Response to an Event at a Medical Cyclotron

1
Response to an Event at a Medical Cyclotron

• Presentation to North Carolina Health Physics
  Society
• By Christopher Martel, CHP
• Director, Radiation Safety Officer
• Brigham and Womens Hospital
• and
• James P. Tarzia, CHP
• Executive Director
• Radiation Safety Control Services, Inc.

2
What happened?

• Radiochemist was exposed to vial containing 1.6
  Curies of F-18 in contact with his upper arm.
• Preliminary dose estimate was 50 100 Rem to
  upper arm (included in definition of Whole Body
  and TEDE).
• Reported to senior management within two hours of
  event and a lot of people decided to jump in.

3
How could this have happened?

• Late May 2009, the delivery line for F-18 was
  redirected to dispense F-18 from Hot Cell 2 to
  Hot Cell 4.
• Line was not returned to original configuration.
• June 30, a F-18 run was made in the Cyclotron,
  and was intended to be dipsensed to Hot Cell 2,
  but because of earlier change, the dose was
  dispensed to Hot Cell 4.

4
How could this have happened?

• Radiochemist was working in Hot Cell 4 setting
  up QC equipment for F-18 while dose was being
  dispensed from Cyclotron.
• 1.6 Curies was dispensed into a vial that was
  held in a container in Hot Cell 4.
• Workers upper arm was in contact with the
  container during and after the F-18 was dispensed.

5
What didnt happen!

• By procedure, no work to be done in any hot cell
  while dose is being delivered!
• He though it was okay he wasnt in cell 2
• Must wear alarming electronic dosimeter when
  working in production area!
• Didnt like it. Alarm level too low! Kept
  alarming!
• Must have working meter in area while working in
  production area!
• Plug in meter inoperable. Replaced with handheld
  and left on, so batteries died.

6
Where it was supposed to go

• FASTlab located in Hot Cell 2

7
Where it went
8
Closer view
9
Showing scale and perspective
10
During re-enactment
Type of vial in container
11
Timeline Day 0

• Call received at 1230 that at 1130 the event
  occurred. The cyclotron engineer estimated dose
  to worker as about 500 millirem.
• RSO investigated.
• Verified activity
• Removed all activity from hot cell
• Worker showed us what he did (reenactment)
• Dose appeared to occur to upper arm
• Dosimetry collected and sent in for immediate
  processing. No more access to restricted areas.

12
Initial Information Gathered

• Initial reported information
• Activity 1.6 Curies F-18
• Delivery time into vial lt 30 seconds
• Exposure time 4 minutes
• Distance to upper arm 2 cm from vial with
  shielding from with container
• Distance to upper arm 4 cm no shielding if arm
  on top of container

13
Estimated Dose

• Using data from preliminary information

3mm Lead shielding in container
Uh oh!!
14
Next sequence of events

• Reported to Senior Management that an event
  occurred.
• Under current regs, it may very well be an
  overexposure. Dose may be 100 Rem to upper arm.
  So, if
• extremity dose 2 times annual limit 100 Rem
• whole body dose 20 times annual limit
• At 430 report to state (and hope no one is
  around!) Too bad! One lone person in office.

15
We made the NRC event page!
General Information or Other Event Number 45176
Rep Org MA RADIATION CONTROL PROGRAMLicensee BRIGHAM AND WOMEN'S HOSPITALRegion 1City BOSTON State MACounty License 44-0004Agreement YDocket NRC Notified By TONY CARPENITOHQ OPS Officer VINCE KLCO Notification Date 06/30/2009Notification Time 1727 ETEvent Date 06/30/2009Event Time 1130 EDTLast Update Date 06/30/2009
Emergency Class NON EMERGENCY10 CFR Section AGREEMENT STATE Person (Organization) JAMES TRAPP (R1DO)DUNCAN WHITE (FSME)
Event Text
AGREEMENT STATE REPORT - POTENTIAL WORKER OVEREXPOSURE The following information was received via facsimile "A worker was working in a hot cell when a F-18 radio-isotope was mistakenly delivered to the hot cell. The initial estimated worker dose was 100 Rad extremity dose and 20 Rad to the whole body (upper arm). The dosimeter has been sent to Landauer for immediate processing. The worker has been taken off Rad work and is being monitored" A Commonwealth of Massachusetts investigation is pending.
16
Nightmare!

