ESTRO 2022 - Abstract Book

S543

Abstract book

ESTRO 2022

Materials and Methods The VSP geometry is invariant under couch and gantry rotation which simplifies dose reconstruction. The SRS/hyperarc plan is transferred to the VSP in the treatment planning system (TPS) with couch angles and dose calculated (Eclipse V15.6.05/15.6.8). EPID image frames are captured on Varian Truebeam linear accelerators in-air (without couch rotations) with aS1200 EPID and used to construct cine EPID images after insertion of the arc couch angle in the image header. For each acquired image, 3D dose is calculated in the VSP at the corresponding gantry and couch angle using a depth-dependent EPID to dose conversion model and all image doses summed. TPS and reconstructed dose are compared using 3D gamma analysis. To experimentally validate the method, EPID reconstructed dose was compared to measured gafchromic film measurement in the Lucy SRS phantom (14 cm diameter) for a 5-lesion (each 10 mm diameter) HyperArc test plan after recalibrating the method to estimate dose in the Lucy phantom. Clinical results for EPID reconstructed dose distributions compared to TPS were obtained for 36 patients (52 plans) for 6FFF energy using 3D gamma analysis in the VSP. Results The comparison of film and EPID reconstructed dose in the Lucy phantom is shown in Figure 1, along with profile comparison to the TPS dose. A 2D gamma pass-rate of 97.1% was obtained with 5%, 1 mm criteria and 10% dose threshold. The pass- rate for measured compared to TPS dose was 99.9%. The EPID method calculates for a perfect uniform density phantom while the Lucy phantom has significant shape and density variations resulting in greater differences than for the TPS, however the results confirm that the method is reproducing measured dose. An example of a clinical verification result is shown in Figure 2. The comparisons between EPID reconstruction and TPS gave gamma pass-rates (mean ± 1 SD) of 99.3% ±0.59% and a minimum of 97.7% at 3%, 1 mm, 10% dose threshold criteria.