SU-E-T-49: A Method of 3D Dose Reconstruction Using MapCHECK Measurement for IMRT QA

Abstract

Purpose: A method to reconstruct point dose in an IMRT phantom using 2D dose measurement to replace ion chamber measurement for routine prostate IMRT QA.Methods: IMRT treatment plans for a cohort of 46 patients with prostate cancer were generated by Pinnacle V8.0. The planar dose for each IMRT field was measured with MapCHECK 2 mounted at an isocentric jig positioned at SAD of 100.4 cm with water equivalent build up of 5 cm. The 2D MapCHECK measurements with corresponding gantry and collimator angles were subsequently imported into an in‐house code for 3D dose reconstruction in a virtual water equivalent cylindrical phantom (diameter: 20cm, length: 18 cm). The dose deposition to the cylindrical phantom was calculated with ray tracing with percent depth dose (PDD) applied based on the MapCHECK measurements. Separately, a volumetric ion chamber (PTW TN30011‐1) measurement in low dose‐gradient region was made in a water‐equivalent cylindrical phantom of the same size for each IMRT plan. The cumulative dose values measured by the ion chamber were compared with the calculated doses with the proposed method.Results: The cumulative dose was compared among the three different methods. The mean and standard deviation (SD) of the differences were found to be 0.10±1.22% in the comparison of Pinnacle vs ion chamber, 0.10±1.22% in Pinnacle vs proposed method, and 0.26±1.24% in ion chamber vs proposed method, respectively. Bland‐Altman analysis in the cumulative dose between ion chamber and proposed method suggested that there was negligible systematic bias (mean difference: 0.57 cGy) between the two methods and the 95% limits of agreement (−5.42 to 6.57 cGy) was acceptable Conclusions: The study showed the feasibility of replacement of a point dose measurement by ion chamber with the proposed 3D dose reconstruction method for prostate IMRT QA.