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Impact of Beam Energy Drifts On IMRT Delivery Accuracy

S Goddu

S Goddu1*, G Kamal2 , A Herman3 , S Edwards4 , S Yaddanapudi5 , S Oddiraju6 , B Cai7 , D Rangaraj8 , S Mutic9 , (1) Washington University, St. Louis, MO, (2) Washington University School of Medicine, St. Louis, MO, (3) Washington University School of Medicine, St. Louis, MO, (4) Washington University School of Medicine, St. Louis, MO, (5) Washington University in St. Louis, St. Louis, MO, (6) Baylor Scot & White, Temple, TX, (7) Washington University School of Medicine, St. Louis, MO, (8) Baylor Scott & White Health, Temple, TX, (9) Washington University School of Medicine, Saint Louis, MO


SU-F-T-279 (Sunday, July 31, 2016) 3:00 PM - 6:00 PM Room: Exhibit Hall

According to TG-40 percent-depth-dose (PDD) tolerance is ±2% but TG-142 is ±1%. Now the question is, which one is relevant in IMRT era? The primary objective of this study is to evaluate dosimetric impact of beam-energy-drifts on IMRT-delivery.

Beam-energy drifts were simulated by adjusting Linac’s bending-magnet-current (BMC) followed by tuning the pulse-forming network and adjusting gun-current. PDD change of -0.6% and +1.2% were tested. Planar-dosimetry measurements were performed using an ionization-chamber-array in solid-water phantoms. Study includes 10-head-and-neck and 3-breast cancer patients. en-face beam-deliveries were also tested at 1.3cm and 5.3cm depths. Composite and single-field dose-distributions were compared against the plans to determine %Gamma pass-rates (%GPRs). For plan dose comparisons, changes in %Gamma pass-rates (cPGPRs) were computed/reported to exclude the differences between dose-computation and delivery. Dose distributions of the drifted-energies were compared against their baseline measurements to determine the %GPRs. A Gamma criteria of 3%/3mm was considered for plan-dose comparisons while 3%/1mm used for measured dose inter-comparisons.

For composite-dose delivery, average cPGPRs were 0.41%±2.48% and -2.54%±3.65% for low-energy (LE) and high-energy (HE) drifts, respectively. For measured dose inter-comparisons, the average %GPRs were 98.4%±2.2% (LE-drift) and 95.8%±4.0 (HE-drift). The average %GPR of 92.6%±4.3% was noted for the worst-case scenario comparing LE-drift to HE-drift. All en-face beams at 5.3 cm depth have cPGPRs within ±4% of the baseline-energy measurements. However, greater variations were noted for 1.3cm depth. Average %GPRs for drifted energies were >99% at 5.3cm and >97% at 1.3cm depths. However, for the worst-case scenario (LE-drift to HE-drift) these numbers dropped to 95.2% at 5.3cm and 93.1% at 1.3cm depths.

The dosimetric impact of beam-energy drifts was found to be within clinically acceptable tolerance. However, this study includes a single energy with limited range of PDD change. Further studies are on going and the results will be presented.

Funding Support, Disclosures, and Conflict of Interest: Received funding from Varian Medical Systems, Palo Alto, CA

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