SU-E-T-710: Study of Dosimetric Leaf Gap and Transmission Factor Variations Affecting Common Clinical QA Tools

Authors


Abstract

Purpose:

To determine if clinical variations in the Dosimetric Leaf Gap (DLG) or Transmission Factor (TF) are detectable in a 3D secondary calculation, 1D secondary calculation, log file-based analysis or hardware-based quality assurance of realistic patient plans.

Methods:

DLG values and TF values were selected based on clinical surveys of 17 clinics. The minimum, maximum, and representative values were used in the generation of 30 6MV plans. Six patients were created in Eclipse 11 and planned for three sites: SBRT Lung, Prostate, and Prostate with Lymph Nodes(P/S/L). Each site was planned utilizing two different techniques: IMRT and VMAT. Variations in the DLG used were 0.02cm, 0.1748cm, 0.17208cm (SBRT), and 0.2cm while holding the TF at 0.015 and 0.0148 (SBRT). The TF values used were 0.0135, 0.015, 0.0148 (SBRT), and 0.0215 while holding the DLG at 0.1748cm or 0.17208cm(SBRT).Plans used the Analytic Anisotropic Algorithm. Consistency in planning included controlling objectives, iterations, priorities, and normalizations. Plans were delivered on a Varian Trilogy with Millennium 120 MLCs. Plans were analyzed with: Mobius3D using Collapsed Cone Convolution/Superposition Algorithm (CCCS) reference beam model, RadCalc using a modified Clarkson scatter integration lookup tables, a gantry-mounted MapCHECK2 using a 2-D diode array, and MobiusFX log file-based CCCS analysis. Gamma analysis in Mobius3D, MobiusFX, and MapCHECK2 used a gamma criteria of 95%/3%/3mm. No efforts were made to improve any outcome by searching for improved agreement.

Results:

The prostate IMRT plans were not affected by any variation. IMRT Lung showed mixed results. VMAT Prostate plans were not affected. The VMAT P/S/L was flagged by Mobius3D and Fx, while the VMAT Lung DLG 0.02cm was flagged by RadCalc as well as the IMRT Lung TF=0.0215.

Conclusion:

The variations in clinical DLG values and TF values were only detectable in extreme cases (DLG=0.02cm or TF=0.0215) in a few select plan variations tested here.

Mobius Systems supplied by Mobius Medical, LP. C Bull is an employee of Mobius Medical, LP

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