A Modified Failure Mode and Effects Analysis for Linear Accelerator Quality Assurance

Failure mode and effects analysis (FMEA) is a proactive method for evaluating failure modes and the consequences of those failures. In radiation therapy, a risk-based approach such as this can be used to inform and drive the quality assurance (QA) program, help prioritize QA, evaluate the impact of any changes to the QA process, and raise awareness of the potential failure modes. A classical FMEA can result in identical risk priority number (RPN) values for different combinations of occurrence, severity, and detectability. This assumes that these 3 values hold the same significance. In radiation therapy, high-severity failure modes can lead to severe clinical consequences and so should be prioritized.

A classical FMEA was carried out for the Varian TrueBeam linear accelerator QA at Peterborough City Hospital. This highlighted the issue of high-severity failure modes appearing relatively low in the FMEA table because of low overall RPN values. Therefore, a modified FMEA using a new clinical severity factor (S²) with hybrid RPN values was proposed and performed.

None of the failure modes resulted in very high RPN values. Some failure modes with high severities, such as gating system beam hold, breathing trace display, and backup timer operation, had relatively low overall RPN scores. The new modified FMEA minimized this effect by moving these failure modes up the FMEA table. For the modified FMEA, the higher scoring failure modes all relate to machine parameters associated with beam delivery and are important for techniques used for most patients. Therefore, the modified FMEA proposed gave clinically relevant results.

A new modified FMEA is proposed for linear accelerator QA in radiation therapy. This method is recommended to prevent high-severity failure modes from appearing low in the FMEA table and thus being given a low overall priority.