A novel audit methodology to improve detection of planning and delivery errors in passive motion management

End-to-end dosimetry audits, including respiratory motion, typically compare film measurements acquired in a moving frame directly to planned dose distributions that do not resolve the applied motion. Inherent motion-induced dose differences cannot be distinguished from error-induced differences in this analysis. An alternate comparison approach is proposed utilising both motion-coupled and decoupled film measurements to overcome the motion-induced differences, and improve error detection.

Reference internal target volume-based plans were created for lung and liver SBRT treatment sites in the ACDS motion phantom. A series of planning and delivery errors were introduced, and 4D dose accumulation was used to estimate the delivered dose. All plans were delivered to the phantom loaded with two films, analysed using gamma criteria ranging from 3%/1mm to 7%/5mm. A receiver operator characteristic decision matrix based on the gamma passing rate and 4D dose difference to reference ( $\left(\Delta 4D\right)$ ) was used to calculate sensitivity/specificity using the standard and proposed (test) comparison approaches.

7/13 introduced errors had $\left(\Delta 4D\right)$  ≥ 5% (range: -21.0% to + 3.0%). 3/25 combinations in the standard arm had sensitivity and specificity ≥ 0.5, versus 9/25 in the test arm. Standard analysis performed optimally using 5%/3mm criteria (sensitivity = 1.0, specificity = 0.62). The test arm had equal sensitivity with improved specificity (0.75) using 4%/2mm criteria.

Film measurements acquired in a moving frame are over-sensitive with low specificity. Eliminating the impact of motion using the proposed measurement/analysis approach enables more stringent gamma criteria (4%/2mm versus 5%/3mm) for better classification accuracy.