Purpose: This study presents a sensitivity analysis of the peripheral dosimetric leaf gap (pDLG) and the central dosimetric leaf gap (cDLG) for a double-stacked multileaf collimator.
Methods: A Farmer-type ionization chamber was used to measure pDLG and cDLG in a water-equivalent plastic phantom by sweeping 2, 4, 6, 10, 14, 16, and 20 mm field size gaps across the position of an ion chamber. To determine the pDLG, measurements were performed at distances of 0.5 cm to 4.5 cm (in 0.5 cm steps) from the central axis of the beam along the direction of the sweeping gap. Measurements were repeated at source to surface distances (SSDs) of 90 cm and 100 cm for both 2 cm and 10 cm depths to assess the sensitivity of the measured pDLG and cDLG to these parameters.
Results: The range of the measured pDLG and cDLG were 0.13 mm to 0.31 mm and 0.26 mm to 0.33 mm, respectively. Values of the pDLG decreased with increasing distance from central axis (range 0.04 mm to 0.14 mm), and the rate of decrease was steepest for the 90 cm SSD at 2 cm depth. The maximum measured pDLG and cDLG were obtained for 90 cm SSD at 10 cm depth, whereas, 100 cm SSD and 2 cm depth revealed the minimum pDLG and cDLG values.
Conclusion: These data sug¬gest that variations of pDLG and cDLG for different SSDs and depths can be considerable for a double-stacked multileaf collimator. Therefore, incorporating the sensitivity of pDLG and cDLG in treatment planning software might improve dose calculation accuracy and quality assurance results, especially when a portion of the target is far from the central axis for the highly modulated clinical treatment plans.