• Someone tells others possible 100 rem whole body
  dose!
• Occupational Medicine gets involved.
• Hears 100 rem whole body dose.
• Requests assistance from Radiation Oncology
  physician.
• He tells occupational medicine We need to get
  this person into the emergency room stat and pull
  blood samples!

17
Day 1

• Big Meeting
• Senior Management
• Risk Management
• Occupational Medicine
• Workers involved in event
• Me
• Radiation Oncologist stated that what I did
  (worker interview and re-enactment) was harsh
  and that I caused psychological trauma to the
  individual.

18
Day 1 (continued)

• More information gathered.
• Time of actual exposure was less, probably on the
  order of two minutes based on sequence of events
  (dose delivery versus call from cyclotron
  engineer to have chemist verify dose was
  delivered).
• Could the distance from vial to arm have been
  further? Deep dose 1 cm into arm?
• Should I use upper arm as extremity or whole body?

19
End of Day 1

• Im on dangerous ground!!
• I better hire someone else to do this so I dont
  unintentionally bias, or give the impression that
  I manipulated the results!
• Hire someone else and let them do an independent
  evaluation!

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Independent Dose Evaluation Our Mission

• To calculate a legally defensible dose to the
  worker which represents accurate risk and
• Is derived from an accurate representation of the
  exposure situation,
• Has a sound technical basis founded on accurate
  parameters and current acceptable dose
  calculation methodologies, and
• Contains an appropriate and defendable amount of
  conservatism

21
Analyzing the Event

• Interview with RSO
• Interviews with workers
• Determination of physical parameters
• Vial and line volumes, shield thicknesses, etc.
• Determination of time and distance parameters
• Detailed controlled mockup of transfer and worker
  position

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Event Facts Discovered from Analysis Delivery
Line

• The expected radioactivity transferred to the
  vial was 1.6 Curies which was confirmed by assay.
• The internal diameter of the delivery tube was
  1/32 inch (0.079cm) and a critical length of less
  than 3 feet (91.4 cm)
• Maximum possible in tube 0.4826 Ci
• The delivery tube extended approximately 12
  inches (30.48) up from the top of the vial and
  then looped back toward the rear of the hot cell.
• Whole body exposure to the worker from the rear
  line loop was most likely not closer than 15 cm.
• Total time to transfer liquid to the vial was 40
  /- 3 seconds.

23
Event Facts Discovered from Analysis Receptor
vial

• The vial contained 10 ml of non-radioactive
  aqueous solution prior to the transfer.
• An additional 1.5 ml of radioactive solution was
  added to the vial containing the F-18.
• The total radioactive solution in the vial was
  contained in a cylindrical shape with a radius of
  1.64 cm and 1.90 cm tall.
• The vial shield was composed of 0.40 cm of lead
  and 0.30 cm of steel.
• Maximum possible air gap between the vial and the
  inside of the shield was approximately 2 cm and
  the minimum air gap possible was approximately
  0.3 cm.

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Event Facts Discovered from Analysis The Worker

• The work performed in Hot cell 4 required the
  workers upper left arm to be positioned against
  the shield that contained the vial to which the
  F-18 was dispensed.
• Distance from the vial to the workers chest
  dosimeter was approximately 15 inches (38.1 cm).
• The hot cell worker was performing maintenance in
  Hot Cell 4 for the entire duration of the
  radioactive solution delivery.

