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A Programmable Moving Insert for the ArcCHECK Phantom for Dose Verification of Respiratory-Gated VMAT

S Gaede

S Gaede1,2*, H Young3 , M Mulligan1 , K Jordan1,2 , (1) London Regional Cancer Program, London, ON, (2) Western University, London, ON, (3) University of Waterloo, Waterloo, ON


SU-E-T-64 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To present a customized programmable moving insert for the ArcCHECK™ phantom that can, in a single delivery, check both entrance dosimetry, while simultaneously verifying the delivery of respiratory-gated VMAT.

Methods: The cylindrical motion phantom uses a computer-controlled stepping motor to move an insert inside a stationery sleeve. Insert motion is programmable and can include rotational motion in addition to linear motion along the axis of the cylinder. The sleeve fits securely in the bore of the ArcCHECK™. Interchangeable inserts, including an A1SL chamber, optically-stimulated luminescence dosimeters, radiochromic film, or 3D gels, allow this combination to be used for commissioning, routine quality assurance, and patient-specific dosimetric verification of respiratory-gated VMAT. Before clinical implementation, the effect of a moving insert on the ArcCHECK™ measurements was considered. First, the measured dose to the ArcCHECK™ containing multiple inserts in the static position was compared to the calculated dose during multiple VMAT treatment deliveries. Then, dose was measured under both sinusoidal and real-patient motion conditions to determine any effect of the moving inserts on the ArcCHECK™ measurements. Finally, dose was measured during gated VMAT delivery to the same inserts under the same motion conditions to examine any effect of various beam “on-and-off” and dose rate ramp “up-and-down”. Multiple comparisons between measured and calculated dose to different inserts were also considered.

Results: The pass rate for the static delivery exceeded 98% for all measurements (3%/3mm), suggesting a valid setup for entrance dosimetry. The pass rate was not altered for any measurement delivered under motion conditions. A similar result was observed under gated VMAT conditions, including agreement of measured and calculated dose to the various inserts.

Conclusion: Incorporating a programmable moving insert within the ArcCHECK™ phantom provides an efficient verification of respiratory-gated VMAT delivery that is useful during commissioning, routine quality assurance, and patient-specific dose verification.

Funding Support, Disclosures, and Conflict of Interest: Prototype phantom development and testing was performed in collaboration with Modus Medical Devices Inc. (London, ON). No financial support was granted.

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