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A Comprehensive Dosimetric Characterization of the New 160 Leaf Elekta Agility Collimator System

J Cui

J Cui1*, C Liu2, L Meng1, S Benedict1, (1) UC Davis Medical Center, Sacramento, CA, (2) Univ Florida, Gainesville, FL

SU-E-T-128 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: A new MLC has been released that includes 160 leaves of 5mm width at isocenter that allows 40x40 cm², rapid leaf speed, interdigitation, and reduced leaf transmission. We evaluate the dosimetric characteristics of this new Agility Collimator and its impacts on clinical applications.

Methods: A dosimetric evaluation of the Agility and the MLCi2 collimator system is performed using a Wellhofer water tank, diode and ion chamber detectors, edge detectors, and rotational delivery QA tools. Characterization includes a comprehensive verification of beam quality (6, 10, and 15 MV), MLC transmission, and delivery efficiency and accuracy of IMRT and VMAT.

Results: The Agility system has one set of diaphragm and the MLC replaces the entire X diaphragm, while the older MLCi2 has two sets of diaphragms. Examination of percent depth dose of the photon beam quality is identical except in the buildup region. MLCi2 has a slightly higher dose in the buildup region; attributed to more collimation material in the beam path. For Agility, the averaged MLC leakage is 0.003, and for MLCi2 the averaged leakage is 0.013. Agility (MLC along) is equivalent to MLCi2 (with backup diaphragm) to achieve low out-of-field radiation. With equivalent quality Philips Pinnacle³ IMRT and SmartArc/VMAT the delivery time is shorter on AGILITY because of higher MLC speeds and improvements in linac control software (Integrity R3.0) which supports continuously variable dose rate.

Conclusion: This work demonstrates that the new Agility collimator assembly has similar beam quality as the original MLCi2 collimator system, except in the buildup region. Agility has less MLC leakage and increased delivery efficiency compared to MLCi2. These findings allow for an expedited commissioning of upgrade to the Agility from the MLCi2. Further features of the leaf speed are under investigation, including their potential for higher modulation in VMAT.

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