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Hot Cell 4 with Shielded Vial and Delivery Tube
26
Dose Calculation Process

• Validate initial bounding calculation performed
  by RSO
• Perform detailed modeling of dose delivery
• Evaluate calculated results against dosimetry
  results and risk to worker
• Refine dose calculations using more appropriate
  risk-based techniques

27
Validation Result

• Bounding calculation performed by RSO was
  validated using modeling techniques but
  overestimated worker risk from TEDE
• Bounding calculation used TEDEDDE (upper arm)
• Not a uniform exposure to the whole body
• Dose to upper arm is not consistent with actual
  whole body risk for non-uniform exposure NRC
  Guidance

28
NRC 10CFR20 Guidance

• The NRC states in 10CFR20 that the organ dose
  weighting factors prescribed in 10CFR20.1003 may
  be used for external exposures on a case by case
  basis until specific guidance is issued

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NRC Regulatory Information Summary
In February 2003, NRC issued a Regulatory
Information Summary encouraging licensees to use
the effective dose equivalent in place of the DDE
in selected situations that include cases where
the doses are calculated rather than measured
with personnel dosimetry.
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TEDE Calculation using EDE

• Received concurrence from State on approach
• Used methodologies developed by Dr. X. George Xu
  at Rensselaer Polytechnic Institute
• Monte-carlo calculations of the EDE rate for
  photon emitting particles located on 74 different
  skin locations of the body
• Calculation determined organ doses for
  significantly exposed organs and sumed them to
  calculate EDE
• Results in EDE conversion factors in urem/hr per
  uCi

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Organ Dose Conversion Factors (Sv/photon) for
Selected Organs
32
Conservatism Applied

• Calculations do not take into consideration
  reductions due to vial shield
• Due to the complex line geometry, the highest EDE
  conversion factor for all 74 body locations was
  selected
• The calculations assumed the line was completely
  full with fluid during entire transfer

33
Organ Doses
34
The Result
35
Skin Dose Calculations

• Conservatively modeled using Microshield.
• 10 concentric rings considered for 10 square
  centimeter calculation
• Vial assumed to be against the upper arm

36
Conservatism Applied to Skin Dose

• No consideration was given to the attenuation or
  distance afforded by clothing worn.
• Method assumes that the entire 10 square
  centimeters of skin was in contact with the
  source during the entire exposure period,
• No adjustments were made to account for curvature
  of the cylindrical source which would have added
  considerable distance to portions of the 10
  square centimeter skin area.

37
Shallow Dose to Ring Segments
Ring Number Center Radius (cm) Area of Inner Circle (cm2) Area of Outer Circle (cm2) Area of Ring (cm2) Max Dose Rate (mrem/hr) Contribution to 10 cm2 Dose Rate (mrem/hr) Contribution to 10 cm2 Dose (mrem)
1 0.089 0.000 0.100 0.100 1.083E06 1.080E04 3.602E02
2 0.267 0.100 0.400 0.300 1.067E06 3.199E04 1.066E03
3 0.446 0.400 0.899 0.500 1.038E06 5.188E04 1.729E03
4 0.624 0.899 1.599 0.700 9.974E05 6.979E04 2.326E03
5 0.803 1.599 2.499 0.900 9.456E05 8.508E04 2.836E03
6 0.981 2.499 3.599 1.100 8.869E05 9.754E04 3.251E03
7 1.159 3.599 4.898 1.300 8.258E05 1.073E05 3.578E03
8 1.338 4.898 6.398 1.500 7.625E05 1.144E05 3.812E03
9 1.516 6.398 8.098 1.700 7.001E05 1.190E05 3.967E03
10 1.695 8.098 9.998 1.900 6.398E05 1.215E05 4.052E03
38
Maximum Shallow Dose

• Calculated by weighting the dose rate of each
  ring by the area of the ring
• All rings summed
• Total maximum SDE Calculated 26.9 rem

39
Conclusions

• Effective Dose Equivalent is the appropriate dose
  to use under circumstances of non-uniform
  exposures
• Results correlated with workers whole body and
  finger dosimetry
• Assignment of a grossly overestimated TEDE using
  DDE to the upper arm would have significantly
  overestimated risk to the worker and liability to
  the employer

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Lessons Learned

• Everyone wants numbers let them wait!
• Spin Control Establish information flow to
  senior management, risk management and RSC chair
  through one point of contact.
• Establish a reporting procedure before something
  happens.
• Let someone else have the fun to do the dose
  assessment, and review what they do, but dont
  guide it.