Abstract
Partial transmission through rounded leaf ends of Varian multileaf collimators (MLC) is accounted for with a parameter called the dosimetric leaf gap (DLG). Verification of the value of the DLG is needed when the dose delivery is accompanied by gantry rotation in VMAT plans. We compared the doses measured with GAFCHROMIC film and an ionization chamber to treatment planning system (TPS) calculations to identify the optimum values of the DLG in clinical plans of the whole brain with metastases transferred to a phantom. We noticed the absence of a single value of the DLG that properly models all VMAT plans in our cohort (the optimum DLG varied between 0.93±0.15 mm and 2.2±0.2 mm). The former value is considerably different from the optimum DLG in sliding window plans (about 2.0 mm) that approximate IMRT plans. We further found that a single‐value DLG model cannot accurately reproduce the measured dose profile even of a uniform static slit at a fixed gantry, which is the simplest MLC‐delimited field. The calculation overestimates the measurement in the proximal penumbra, while it underestimates in the distal penumbra. This prompted us to expand the DLG parameter from a plan‐specific number to a mathematical concept of the DLG being a function of the distance in the beam's eye view (BEV) between the dose point and the leaf ends. Such function compensates for the difference between the penumbras in a beam delimited with a rounded leaf MLC and delimited with solid jaws. Utilization of this concept allowed us generating a pair of step‐and‐shoot MLC plans for which we could qualitatively predict the value of the DLG providing best match to ionization chamber measurements. The plan for which the leafs stayed predominantly at positions requiring low values of the DLG (as seen in the profiles of 1D slits) yielded the combined DLG of 1.1±0.2 mm, while the plan with leafs staying at positions requiring larger values of the DLG yielded the DLG 2.4±0.2 mm. Considering the DLG to be a function of the distance (in BEV) between the dose point and the leaf ends allowed us to provide an explanation as to why conventional single‐number DLG is plan‐specific in VMAT plans.PACS numbers: 87.56.jf, 87.56.nk
Highlights
68 Szpala et al.: dosimetric leaf gap (DLG) in volumetric-modulated arc therapy (VMAT) former effect is referred to as the leaf position offset (LPO), and is defined as the difference between the optical field size and the nominal leaf position
The LPO is corrected for in the multileaf collimators (MLC) controller software using a table of corrections to the nominal leaf positions depending on the distance from the central axis.[1,2] The partial transmission of radiation through rounded leaf ends is modeled in Eclipse treatment planning system (TPS) (Varian Medical Systems) through increasing the distance between the opposite leafs, following work of Wang et al[3] The increase in the field size along the direction of leaf movement from the field size set by the ends of the leafs is referred to as the dosimetric leaf gap (DLG).(4) The DLG is related to the radiation field offset (RFO), and numerically is twice as large.[2]
Film measurements The superior–inferior profile measured with film passing through two planning target volume (PTV) of a VMAT plan transferred to the cube phantom is shown in Fig. 4, together with the corresponding profiles calculated in Eclipse for various values of the DLG ranging from 0 to 3 mm
Summary
68 Szpala et al.: DLG in VMAT former effect is referred to as the leaf position offset (LPO), and is defined as the difference between the optical field size and the nominal leaf position. Rangel and Dunscombe[5] estimated consequences of systematic errors in leaf positioning in dynamic intensity-modulated radiation therapy (IMRT) plans, which produce a similar effect in matching the calculated to the measured dose distribution as modifying the value of the DLG. They simulated plans in Eclipse, and concluded that 2% error in the PTV dose is introduced when all leafs are opened up by about 0.3 mm in head and neck (H&N) plans and about 0.7 mm in prostate plans. They obtained the value of the DLG of 1.7 ± 0.1 mm for a 6 MV photon beam
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