N
Patient 1
THE MANUFACTURER'S INVESTIGATION CONCLUDED THAT THERE WAS THE POTENTIAL FOR AN INCORRECT REFERENCE POSITION TO BE MEASURED DURING A CBCT SCAN. THE HDMM PATIENT DISPLACEMENT USES THE REFERENCE POSITION WHEN MONITORING THE PATIENT MOVEMENT IN THE DOSE CONTROL FUNCTION. THE REASON FOR THIS IS THAT THE REFERENCE PATIENT POSITION DURING A SCAN IS ALSO CALCULATED AFTER THE SCAN IS FINISHED AS LONG AS THE START/ENABLE BUTTONS ARE ACTIVE. IF THE PATIENT MOVES DURING THIS TIME INTERVAL BETWEEN THE LAST CBCT PROJECTION AND RELEASE OF THE START/ENABLE BUTTONS, THE PATIENT POSITION REFERENCE WILL BE AFFECTED IN AN INCORRECT WAY; THE PATIENT POSITION REFERENCE SHOULD ONLY BE MEASURED WHILE CBCT PROJECTIONS ARE BEING TAKEN. THIS COULD LATER DURING TREATMENT HIDE A PATIENT POSITION SHIFT AND DELAY THE DOSE CONTROL FUNCTION SHUTTING OFF THE RADIATION. INVESTIGATION AND ROOT CAUSE ANALYSIS: THE SYSTEM RELIES ON THE RELATIVE MEASUREMENTS BETWEEN A REFERENCE POINT AND THE CURRENT POSITION OF THE PATIENT MARKER. THIS DISTANCE IS CALLED THE PATIENT DISPLACEMENT. THE DISPLACEMENT IS USED TO PRESENT A GRAPH TO THE USER AND AS A CRITERIA FOR THE IFMM (INTRA-FRACTION MOTION MANAGEMENT) ALARM. THE REFERENCE POINT IS CRUCIAL TO IFMM. THE REFERENCE POINT IS DEFINED AS THE AVERAGE POSITION OF THE PATIENT MARKER DURING A CBCT SCAN. THE PRINCIPLE IS THAT THE PATIENT POSITION SHOWN IN THE CBCT IMAGE SHALL BE CAPTURED AS THE REFERENCE POINT FOR THE PATIENT DISPLACEMENT CALCULATION. THE PROBLEM INVESTIGATED HERE IS THAT THE REFERENCE POINT IS CALCULATED, USING PATIENT POSITION ALSO AFTER THE CBCT SCAN IS FINISHED UNTIL THE START/ENABLE BUTTONS ARE RELEASED. RISK ASSESSMENT: THE RISK OF AN INCORRECT REFERENCE POINT BEING CALCULATED DEPENDS ON THE FOLLOWING: 1. THE PATIENT MOVES AFTER LAST PROJECTION IS FINISHED BUT BEFORE THE START/ENABLE IS RELEASED. 2. THE MOVEMENT OF THE PATIENT IS NOT LARGER THAN 4 MM, OTHERWISE A WARNING IS SHOWN. THE WORST CASE EFFECT IS TREATMENT IN THE WRONG POSITION, HOWEVER FOR THE ISSUE TO OCCUR, A COMBINATION OF PATIENT MOVEMENT DURING A SMALL TIME FRAME AND THE START/ENABLE NOT BEING RELEASED AFTER THE SCAN HAS FINISHED, TOGETHER WITH THE WARNING IF THE PATIENT MOVEMENT IS TOO BIG WAS DETERMINED TO BE IMPROBABLE. THE PROBLEM HAS BEEN FIXED IN SOFTWARE VERSION 11.1, WHICH HAS NOT BEEN RELEASED TO DATE. A FIELD SAFETY CORRECTIVE ACTION IS NOT REQUIRED TO UPDATE THE SOFTWARE OR PRODUCT DOCUMENTATION FOR THE RELEASED VERSIONS AS THERE WAS NO NEW RISK IDENTIFIED. AT THE TIME OF THIS REPORT, THERE WERE NO INSTALLATIONS OF THE LEKSELL GAMMA KNIFE ICON IN THE US.