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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 Women’s Hospital and James P. Tarzia, CHP Executive Director Radiation Safety & Control Services, Inc.

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Response to an event at a medical cyclotron

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 Women’s Hospital

and

James P. Tarzia, CHP

Executive Director

Radiation Safety & Control Services, Inc.


What happened
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.”


How could this have happened
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.


How could this have happened1
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.

  • Worker’s upper arm was in contact with the container during and after the F-18 was dispensed.


What didn t happen
What didn’t happen!

  • By procedure, no work to be done in any hot cell while dose is being delivered!

    • He though it was okay – he wasn’t in cell #2

  • Must wear alarming electronic dosimeter when working in production area!

    • Didn’t 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.


Where it was supposed to go
Where it was supposed to go

  • FASTlab located in Hot Cell #2





During re enactment
During re-enactment

Type of vial in container


Timeline day 0
Timeline – Day 0

  • Call received at 12:30 that at 11:30 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.


Initial information gathered
Initial Information Gathered

  • Initial reported information:

    • Activity – 1.6 Curies F-18

    • Delivery time into vial – < 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


Estimated dose
Estimated Dose

  • Using data from preliminary information:

3mm Lead shielding in container

Uh oh!!


Next sequence of events
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 4:30 – report to state (and hope no one is around!) Too bad! One lone person in office.



Nightmare
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!”


Day 1
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.


Day 1 continued
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?


End of day 1
End of Day 1

  • I’m on dangerous ground!!

  • I better hire someone else to do this so I don’t unintentionally bias, or give the impression that I manipulated the results!

  • Hire someone else and let them do an independent evaluation!


Independent dose evaluation our mission
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


Analyzing the event
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


Event facts discovered from analysis delivery line
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.


Event facts discovered from analysis receptor vial
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.


Event facts discovered from analysis the worker
Event Facts Discovered from Analysis – The Worker

  • The work performed in Hot cell #4 required the worker’s 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.



Dose calculation process
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


Validation result
Validation Result

  • Bounding calculation performed by RSO was validated using modeling techniques but overestimated worker risk from TEDE

    • Bounding calculation used TEDE=DDE (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


Nrc 10cfr20 guidance
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”


Nrc regulatory information summary
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”.


Tede calculation using ede
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



  • Conservatism applied
    Conservatism Applied Organs

    • 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


    Organ doses
    Organ Doses Organs


    The result
    The Result Organs


    Skin dose calculations

    10 Organs

    9

    Skin Dose Calculations

    8

    7

    6

    5

    4

    3

    • Conservatively modeled using Microshield.

    • 10 concentric rings considered for 10 square centimeter calculation

    • Vial assumed to be against the upper arm

    2

    1


    Conservatism applied to skin dose
    Conservatism Applied to Skin Dose Organs

    • 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.



    Maximum shallow dose
    Maximum Shallow Dose Organs

    • 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


    Conclusions
    Conclusions Organs

    • 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


    Lessons learned
    Lessons Learned Organs

    • 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 don’t guide it.


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