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Effect of Dose Plane Resolution On IMRT Plan Verification

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N Manohar

N Manohar*, J Rosenfield , E Elder , A Dhabaan , Emory University, Atlanta, GA


SU-I-GPD-T-283 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To assess the effects of calculated dose plane resolution and assignment of reference dose distribution on gamma analysis results for IMRT plan verification.

Methods: IMRT plan verification using gamma analysis involves comparison of reference and evaluated dose distributions. The original formulation recommends setting the measurement as the evaluated distribution and the calculation as the reference. However, the converse has also been implemented clinically. In this study, 10 VMAT plans were delivered to a 24.4×24.4 cm² ion chamber array with 1020 detectors (~32×32 array), corresponding to a detector spacing of 0.762 cm. The equivalent dose plane from the Eclipse treatment planning system (TPS) was exported at various resolutions (1.575-0.046 cm, corresponding to 16×16-512×512 pixels). Gamma analysis (3% / 3 mm) was performed and the mean pass rates and average gamma values were examined for each resolution setting using the TPS calculation as the reference distribution and then as the evaluated distribution.

Results: Gamma pass rate was less sensitive to exported dose plane resolution when the TPS calculation was used as the reference. The maximum pass rate and the minimum average gamma value occurred when the TPS resolution equaled that of the measurement (32×32 pixels). Setting the measurement as the reference, the pass rate was generally higher but decreased significantly (with an associated increase in average gamma) when the TPS resolution was less than that of the measurement. The pass rate and the average gamma were nearly identical when the measurement and TPS resolutions matched, independent of which distribution was selected as the reference.

Conclusion: When performing IMRT plan verification using gamma analysis, the choice of reference and evaluated dose distributions should be carefully considered, along with resolution of the calculated dose plane. In general, the calculated dose plane should have a resolution at least equal to that of the measurement.

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