SU‐C‐213CD‐04: Evaluation of IsoCal Imaging Isocenter Calibration System for Varian OBI Machines

Purpose: To evaluate the accuracy and reproducibility of the IsoCal geometric calibration system for Varian OBI kV and EPID imaging systems. Methods: IsoCal consists of hardware (IsoCal phantom, collimator plate) and software. IsoCal calibration starts by imaging the phantom and collimator plate using MV images with different collimator angles as well as MV and kV images at different gantry angles. The software then identifies objects on collimator plate and in the phantom to determine the location of the treatment isocenter and its relation to MV/kV imaging centers. It calculates offsets of the position of the imaging panels versus the treatment isocenter as function of gantry angle and writes a correction file that can be applied to MV/kV system to correct for offsets in the position of the panels. We performed IsoCal calibration three times in a row on 5 different linacs, each time with an independent setup. We also compared the IsoCal calibrations with an in‐house Winston‐Lutz‐based (WL) system as well as a Varian cube‐based (VC) system. Results: The IsoCal corrections ranged from 1.1 to 2.3 mm for MV and 1.1 to 1.8 mm for kV imagers across the 5 linacs. The variations in the three calibrations for each linac were within ±0.2 mm. The differences between IsoCal and the WL and VC systems were within 0.5 mm and 1.0 mm respectively. Without IsoCal the WL results showed discrepancies between the treatment isocenter and MV/kV imaging center ranging from 0.9 to 1.6 mm (MV) and 0.5 to 1.1 mm (kV), with IsoCal applied they were reduced to 0.2 to 0.4 mm (MV) and 0.3 to 0.4 mm (kV) across the 5 linacs. Conclusions: This study demonstrates that IsoCal is an accurate and consistent method for MV/kV image system calibration and periodic quality assurance.