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Sensitivity of Patient Specific IMRT QA to Errors in the Dosimetric Leaf Gap

D Bollinger

D Bollinger*, B Sun , Washington Univ. in St. Louis, St. Louis, MO


SU-I-GPD-T-274 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Precise knowledge of the dosimetric leaf gap (DLG) is critical for accurate delivery of dynamic IMRT plans. The purpose of this work is to evaluate the sensitivity of various patient-specific IMRT QA methods to DLG errors.

Methods: Ion chamber (IC) measurements were used to derive the DLG for three different photon energies on a TrueBeam and compared with the DLG values from the treatment planning system (TPS). Three different IMRT approaches were used to evaluate the dosimetric effect of the DLG: Matrixx (2d ion chamber array) measurements, point dose measurements with an IC, and portal dosimetry. To encompass a variety of levels of intensity modulation, seven patient plans of different treatment sites were chosen.

Results: The DLG values were derived at central axis to be 0.9, 1.1, and 0.7 mm for 6MV, 10 MV and 6FFF, respectively. These values were 1 mm less than the configured values in the TPS. The average gamma passing rate using Matrixx for the 7 IMRT plans was 92.89% with 1-2% scalling. The average point dose measured with the ion chamber was 4.7% lower than the calculated value from the TPS. The average passing rate using portal dosimetry was 73.9% for the 2%/ 2mm criteria and 92.6% for the 3%/ 3mm criteria. After the DLG was adjusted to match the values in the TPS, the average passing rate of Matrixx measurements increased to 97.6%, and the passing rate in portal dosimetry increased to 97.6% and 99.3% for 2% /2 mm and 3%/3mm respectively.

Conclusion: It is estimated that a systematic MLC leaf gap error of 1 mm will result in a 4.7% dose difference between the TPS predicted and measured dose. Both point dose measurements with an ion chamber and portal dosimetry show higher sensitivity to the errors in the DLG.

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