United States Nuclear Regulatory Commission - Protecting People and the Environment

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
                          WASHINGTON, D.C. 20555-0001

                                April 10, 1996

                               DOOR INTERLOCK CIRCUIT ON NUCLETRON HIGH-DOSE   
                               RATE AND PULSED DOSE RATE REMOTE AFTERLOADING   
                               BRACHYTHERAPY DEVICES


All U.S. Nuclear Regulatory Commission Medical Licensees authorized to use
brachytherapy sources in high- and pulsed-dose-rate remote (HDR/PDR)


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

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

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                                                            April 10, 1996
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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.


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

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

                     James A. Smith, NMSS
                     (301) 415-7904

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.
Page Last Reviewed/Updated Thursday, November 21, 2013