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The Impact of Measurement Uncertainty On the Dose Map Analysis of VMAT Verification Plans

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G Sandhu

G Sandhu*, K Kohli , S Szpala , Fraser valley cancer center, BC Cancer Agency, 13750, 96th Avenue, Surrey, British Columbia, V3V 1Z2


SU-I-GPD-T-286 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To investigate the impact of measurement uncertainty on dose map analysis of volumetric modulated arc therapy (VMAT) verification plans.

Methods: Thirty seven VMAT patient verification plans (anal canal (22), gynecological (5), prostate (4) and pelvis (6)) were generated on a cylindrical water phantom, using Eclipseᴿ, v11.031 (Varian Ltd, USA) treatment planning system (TPS) for Varianᴿ TrueBeam/clinac ix (Varian Ltd, USA) machines. Dose calculations were performed using the AAA dose calculation algorithm (version 11.031), grid size 0.25 cm and with inhomogeneity corrections. These plans were delivered on ArcCHECKᴿ (Sun Nuclear Corporation (SNC), USA) device and acquired dose maps were compared against the TPS reference plans using SNC patient software. Various gamma indices γ₃(3%, 3mm), γ₂(2%, 2mm), γ₁(1%, 1mm) were analyzed by enabling/disabling the measurement uncertainty correction (an option available within software). To study the effect of uncertainty correction on dose and distance parameter of gamma index following γ indices were analyzed: (3%, 3 mm), (2%, 3mm), (1%, 3mm) and (3%, 3 mm), (3%, 2mm), (3%, 1mm).

Results: It is found that disabling the uncertainty correction lowers the gamma passing rate. Average decrease is found to be: less than 1% in γ₃, up to 3% in γ₂ and up to 15% in γ₁. 1% decrease in dose parameter lowers the passing rate up to 3%. 1 mm decrease in the distance parameter lowers the gamma passing rate by less than 1%.

Conclusion: Uncertainty correction has less effect on γ₃ and more significant effect on γ₂ and γ₁ indices. Effect of uncertainty correction observed to be more pronounced on dose parameter than distance parameter of the gamma matrix. This effect need to be accounted for while setting the threshold for gamma passing rate. With disabled uncertainty correction, lower threshold passing rate need to be set for the effected gamma indices.

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