The dosimetric impact of control point spacing for sliding gap MLC fields

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

Commissioning and quality assurance for VMAT delivery systems: An efficient time‐resolved system using real‐time EPID imaging

“…Despite the lower standard deviation of log files and the reasonable agreement with EPID in this study, it is not recommended to rely solely on log files as the readout is sourced directly from the linac control system. In particular, care should be taken when using log files for MLC QA as the positions recorded may not reflect the actual location of the MLCs as other authors have demonstrated . In this study the log file analysis appeared to be sensitive to delivery errors introduced into Dose Test 2 and MLC Test 1, however this is due to the fact that errors were introduced into the DICOM plan file rather than the MLC, gantry angle or dose calibration.…”

“…In particular, care should be taken when using log files for MLC QA as the positions recorded may not reflect the actual location of the MLCs as other authors have demonstrated. 16,23,24 In this study the log file analysis appeared to be sensitive to delivery errors introduced into Dose Test 2 and MLC Test 1, however this is due to the fact that errors were introduced into the DICOM plan file rather than the MLC, gantry angle or dose calibration. As the EPID measurements are a direct independent measurement of the MLC-defined radiation field and output they are not subject to the same inadequacies as log files and may be used as a gold standard for both MLC and dose delivery accuracy.…”

“…5,[15][16][17][18][19][20][21][22] Machine log files offer a number of advantages such as the ability to streamline and automate QA processes, however, a number of recent studies have shown that log files are insensitive to some types of delivery errors. 16,23,24 This insensitivity arises from the fact that the parameters recorded by log files (i.e., MLC positions, gantry angle and dose fraction) are sourced from the linac control system itself and hence are not independent measurements. This was demonstrated by Agnew et al 16 who showed that Varian Dynalog files were not sufficient to detect MLC leaf calibration errors and backlash.…”

Toward real‐time verification for MLC tracking treatments using time‐resolved EPID imaging

“…Retrospective assessment of MLC accuracy and dose evaluation has been previously performed using machine log files for conventional radiotherapy and MLC tracking 6,9,12,13,30,31 . While use of log files is efficient and convenient, several studies have shown that they may be insensitive to MLC errors and are not truly independent of the linac control system 32‐34 . Agnew et.…”

“…Zwan et. al 34 . showed that log files can be insensitive to errors introduced into the planned MLC positions prior to communication with the MLC controller.…”