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Information Notice No. 96-21: Safety Concerns related to the Design of the Door Interlock Circuit on Nucletron High-Dose Rate and Pulsed Dose Rate Remote Afterloading Brachytherapy Devices
UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR MATERIAL SAFETY AND SAFEGUARDS WASHINGTON, D.C. 20555-0001 April 10, 1996 NRC INFORMATION NOTICE 96-21: SAFETY CONCERNS RELATED TO THE DESIGN OF THE DOOR INTERLOCK CIRCUIT ON NUCLETRON HIGH-DOSE RATE AND PULSED DOSE RATE REMOTE AFTERLOADING BRACHYTHERAPY DEVICES Addressees All U.S. Nuclear Regulatory Commission Medical Licensees authorized to use brachytherapy sources in high- and pulsed-dose-rate remote (HDR/PDR) afterloaders. Purpose The NRC is issuing this information notice to alert addressees to the recent discovery that the treatment room door interlocks used with the Nucletron HDR and PDR devices are rendered inoperative by the failure of either the control unit or by a loss of communications between the control and treatment units. It is expected that recipients will review the information for applicability to their facilities and consider actions, as appropriate, to avoid similar problems. However, suggestions contained in this information notice are not NRC requirements; therefore, no specific action nor written response is required. Description of Circumstances During the administration of the third of a three-fraction vaginal treatment of a patient on January 9, 1996, the treatment console of the Nucletron HDR 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 termin- ate 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 treat- ment 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 retrac- tion mechanism. The treatment unit sensed this movement/resistance and automatically executed an emergency retraction of the source. The original treatment plan was to deliver three fractions of 7 gray (Gy) (700 rads) each at a distance of 0.5 centimeters (cm) from the surface of a 3 cm diameter vaginal cylinder, using 11 dwell positions of 0.5 cm spacing. Thus, the full course of therapy would have delivered 21 Gy (2100 rads) to the 9604040106. IN 96-21 April 10, 1996 Page 2 of 3 prescription point according to the plan. As a result of the interrupted treatment during the third fraction, the full course of therapy actually delivered 18.88 Gy (1888 rads) to the prescription point, a deviation from the planned dose of 10.1 percent. Discussion Before this incident on January 9, 1996, there had been at least two other reported failures of the Nucletron HDR console resulting in aborted patient treatments. Unlike the present failure, these treatments were properly aborted by use of the device's �Emergency Stop� button, which activates the emergency source retract mode via a direct-wire connection to the treatment unit's emergency source retraction circuit. After these earlier failures, Nucletron revised the oscillator circuit on the console processor circuit board to correct the failures. Since then, NRC has received no further reports of console failures caused by this console oscillator failure. The most recently reported failure is believed to be caused by a loss of the communications link, for unknown reasons, between the control unit and the treatment unit. In the latest console failure incident, the licensee�s attempt to use the door interlock in lieu of the �Emergency Stop� button, to abort the treatment, alerted the NRC to the fact that the door interlock was not functioning in a manner that provided adequate protection for attempted entries into the treatment room with the source exposed. NRC believes that the door interlock protection system should be independent of the proper functioning of other circuitry in the performance of its essential safety function. This can be achieved by designing the circuitry in such a manner that the emergency source retraction cycle is immediately implemented on opening the treatment room door. After this incident, Nucletron-Oldelft Corporation issued a �Safety Alert� (Attachment 1) to all of its HDR/PDR device customers, on March 4, 1996, which sets forth a series of four steps to be taken if the Control Unit stops updating the status of the treatment in progress, for any reason. These steps should ensure that the treatment is interrupted and the source retracted before licensee personnel enter the treatment room. Discussions between Nucletron-Oldelft (a State licensee) and the State of Maryland are ongoing, at this time, on the development of an appropriate corrective action by Nucletron for this door interlock design deficiency. It should be noted that NRC is uncertain whether similar defects in the design of the door interlock circuitry of other makes and models of HDR devices may exist. Unless established by test or vendor documentation, all users of these devices should consider that the door interlock protection circuit could be rendered inoperative by various device component failures. NRC is concerned about the possibility of either patients or licensee staff receiving excessive exposures to radiation if an HDR/PDR source fails to retract as expected when the treatment room door is opened. Because this failure mode is documented . IN 96-21 April 10, 1996 Page 3 of 3 for the Nucletron devices, all affected licensees are encouraged to follow the recommendations of the device vendor, as contained in its �Safety Alert� of March 4, 1996. This information notice requires no specific action nor written response. If you have any questions about the information in this notice, please contact the technical contacts listed below or the appropriate regional office. signed by Donald A. Cool, Director Division of Industrial and Medical Nuclear Safety Office of Nuclear Material Safety and Safeguards Technical contacts: Robert L Ayres, NMSS (301) 415-5746 Internet:firstname.lastname@example.org James A. Smith, NMSS (301) 415-7904 Internet:email@example.com Attachments: 1. Nucletron Safety Alert dated March 4, 1996 2. List of Recently Issued NMSS Information Notices 3. List of Recently Issued NRC Information Notices Note: Attachments not included in electronic version.
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