SU-E-T-368: Effect of Oblique Beam Incidence On 3D Dose Reconstruction for Small Field IMRT QA.

PURPOSE To compare dose reconstruction accuracy with different oblique beam angles when 2d detector arrays are used for small field IMRT QA. METHODS 2D detector arrays are convenient to use but their low resolution limits the number of sampling points inside small radiation fields. In this study, we demonstrate that with oblique beam incidence, more points can be measured and 3D dose volume can be reconstructed more accurately. QA plans were simulated and measured using four oblique angles: 90 degree (straight incidence), 60 degree, 30 degree, and 15 degree. 3D dose volume was reconstructed based on dose measurements through back-projection and inverse attenuation correction. Total delivering dose was calculated after rotating individual beam doses into actual beam angles. Two IMRT QA devices were used in our study, including a PTW /Octavius phantom and a Delta4 QA device. A spine SBRT plan was chosen as the study case. Gamma passing rate, mean PTV dose, and maximum dose received 2cc of spinal cord were calculated to compare reconstructed dose to original planning dose. Effects of measurement error and detector resolution were also investigated. RESULTS Oblique beam incidence will improve the sampling frequency of dose measurments. When the detector spacing is 10mm, 30 degree is the optimal angle for the Delta4 phantom and 15 degree is the opitmal angle for the PTW/Octavius phantom. Compared with standard straight incidence setup, QA with the optimal angle has 10% higher gamma passing rate (with 2mm/2% criteria) in both simulation and actual measurement. However, oblique incidence didn' t improve QA accuracy consistently when the detector spacing is 5mm. CONCLUSION Oblique incidence can be used to improve IMRT QA accuracy for 2d-array devices which don' t have sufficient spatial resolution for small field measurement.