United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 95-25: Valve Failure During Patient Treatment With Gamma Stereotactic Radiosurgery Unit


May 11, 1995

                               GAMMA STEREOTACTIC RADIOSURGERY UNIT  


All U.S. Nuclear Regulatory Commission Medical Licensees.


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to an incident in which the patient couch of a
Leksell Gamma System Model 23016 ("gamma knife") unit failed to retract from
the treatment position because of a failure of a two-position, solenoid-
operated valve on the hydraulic system of the unit.  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 new NRC requirements;
therefore, no specific action nor written response is required.

Description of Circumstances

NRC was notified of an incident that occurred at an Agreement State licensee
in which a patient undergoing gamma stereotactic radiosurgery received a dose,
for a single fraction, that was 127 percent greater than the dose prescribed
for that fraction.  On October 25, 1994, a patient was prescribed to receive a
series of 10 exposures in a Leksell Gamma System Model 23016 ("gamma knife")
unit.  At the end of the sixth exposure, the patient couch failed to retract
from the treatment position because of a failure of a two-position, solenoid-
operated valve on the hydraulic system of the unit.  

The licensee's staff attempted to:  (1) manually pump the hydraulic system;
and (2) shut the unit off.  The latter action would normally turn the pump on
and direct the pressure to allow the bed to retract.  However, in this case,
the valve was stuck in the `bed-in' position and the internal spring could not
reset the valve to allow the bed to move.  The valve failure disabled both the
normal and primary emergency patient retraction systems on the unit, resulting
in the patient being irradiated for 3.8 minutes longer than the intended 
3-minute treatment time.  Medical personnel entered the room, pulled a
pressure equalization latch on the bed, and were able to move the bed
approximately 50 centimeters (20 inches).  Subsequently, they manually
disconnected the helmet from the unit to remove the patient from the treatment
room.  The U.S. distributor, Elekta Radiosurgery, Inc., was notified of the
event and subsequently replaced the valve.  The distributor also notified all
its customers of the event and attributed it to a valve failure, with no
specific information on the cause of the failure.  

9505050171.                                                            IN 95-25
                                                            May 11, 1995
                                                            Page 2 of 3

At the request of the State of Georgia (Elekta Radiosurgery, Inc., is located
in Georgia), NRC, through a contract with the Idaho National Engineering
Laboratory (INEL), conducted a root-cause analysis of the incident.  The INEL
team was comprised of a mechanical engineer, risk analyst, medical physicist,
and radiation oncologist.  


The findings of the INEL report indicate that the cause of the valve failure
was the existence of metal contaminants in the hydraulic fluid system, which
either became caught between the valve spool and valve body or scored the
spool, thereby locking the valve in the `bed-in' position.  It appears that
during installation, several months before, pieces of dirt, metal, and rubber
that are typically found in new hydraulic hoses were not properly cleaned from
one of the hoses.  The report concluded that other machines that were
installed before and have been cycled repeatedly are less likely to have a
similar failure; however, this type of failure remains a concern for the
installation of new machines.  

When the patient couch failed to retract, the facility staff released the
latch at the foot of the couch, thereby dropping the helmet to the lowest
position corresponding to the low point of the couch track.  When the helmet
is at the low point, the maximum dose rate at the focus of the primary
collimator through the helmet is approximately 10 percent of the dose rate at
the treatment position because of the lack of alignment with the helmet
openings.  Although the one exposure delivered a 127 percent overdose, it was
delivered to a partial volume of the complete target volume with the result
that there was a slight increase in the percentage of the target within the 
45 percent isodose.  However, changes in the isodose contour were minor at the
20 percent isodose contour.  The maximum total dose delivered to the patient
was approximately 33.5 Gray (Gy) (3350 rads) for all 10 exposures (fractions),
compared with a planned dose of 33.33 Gy (3333 rads), therefore the medical
consequences of this incident are minimal.  Furthermore, it appears that the
medical staff who responded to the emergency all received less than 0.03 mSv
(3 mrem) each.  

The report provides several recommendations:  1) to further reduce the
possibility of a repeat failure of the kind experienced at the facility, any
proposed design change should include installation of a 20-micron filter; 
2) a one-time laboratory analysis of hydraulic fluid after each new unit
installation in addition to such analysis of, at least five other gamma knife
units currently in operation; 3) all gamma knife users should obtain a second
emergency tool to disengage the head frame from the unit; 4) all gamma knife
users should review their emergency procedures to ensure they include
provisions in the event the bed fails to retract; and 5) review and retraining
of all emergency procedures with operating staff.  

.                                                            IN 95-25
                                                            May 11, 1995
                                                            Page 3 of 3

As part of the licensing process, license applicants are requested to submit
procedures for emergencies which could result in radiation exposure of
patients, workers, and the general public.  Emergency procedures for the gamma
knife unit should address situations in which the hydraulic pressure is within
normal operating range, but the bed fails to retract when the timer reaches
0.0.  In addition, the emergency procedures should include procedures for
releasing the helmet from the trunnions in the housing, using the long-handled
emergency tools provided by the manufacturer.  Licensees who may be installing
a gamma knife unit should be aware of the potential for contamination of the
hydraulic fluid.  

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.

                              /s/'d by E. William Brach/for

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

Technical contact:  Patricia K. Holahan, NMSS
                    (301) 415-7847

1.  List of Recently Issued NMSS Information Notices
2.  List of Recently Issued NRC Information Notices

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