P207] Star-shot analysis of radiation isocenter wobble: Quality control of quality control tools

Abstract

Purpose To devise a secondary cross-check for the software used in the quality control (QC) of the isocenter wobble during the collimator, couch and gantry rotation. Methods Monthly QC of the radiation isocenter wobble is performed with 3 slit-like fields spaced 120° apart, exposing cassettes with phosphor imaging plates, which are scanned using Kodak ACR-2000 tabletop computed radiography system. The ensuing DICOM images are analyzed both with Pipspro 4.4 (Standard Imaging, Middleton, WI, USA) and Pylinac 2.0.1 (James Kerns, https://pypi.python.org/pypi/pylinac ) starshot module. The diameter of the circle which touches the projections at the point of intersection is taken as an estimate of the isocenter wobble. Results Preliminary results obtained on data for a single treatment machine show an agreement which is below our expectations. The means (±standard deviations) for the difference of values obtained by the two methods are −0.014 mm (±0.268 mm) for the collimator, −0.162 mm (±0.229 mm) for the couch, and −0.064 mm (±0.180 mm) for the gantry. Conclusions The results show that at least one of the programs is performing sub-optimally. As a next step, we intend to test both systems with a set of synthetic images with varying ”wobble“. Our intention is to establish Pylinac as a primary standard, as it is an open-source solution, the correctness of which can be verified independently by analyzing the source code, and test the proprietary ”black-box“ solutions against it.