Table 3.

Failure modes with the highest RPN scores for Gamma Knife Radiosurgery

No.	Step	Potential failure modes	Potential causes of failure	Potential effects of failure	RPN	Notes and examples of causes and failures
65	Frame adapter attachment	Frame adapter not attached properly	1. Inadequate training/orientation 2. Personnel omission	Inconvenience Inaccurate dose delivery	122.55	On the Perfexion, the frame adaptor should be attached properly. Failure to do so may result in an interruption in the treatment process or an imperfect dose delivery. Elekta has a field notice about this.
7	Frame/fiducial box check	Bad fiducial box assembly	1. Lack of standardized procedures 2. Personnel omission 3. Inadequate training/orientation	Inaccurate volume Inaccurate dose distribution Imaging may need to be repeated	61.70	A fiducial box is not assembled correctly, i.e. screws are loosen, frontal/back or left/right pieces are switched etc.
73	Final check	Inserts/plugs not secured	1. Personnel omission 2. Lack of double check	Interrupted treatment	60.38	An insert or plug is not firmly attached to a helmet. The insert/plug drops during a treatment. Cause the treatment to stop.
38	Image definition/levelling	Inadequate leveling	Inadequate training	Wrong volumes	59.72	Images are not levelled appropriately. The boundary of a treatment area is blurred. Certain areas are overlooked because of poor image contrast.
28	Machine daily QA	Undetected mechanical failure	1. Lack of standardized procedures. 2. Human error	Inconvenience Delayed or partially completed treatment	48.71	There is a mechanical failure in the treatment unit (APS problem, helmet micro switch problem, helmet hoist malfunctioning, sector switch malfunctioning, couch problem etc.) but not identified during the morning QA process. Treatment can not start or may have to be stopped in the middle.