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A Novel High-Resolution Quality Assurance Phantom System for Modulated Arc Stereotactic Body Radiation Therapy

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T Knewtson

T KNEWTSON1,2*, A WILES1,2 , K BRINK1 , E W IZAGUIRRE1,3 , (1) Methodist Le Bonheur University Hospital - West Cancer Center, Memphis, Tennessee, (2) University of Missouri, Columbia, Missouri, (3) University of Tennessee Health Science Center, Memphis, Tennessee


SU-I-GPD-T-555 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: We have developed a high-resolution cylindrical phantom system that utilizes flexible multiplane film and correlated small volume ion chamber measurements to assure submillimeter dose distribution and absolute dosimetric accuracy for conformal arc and volumetric modulated arc therapy (VMAT) stereotactic treatments.

Methods: A system consisting of two spatially correlated phantoms, one for multiplane film dosimetry and another for small ion chamber dosimetry were developed specifically for stereotactic body arc therapy. Each phantom is composed of a series of acrylic disks with varying thicknesses stacked together to form a cylindrical phantom. Multiple circular films (up to 12) can be placed between any two disks to determine the dose distribution in multiple planes with submillimeter resolution. The films are scanned using the RIT Classic QA software, registered with the planar dose exported from the TPS, and analyzed using the gamma test to compare absolute dose and distance-to-agreement with submillimeter sampling. Our dosimetry system includes an interpolation algorithm to compute dose distributions between film planes for volumetric dose reconstruction. The second phantom utilizes an A12S ion chamber that is spatially correlated with the film phantom for absolute dose calibration.

Results: Conformal arc and VMAT plans were used to validate the phantom system. Film quality assurance (QA) passed the 3%, 3mm gamma criteria in all cases by 97.5% or better with an average gamma of 98.4%. Similarly, in all cases the point dose measurements were within 2.25% of the absolute dose calculated in the TPS with an average of 1.1% difference.

Conclusion: The developed QA phantom system is an accurate conformal arc and VMAT QA device with submillimeter dose distribution and positioning accuracy. It’s resolution surpasses commercial array detectors, and it can be used as the reference standard for commissioning SBRT modulated arc therapy and commercial QA arrays for patient specific QA.

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