Resolution of Generic Safety Issues: NMSS-0001. Door Interlock Failure Resulting from Faulty Microselectron High Dose Rate Remote Afterloader ( NUREG-0933, Main Report with Supplements 1–34 )
During the administration of the third of a three-fraction vaginal treatment of a patient on January 9, 1996, the treatment console of a Nucletron high-dose-rate (HDR) remote afterloader unit locked up, and the console alarm sounded when the afterloader was nearly finished treating the seventh dwell position of an 11-position treatment. At the sound of the alarm, the physicist directed the device operator to terminate the treatment. The operator pushed the console-mounted "Treatment Interrupt" button, but the treatment unit did not respond by withdrawing the source. The physicist then instructed the device operator to open the treatment room door slightly so that the door interlock would cause the source to withdraw. This was done, but the source did not retract into the storage safe as expected. At this point, the physicist entered the treatment room, opened the top of the treatment unit, and started to turn the manual source retraction mechanism. The treatment unit sensed this movement/resistance and automatically executed an emergency retraction of the source. As a result of the interrupted treatment during the third fraction, the full course of the therapy actually delivered 1888 rads to the prescription point, a deviation of 10.1 % from the planned dose. This issue was identified1700 by NMSS.