Information Notice No. 96-72: Undetected Failures that may occur during Patient Treatments with Teletherapy Devices

                                          UNITED STATES
                                  NUCLEAR REGULATORY COMMISSION
                                     WASHINGTON, D.C.  20555

                                        December 24, 1996



All teletherapy licensees.


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to a recently reported failure of an AECL Theratron
780 teletherapy device to place the source in the exposure position during
patient treatment, and the potential for similar failures in all older AECL
teletherapy devices that were manufactured prior to 1985.  These include the
Theratron T765, T80, and T60 model numbers, and the Eldorado E78, E76, E8, and
E6 model numbers.  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

On November 29, 1996, a licensee's teletherapy physicist notified NRC that he
was experiencing electrical problems with an AECL Theratron 780 teletherapy
unit.  On  November 27, 1996, the licensee's radiation therapy technologist
observed that the cobalt-60 teletherapy source was not in the exposure (beam-
on) position during a patient treatment, even though the treatment timer was
running.  The running treatment timer was an indication of in-progress patient
treatment.  However, the beam-on indicator light was not illuminated, nor was
the room radiation monitor indicating the presence of radiation.  Thus, the
information provided by the operating treatment timer conflicted with that
from both the beam-on indicator and room radiation monitor.  Upon observation
of this problem, the .                                                                           IN 96-72
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treatment was terminated by licensee staff.  When the teletherapy physicist
subsequently evaluated the control panel operation he discovered a sporadic
failure of the source to move to the exposed position, despite normal
operation of the treatment timer.

On December 2, 1996, a service representative from Theratronics International
Corporation arrived at the licensee's site.  The service representative
concluded that a loose wire connection, between the timer and the solenoid
driver circuit board, was causing a sporadic circuit interruption which, in
turn, prevented the activation of the driver necessary to move the source into
the exposed position.  The control console lights were found to be functioning
normally; however, since the timer was not connected to any source position
indicators, it continued to run even though the source was not in the exposed
patient treatment position.  Following repair of the loose wire connection to
the circuit board the Theratronics service representative determined the unit
was fully operational. 

The device subsequently failed again on December 9, 1996, in a manner similar
to that of the previously reported November 27, 1996, failure.  This second
failure has been attributed to a faulty solenoid driver circuit board, which
was replaced by the Theratronics service representative.  The first test of
the unit, with the replacement circuit board in place, also resulted in the
same system failure.  However, neither the service representative or
inspectors from both the Food and Drug Administration (FDA) and NRC have been
able to reproduce this latest failure following replacement of the circuit
board.  Thus, the root cause of these failures has yet to be determined and
efforts are continuing to make this determination.


Review of this incident reveals that the malfunctioning unit was an early
model Theratron 780 using a single channel timer.  These single channel timers
can be of either mechanical or digital designs.  With this type of timer,
originally installed on all AECL/Theratronics teletherapy units built before
1985, patient treatments are initiated by starting the treatment timer which,
in turn, generates the signal to move the source to the exposed treatment
position.  If, for any reason, the source fails to move to the exposed
treatment position, the timer will continue to run without interruption.  This
would result in the patient receiving less than the intended dose for that
treatment fraction.  Additionally, this can occur not only upon the initiation
of treatment, but at any time the source exposure signal pathway is
interrupted.  Potential harm to the patient could occur if the failure of the
source to move to the fully exposed treatment position is not observed, and
subsequently corrected, by the licensee.

