(Saturday, 4/7/2018) 10:30 AM - 12:30 PM
Room: Marquis Ballroom 5-8
Purpose: Independent verification of linear accelerator (linac) output is necessary during machine commissioning, and limited third-party services are available for this purpose. In addition, these methods require time (weeks to receive results or up to a day of machine unavailability), which is often limited during the commissioning process and are not without monetary costs. A method for fast and efficient verification of linac output was developed wherein a Gafchromic film is doubly exposed using a linac with verified output and another machine which has been recently calibrated. The procedure has been demonstrated at our facility for validation of recent output calibration during the commissioning process while awaiting third-party verification.
Methods: For all available photon and electron beams of a newly commissioned Varian TrueBeam accelerator (6MV, 6FFF, 10MV, 10FFF, 15MV, 6MeV, 9MeV, 12MeV, 16MeV, and 18MeV), dose of 100 MU was delivered to one side of a piece of Gafchromic film placed in solid water using a 10x10 cm2 field or cone. Immediately afterwards, the film and setup apparatus were transported to a second accredited accelerator, and the same doses were delivered to the opposite half of each film using the second machine. Exposed films were scanned using an Epson XL10000 scanner. The scans were analyzed using in-house MATLAB code to quantify agreement in output between machines. Crossline and inline profiles were extracted and compared within the central 10% of the delivered field size.
Results: Automatic film analysis code was developed to facilitate the output verification. All analyzed profiles passed gamma analysis of 3%/2 mm with passing rates ranging from 99.6% - 100%. Using a 0.4% dose difference criterion, differences greater than or equal to 1 MU were detected with sensitivity of 0.925.
Conclusion: The developed method provided convenient assurance of output calibration prior to independent verification by a third party.
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