SU‐E‐T‐01: 2‐D Characterization of DLG Among All MLC Leaf Pairs

Abstract

Purpose:The aim of this study is to evaluate the variation of dosimetric leaf‐gap (DLG) along the travel path of each MLC leaf pair. This study evaluates whether the spatial variations in DLG could cause dose differences between TPS‐calculated and measured dose.Methods:The 6MV DLG values were measured for all leaf pairs in the direction of leaf motion using a 2‐D diode array and 0.6cc ion chamber. These measurements were performed on two Varian Linacs, employing the Millennium 120‐leaf MLC and a 2‐D‐DLG variation map was created via in‐house software. Several test plans were created with sweeping MLC fields using constant gaps from 2mm to 10mm and corrected for 2‐D variation utilizing in‐house software. Measurements were performed utilizing the MapCHECK at 5.0cm depth for plans with and without the 2‐D DLG correction and compared to the TPS calculated dose via gamma analysis (3%/3mm).Results:The measured DLGs for the middle 40 MLC leaf pairs (0.5cm width) were very similar along the central superior‐inferior axis, with maximum variation of 0.2mm. The outer 20 MLC leaf pairs (1.0cm width) have DLG values from 0.32mm (mean) to 0.65mm (maximum) lower than the central leaf‐pair, depending on off‐axis distance. Gamma pass rates for the 2mm, 4mm, and 6mm sweep plans increased by 23.2%, 28.7%, and 26.0% respectively using the 2‐D‐DLG correction. The most improved dose points occur in areas modulated by the 1.0cm leaf‐pairs. The gamma pass rate for the 10mm sweep plan increased by only 7.7%, indicating that the 2D variation becomes less significant for dynamic plans with larger MLC gaps.Conclusion:Fluences residing significantly off‐axis with narrow sweeping gaps may exhibit significant variations from planned dose due to large differences between the true DLG exhibited by the 1.0cm leaf‐pairs versus the constant DLG value utilized by the TPS for dose calculation.