Since these older generation devices have this known single-point failure
mode, it is important that licensees be especially vigilant in monitoring all
status information available on the initiation, and throughout the course, of
treatment.  These would include not only the .                                                                          IN 96-72
                                                                          December 24, 1996
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timer count down, but the source position indicators and, if appropriate, the
room radiation monitor alarm output.  The illumination of the "Beam On"
indicator does not necessarily mean that the source is in the exposure
position, but merely indicates the source is not in the fully shielded

For the older (prior to 1985) AECL/Theratron devices with single channel timer
units, the manufacturer presently offers a replacement dual channel timer of
more recent design.  Since some of these units may have been refurbished by
other vendors, they may no longer carry the AECL/Theratronics identification. 
If you have any questions concerning whether or not your unit is susceptible
to this failure, contact either your service provider or refurbisher.  The
replacement timer is designed to operate quite differently than the older
single channel timers it is intended to replace.  Had this dual channel timer
upgrade been installed, it is likely that the failure of the device to move
the source into the fully exposed treatment position would have been more
readily detected by the licensee and the potential for undetected patient
underexposures reduced.  

The vendor (Theratronics International Corporation) has informed NRC that it
intends to notify all its customers having these older single-timer units, 
advising them of the potential problems and the vendor's recommended
corrective actions.  A copy of this vendor's notification is attached
(Attachment 1).  Licensees should contact the vendor directly with questions
related to the notification.

Licensees should consider increasing their preventive maintenance procedures,
replacement of parts, and upgrading of components (when available) for older
devices, such as these teletherapy units, that may be approaching the end of
their safe useful life.  When considering options, licensees may wish to
consider human factors studies that indicate that people are poor at detecting
intermittent or infrequent failures, particularly if one operational
indicator, such as the treatment timer, is monitored more closely than others.

Licensees should also be aware that medical device user facilities are now
subject to mandatory FDA adverse event reporting requirements for medical
devices.  Information concerning FDA's mandatory reporting requirements can be
obtained by contacting the Center for Devices and Radiological Health, Office
of Surveillance and Biometrics, Division of Surveillance Systems at (301) 594-
2735.  Since the FDA mandatory reporting requirements may not be applicable to
all medical device events, FDA also depends on information voluntarily
provided by device users because they are often the first to recognize medical
device related hazards.  Any concerns that licensees may have pertaining to
the safety or quality problems associated with medical devices can be
voluntarily reported to the FDA by calling MedWatch at 1-800-FDA-1088. 
Voluntary reports can be submitted anonymously.

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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 contact listed below or the appropriate NRC regional office.

                                                    signed by F. C. Combs

                                             Donald A. Cool, Director
                                             Division of Industrial and 
                                               Medical Nuclear Safety
                                             Office of Nuclear Material Safety
                                               and Safeguards

Technical Contact:  Robert L. Ayres, NMSS
                    (301) 415-5746

1.  Theratronics Customer Notification

International Limited                                      Attachment 1
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                                                           Page 1 of 1    

                  December 10, 1996

                  Dear Theratron Customer,

                               PRECAUTIONS IN THE OPERATOR'S MANUAL

                 Model: _________

                 S/N:   _________

Theratron units shipped prior to 1985 included single-channel timers, either mechanical or digital. 
Both of these models are designed to begin their timing sequence from the source-on command rather
than directly from the source transit drawer switches as in the more recent Theratron models. With
these older single-channel timers, an electrical or mechanical problem in the source mechanism
should it go unnoticed, could affect the accuracy of treatment time,

Single channel timers have been in use for more than thirty years and reports of problems have
been very rare with no reports of significant dose misadministration.  Users of units having single
channel timers, however, are reminded of the following standard operating precautions:

     1.     As is explained in the Operator's Manual, observe the operation of the Source Position
            Indicator lights on the control console and above the treatment room door while treatment
            is in progress.

      2.    Follow daily, start-up procedures and periodic maintenance instructions outlined in the
            Operator's Manual, and investigate any problems promptly.

Kits are available to install the current Dual Timer on older Theratron models, which will improve
timer accuracy and reliability.  Please contact Theratronics for price and delivery information at
1-800-826-2258 or 1-800-267-7230.

Original signed by

E. S. Martell
Vice President
Quality Assurance and Regulatory Affairs

413 March Road
P.O. Box 13140
Kanata Ontario Canada
     K2K 2B7
(613) 591-2100
FAX (613) 592-3816                                                          Kanata, Ontario. Canada

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