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A Method for Using a Global Dosimetric Leaf Gap and Transmission Factor for Beam Matched Linear Accelerators

D Dean*, M Price , K Homann , School of Medicine at Vanderbilt University, Nashville, TN


(Sunday, 7/14/2019)  

Room: ePoster Forums

Purpose: This study assesses how the utilization of a best fit dosimetric leaf gap (DLG) and transmission value in the treatment planning system (TPS) beam model instead of machine-specific optimized values per machine will impact Intensity Modulated Radiation Therapy (IMRT) quality assurance (QA) gamma passing rates for beam-matched linear accelerators.

Methods: A standardized “chair� pattern created with “sliding window� IMRT delivery was optimized using a default DLG and transmission setting. Two beam-matched linacs delivered this plan in solid water phantom. Planar dose distributions (radiochromic film) and point dose measurements (ion chamber) were taken. These measurements were compared to an array of treatment plans generated by varying the DLG and transmission factor values in the TPS. The DLG and transmission factor combination producing the highest gamma pass rate was designated as the Ideal value for each machine. These values for each machine were averaged to define a Global DLG and transmission factor, which was applied to both machines for dose calculations. The variation of results for these Global values compared to the Ideal for both machines were then analyzed on a clinical VMAT plan using clinical gamma metrics.

Results: Ideal DLG and transmission factors for each linear accelerator were 1.6 mm/2.10% and 1.0 mm/2.50%, respectively. Using the calculated mean values as the Global factor, a head and neck treatment plan analysis showed Ideal and Global comparisons for both film and ion chamber to be within 0.5% for a single linac, and within 4% for gamma and 0.5% for ion chamber between linacs.

Conclusion: Global DLG and transmission factors derived from optimized parameters per machine yield equivalent if not improved results when applied to beam-matched linacs. With beam-matching capabilities becoming increasingly easier to obtain in multi-linac facilities, Global values could provide reliable and standardized metrics to quickly validate IMRT QA processes.


Quality Assurance, Radiation Dosimetry, Film


TH- External beam- photons: Quality Assurance - IMRT

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