Abstract
Dynamic sliding gap multileaf collimator (MLC) fields are used to model MLC properties within the treatment planning system (TPS) for dynamic treatments. One of the key MLC properties in the Eclipse TPS is the dosimetric leaf gap (DLG) and precise determination of this parameter is paramount to ensuring accurate dose delivery. In this investigation, we report on how the spacing between control points (CPs) for sliding gap fields impacts the dose delivery, MLC positioning accuracy, and measurement of the DLG. The central axis dose was measured for sliding gap MLC fields with gap widths ranging from 2 to 40 mm. It was found that for deliveries containing two CPs, the central axis dose was underestimated by the TPS for all gap widths, with the maximum difference being 8% for a 2 mm gap field. For the same sliding gap fields containing 50 CPs, the measured dose was always within ±2% of the TPS dose. By directly measuring the MLC trajectories we show that this dose difference is due to a systematic MLC gap error for fields containing two CPs, and that the cause of this error is due to the leaf position offset table which is incorrectly applied when the spacing between CPs is too large. This MLC gap error resulted in an increase in the measured DLG of 0.5 mm for both 6 MV and 10 MV, when using fields with 2 CPs compared to 50 CPs. Furthermore, this change in DLG was shown to decrease the mean TPS‐calculated dose to the target volume by 2.6% for a clinical IMRT test plan. This work has shown that systematic MLC positioning errors occur for sliding gap MLC fields containing two CPs and that using these fields to model critical TPS parameters, such as the DLG, may result in clinically significant systematic dose calculation errors during subsequent dynamic MLC treatments.PACS number(s): 87.56.nk
Highlights
In external beam radiotherapy, the multileaf collimator (MLC) is used to shape the X-ray beam to produce irregular beam geometries.[1,2,3] In advanced delivery techniques, such as intensitymodulated radiotherapy (IMRT) and volumetric-modulated arc therapy (VMAT), the MLC is employed to dynamically shape the beam during the delivery[4,5,6,7,8] in order to achieve highly conformal dose distributions
Accurate modeling of the MLC properties within the treatment planning system (TPS) is essential to ensure accurate dose delivery for IMRT and VMAT.[12,13,14] For the Varian Eclipse TPS (Varian Medical Systems, Palo Alto, CA), the only MLC properties that may be edited by the user are the MLC transmission factor and the dosimetric leaf gap (DLG).(13,14) The DLG is a parameter used in Eclipse to model the difference between the light field edge and the radiation field edge
We report on the detection of systematic MLC gap errors during the delivery of sliding gap MLC fields created using just two control points (CPs)
Summary
The multileaf collimator (MLC) is used to shape the X-ray beam to produce irregular beam geometries.[1,2,3] In advanced delivery techniques, such as intensitymodulated radiotherapy (IMRT) and volumetric-modulated arc therapy (VMAT), the MLC is employed to dynamically shape the beam during the delivery[4,5,6,7,8] in order to achieve highly conformal dose distributions. Accurate modeling of the MLC properties within the treatment planning system (TPS) is essential to ensure accurate dose delivery for IMRT and VMAT.[12,13,14] For the Varian Eclipse TPS (Varian Medical Systems, Palo Alto, CA), the only MLC properties that may be edited by the user are the MLC transmission factor and the dosimetric leaf gap (DLG).(13,14) The DLG is a parameter used in Eclipse to model the difference between the light field edge and the radiation field edge This difference is caused by X-ray transmission through the rounded end of the MLC.[1,7,15] Eclipse corrects for this by reducing the distance between opposing MLC leaves by a fixed amount at each control point in the plan. It is clear from these works that any errors in measurement of the DLG can potentially result in clinically significant dose delivery errors
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