Program Information
Theory and Clinical Implications of the Constant Dosimetric Leaf Gap (DLG) Approximation
L Kumaraswamy1*, D Bailey2 , Z Xu3 , J Schmitt4 , M Podgorsak5 , (1) Roswell Park Cancer Institute, Buffalo, NY, (2) Northside Hospital, Atlanta, GA, (3) Roswell Park Cancer Institute, Buffalo, NY, (4) RadAmerica, LLC--MedStar Health, Baltimore, MD, (5) Roswell Park Cancer Institute, Buffalo, NY
Presentations
SU-G-BRC-16 (Sunday, July 31, 2016) 4:00 PM - 6:00 PM Room: Ballroom C
Purpose: Commercial dose calculation algorithms incorporate a single DLG value for a given beam energy that is applied across an entire treatment field. However, the physical processes associated with beam generation and dose delivery suggest that the DLG is not constant. The aim of this study is to evaluate the variation of DLG among all leaf pairs, to quantify how this variation impacts delivered dose, and to establish a novel method to correct dose distributions calculated using the approximation of constant DLG.
Methods: A 2D diode array was used to measure the DLG for all 60 leaf pairs at several points along each leaf pair travel direction. This approach was validated by comparison to DLG values measured at select points using a 0.6 cc ion chamber with the standard formalism. In-house software was developed to enable incorporation of position dependent DLG values into dose distribution optimization and calculation. The accuracy of beam delivery of both the corrected and uncorrected treatment plans was studied through gamma pass rate evaluation. A comparison of DVH statistics in corrected and uncorrected treatment plans was made.
Results: The outer 20 MLC leaf pairs (1.0 cm width) have DLG values that are 0.32 mm (mean) to 0.65 mm (maximum) lower than the central leaf-pair. VMAT plans using a large number of 1 cm wide leaves were more accurately delivered (gamma pass rate increased by 5%) and dose coverage was higher (D100 increased by 3%) when the 2D DLG was modeled.
Conclusion: Using a constant DLG value for a given beam energy will result in dose optimization, dose calculation and treatment delivery inaccuracies that become significant for treatment plans with high modulation complexity scores delivered with 1 cm wide leaves.
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