Investigation of the mechanical performance of Siemens linacs components during arc: gantry, MLC, and electronic portal imaging device

Abstract

Background

In radiotherapy treatments, it is crucial to monitor the performance of linac components including gantry, collimation system, and electronic portal imaging device (EPID) during arc deliveries. In this study, a simple EPID-based measurement method is suggested in conjunction with an algorithm to investigate the stability of these systems at various gantry angles with the aim of evaluating machine-related errors in treatments.

Methods

The EPID sag, gantry sag, changes in source-to-detector distance (SDD), EPID and collimator skewness, EPID tilt, and the sag in leaf bank assembly due to linac rotation were separately investigated by acquisition of 37 EPID images of a simple phantom with five ball bearings at various gantry angles. A fast and robust software package was developed for automated analysis of image data. Three Siemens linacs were investigated.

Results

The average EPID sag was within 1 mm for all tested linacs. Two machines showed >1 mm gantry sag. Changes in the SDD values were within 7.5 mm. EPID skewness and tilt values were <1° in all machines. The maximum sag in leaf bank assembly was <1 mm.

Conclusion

The method and software developed in this study provide a simple tool for effective investigation of the behavior of Siemens linac components with gantry rotation. Such a comprehensive study has been performed for the first time on Siemens machines.

Keywords:

• linac
• Siemens
• arc
• sag
• EPID
• gantry

Acknowledgments

The authors gratefully acknowledge Ms Katalin Szep from Siemens Customer Solutions, Erlangen, Germany, for helpful discussions and detailed information on the Siemens linac mechanical constructions, and Mr Terje Pedersen, Radiofysisk Laboratorium, Odense University Hospital, Odense, Denmark, for a stand-alone solution of the Mosaiq verification system to allow EPID image acquisition on the Siemens Primus linac. This work was partly supported by the WA Department of Health Merit Award.

Disclosure

The authors declare that they have no competing interests in this work.