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Can We Use the Same Gamma-Passing Rate When Performing 3-D Analysis as the One From Standard 2-D Comparison?

C Schinkel

C Schinkel*, C Christou , K Prado , B Yi , Univ. of Maryland School Of Medicine, Baltimore, MD


SU-E-T-60 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Increasing numbers of institutions are beginning to utilize calculation-based methods (Monte Carlo or commercial software) of performing patient specific IMRT QA. In such systems the full three-dimensional (3-D) dose distribution may be available. In this work we determine the gamma pass rate calculated using the full 3-D dose distribution that is equivalent to the more conventional 2-D gamma pass rate.

55 IMRT and SBRT plans with a variety of field sizes were selected. All plans were created using Pinnacle v9.0. The delivered dose for each plan was re-computed using Monte Carlo simulation and then compared to the planned dose. 2-D gamma analysis was performed on each of three planes (axial, sagittal and coronal) and the values were compared with those obtained through 3-D gamma analysis. Gamma criteria used included 2%/2mm, 3%/3mm and 5%/5mm.

3-D and 2-D gamma were linearly related (R² = 0.84). The linear relationship held for all three 2-D planes and did not appear to depend on field size or on gamma criterion. For a given 2-D gamma value, the calculated 3-D gamma was always larger. For a 2-D gamma pass rate of 95% (range 92-97%), the average equivalent 3-D gamma was 97.5% (range 94.3 - 99.7%).

For patient-specific IMRT QA involving the full 3-D dose distribution, acceptable plans require a larger percentage of points to pass gamma analysis than for the equivalent 2-D analysis employed during measurement-based QA. If a plan shows a 95% pass rate for a 2-D planar gamma calculation, the corresponding acceptable 3-D plan will have a pass rate of 97.5%. Clinics that use the full 3-D dose distribution for QA will need to adjust their passing criteria accordingly.

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