Evaluating the performance of two matched elekta versa HD machines for agility MLC modeling in VMAT plans using the monaco treatment planning system

Abstract

Author(s): Ibrahim H. Alnidawi*, Mohamed N. Yaseen, Magdy M. Khalil and Hany M. Ammar

The aim of this study was to investigate the accuracy and tolerance of using a single data set to perform agility head Multi-Leaf Collimator (MLC) modelling in Volumetric Modulated Arc Therapy (VMAT) plans using two matched linear accelerators. First, the Express QA package provided by the system manufacturer was measured on two matched Versa HD Linear Accelerators (LINACs) at to evaluate the match credibility for the MLC parameters of the two LINACs assessed. Second, ten patients with head and neck cancer and ten patients with prostate cancer were evaluated, with and without MLC modelling, while maintaining identical optimisation constraints in the VMAT plans. All patient plans were verified using an IBA Dosimetry MatriXXEvolution array inserted into a multi cube phantom. Finally, ten head and neck clinical cases were measured using the MatriXX array on the two matched LINACs. Comparisons were performed using IBA MyQA patient software, including a gamma index and profile-to-profile analysis. During the first evaluation, 3ABUT plan analysis was performed at the first abutting line y1 and the second abutting line y2, which showed significant gamma index differences for the minor offsets between the two LINACs. In FOURL, the gamma index analysis results showed some differences for leaf groove and abutting regions. The abutting region showed a high level of gamma index agreement, whereas the tongue and groove region showed obvious gamma index discrepancies. For the second evaluation, in patients with head and neck and prostate cancers, the differences in the Planned Targeting Volume (PTV) doses were not significant, and most variations were associated with the maximum dose. Although the identified discrepancies were not significant, even small differences could be clinically crucial for accurate and precise radiation treatment planning. During the final assessment, the evaluations of the two matched Versa HD LINACs were compared to investigate their impacts on treatment plans for patients using the VMAT technique. The reference LINAC showed high agreement with the calculated dose, although some variations were reported when compared with the other LINAC. The deviations, in some cases, were greater than 4% for the VMAT plans, with 2% mean differences for all plans. The discrepancies between clinical cases were likely due to one or more MLC parameters differing between the two LINACs. Performing VMAT plans may have increased the chances of these differences appearing. The use of a single set of MLC parameters might not be an optimal approach for matched LINACs. The MLC parameters showed significant discrepancies between the two matched LINACs and should potentially be considered in the matching adjustment.

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