 141

Purpose: To evaluate the dosimetric and geometric accuracy of the gated SBRT delivery using film measurements and intra-fractional kV images with predefined software-markers.

Methods: End-to-end tests of gated SBRT delivery were performed using a Varian TrueBeam™ linac and a CIRS lung motion phantom with targets of 1 and 2 cm in diameter and implanted fiducials. 4DCT images of the phantom were acquired with 1 mm slice thickness. RapidArc plans were created in Eclipse TPS with software-markers placed on the fiducials at maximal expiration. SBRT plans were delivered to the phantom in stationary and in gating mode (2 mm and 5 mm-gating window). Plans were also delivered with 5 mm targets shift with respect to the boney anatomy in stationary mode and 10% phase shift in 2 mm-gating mode. To investigate the dosimetric accuracy, dose was measured by EBT3 film and compared with the TPS calculation (AcurosXB algorithm) using γ-index criteria (3mm distance-to-agreement and 3% dose-difference). The geometric accuracy was evaluated using kV intra-fraction images of the fiducials relative to the predefined software-markers.

Results: Comparing with the stationary mode, the percentage passing γ-index (%γ) for 1 cm target decreased significantly from 98.0% to 94.8% (2 mm-gating window) and 84.3% (5 mm-gating window) while for 2 cm target %γ decreased from 99.5% to 99.2% (2 mm-gating window) and 86.6% (5 mm-gating window). The %γ for the conditions of target shift and phase shift for the 2 cm target were 89.0% and 89.9%, respectively. Geometric deviation was less than 0.05 mm, except for the phase shift condition (0.45 mm).

Conclusion: Higher dosimetric impact from motion was found for the smaller target. With a proper gating window the planned dose can be accurately delivered with the use of kV intra-fraction image and software-maker to ensure the geometric accuracy during treatment.

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