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Evaluate the Dosimetric Impacts On Patient Specific Treatment Plans Due to Set Up Uncertainties During LINAC Annual QA and Commission

V Nguyen

V Nguyen1*, C Shi2 , B Wang1 , (1) James Graham Brown Cancer Center, University of Louisville, Louisville, KY (2) Saint Vincent Medical Center, Bridgeport, CT


SU-E-T-367 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To quantify the dosimetric impact on patient’s specific treatment plans due to set up uncertainties during LINAC commission and annual QA and to determine the maximum set up uncertainty allowance range.
Methods: A 60x60x60 cm³ solid water cube was created in Varian Eclipse TPS. Beam data profiles (crossline and diagonal) and PDDs for field sizes ranging from 2x2 cm² to 40x40 cm² were simulated. Three main uncertainty scenarios were purposely introduced for gantry position tilts (0- 5°), source axis distance changes (100-105 cm), and iso-center position shifts (0-5 mm) during the simulation. A gamma analysis was used to compare the correct simulated profiles with the profiles for each scenario. Two static IMRT treatment plans (H&N and GYN) with tumors at 5 cm and 15 cm depths were compared using similar set up uncertainties.
Results: A gamma analysis using ±3%/±3mm with 90% passing rate criteria is included to show the passing rate for each scenario. Crossline and diagonal profiles showed a gamma passing rating of ≥ 90% at depth ≤10 cm for these scenarios: gantry tilted from 0-5°, SAD changed from 100-105 cm, and iso-center shifted ≤ 4 mm. From 10 to 20 cm depths, all three scenarios failed with gamma passing ≤ 90% excepted for diagonal profiles at Gantry =2° , SAD =1 cm, and iso-center =1 mm off center. Diagonal profiles showed a higher gamma passing rating compared to crossline profiles for all three scenarios. PDD differences also increased as depth increased. For patient’s specific treatment plans, maximum uncertainties allowed to obtain a ≥90% gamma passing rating are: gantry tilts ±1 degree, SAD shifts ±2 cm, and iso-center moves ±3 mm.
Conclusion: This study validated AAPM TG 142 recommendations on the mechanical and dosimetry uncertainties and provided proofs on maximum acceptance tolerances for LINAC annual QA and commission.

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