Estimation of Dosimetric Leaf Gap Correction Factor as Part of Commissioning a Log File Based QA System
U. Sreekala1*, N.S. Shine1, P. Raghukumar2 (1) Banasthali Vidyapith, Jaipur, Rajasthan, INDIA(2) Regional Cancer Centre (RCC), Thiruvananthapuram, Kerala, INDIA
Presentations
PO-GePV-T-194 (Sunday, 7/25/2021) [Eastern Time (GMT-4)]
Purpose: During the commissioning of the Mobius3D (M3D) QA software for TrueBeam (Varian Medical Systems Inc. Palo Alto, CA), an independent, log file-based QA system, it was observed that for more complex plans (IMRT/RapidArc) while performing dose comparison, the target volumes consistently came out as hot. This problem demanded a need for the determination of a Correction Factor (CF) for Dosimetric Leaf Gap (DLG). Even though DLG is a dosimetric parameter hardcoded in the M3D system to account for partial transmission through the rounded leaf ends on the MLC, provisions are incorporated within the software for its optimization by applying additional corrections. A Linear regression method recommended by the vendor was followed to overcome this scenario. This paper explains the steps performed to accomplish this task as part of M3D commissioning.
Methods: The process for determining the DLG CF involved a spreadsheet for the calculation and a verification phantom (VP) provided by the vendor, subtracting MLC transmission contribution from ionization chamber measurements of multiple sweeping gap plans, plotting the resulting differences against the nominal gaps, and using linear regression to finding the optimal DLG CF. MFX, the value from the VP, is compared with the measured values to find the percentage deviation. Seven different plans, IMRT/RapidArc for various sites, were used in this process.
Results: Dosimetric Leaf Gap (DLG) Correction Factor (CF) was determined for the energy 6MV and used these values to configure the M3D system. After the correction, the plan comparison for complex plans showed better results.
Conclusion: This method proved to be very easy and reliable, which could be used as an essential tool for fine-tuning the Dosimetric Leaf Gap (DLG) Correction Factor (CF) for M3D commissioning. This method's approach and principles can be applied to any independent QA verification system's commissioning.
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