SU‐E‐T‐60: Can We Use the Same Gamma‐Passing Rate When Performing 3‐D Analysis as the One From Standard 2‐D Comparison?

Purpose: 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. Methods: 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. Results: 3‐D and 2‐D gamma were linearly related (R 2 = 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%). Conclusion: 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.