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Shielding Design for PET Clinics

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Title: Shielding Design for PET Clinics


1
Shielding Design for PET Clinics
  • Robert L. Metzger, Ph.D.

2
Introduction
  • Positron Emission Tomography is enjoying
    explosive growth due to its ability to stage and
    track cancer lesions.
  • Patients receive 0.55 GBq (15 mCi) of 18F
    labeled 2-Fluoro-2-Deoxy-D-Glucose (FDG) and rest
    in a Quiet Room for 45 minutes to allow the FDG
    to localize in the lesions.

3
A Typical PET Scan
  • After voiding, the patients are then scanned for
    approximately 45 minutes in the PET scanner, or
    more commonly now, a combined PET/CT unit.
  • The PET/CT unit overlays the PET image on the CT
    image to create a precise registration of the
    tumor location.

4
PET/CT Study
5
Malignant Melanoma
6
Malignant Melanoma
7
PET Clinic Layout
Patient Bathroom
Quiet Rooms
Hot Lab
PET/CT Control
PET/CT
8
PET Clinic Layout
  • This clinic layout utilizes distance rather than
    shielding to protect the technologist. The quiet
    areas are gt20 feet distant from the tech at the
    PET/CT control.

9
Example Clinic Layout
10
Shielding Problems in PET
  • There was no published shielding guide for PET
    clinics. The AAPM Guide (handout) just
    published.
  • PET started as a research tool with low patient
    volumes and tightly controlled research
    environments. Little shielding was necessary.
  • PET has now exploded into clinical practice where
    patient volumes are high, and facilities
    frequently crammed into existing spaces.

11
Shielding Problems in PET
  • The 511 keV photons can easily penetrate
    shielding used for conventional Nuclear Medicine
    (140 keV) and diagnostic X-Ray (lt120 kVp).
  • PET patients can fog film stored in dry laser
    printers and dark rooms. Loaded cassettes are
    quite vulnerable.

12
Shielding Problems in PET
  • NCRP 49 has attenuation coefficients for 511 keV
    photons, but they are narrow beam coefficients
    and PET is a broad beam environment. There are
    no buildup factors provided in NCRP 49. This has
    led to undershielding of some facilities (see
    Shielding Factors on pg. 6 of the handout).

13
Shielding Problems in PET
  • The hot lab L block, syringe shields, and
    shadow shields for 511 keV photons are much more
    costly than those available for 140 keV Tc-99m.
    Some facilities have substituted Tc syringe
    shields and L blocks to save money.
  • These problems have resulted in doses gt5 rem in
    one month for a technologist and gt 2 rem per year
    for adjacent tenants in some facilities.
  • These doses have also resulted in a regulatory
    crack-down in some states.

14
AAPM Shielding Guide
  • Provides conservative shielding estimates for
    clinics using Monte Carlo derived broad beam
    shielding curves for lead, concrete, and iron.
  • Considers patient self attenuation, decay of 18F
    throughout the scanning process.
  • Uses conservative usage for the quiet rooms and
    scanner.
  • Uses NCRP 147 occupancy factors for surrounding
    areas.

15
Design Dose Limits
  • Effective Dose Equivalent in Unrestricted areas
    must not exceed 1 mSv/yr (100 mrem/year).
  • The occupational limit is 50 mSv/yr, but AAPM
    uses a design guide of 5 mSv/yr (500 mrem/yr)
    based on ALARA and pregnant worker limits.
  • NCRP 147 uses the same criteria.

16
Source Term
  • AAPM Draft Guide uses a patient dose rate of
    0.092 ?Sv m2/MBq hr or 5.1 mrem per hour at 1
    meter for a freshly dosed patient with 15 mCi of
    FDG (pg. 8).
  • Represents about 65 of the point source gamma
    constant and is consistent with anterior patient
    measurements.

17
Scanner Source Term
  • After the patient rests for approximately 45
    minutes, they void, and are then scanned. The
    radioactive decay for one hour resting is 0.74
    and the patient voids 15 of the dose, and the
    reduction during the scan is 0.91, leaving about
    ½ of the delivered dose.
  • AAPM uses mean doses to come to the same number
    (pg 10).
  • 30 of the dose is in the patients head with the
    remainder distributed in the body.

18
Scanner Attenuation
  • AAPM uses a 15 reduction in dose due to scanner
    attenuation, but does not use this factor in the
    example problems.
  • AAPM Guide was written before PET/CT became
    prevalent.
  • Attenuation provided by the PET/CT double gantry
    is significant.
  • Use manufacturers data for scanner attenuation.

19
Usage
  • AAPM Guide assumes 100 occupancy of the quiet
    room with a dosed patient.
  • It does not consider multiple quiet rooms that
    are now common.
  • If multiple rooms are used, the occupancy cannot
    be 100.
  • Scanners can scan a patient every 30 to 45
    minutes, so, at maximum, the scanner cannot do
    more than 16 patients per 8 hour shift.
  • Practically, 10 is the maximum given patient
    setup times.

20
Usage
  • Quiet rooms can produce no more than one patient
    every 45 minutes or 10.7 (call it 10) per shift.
  • When multiple rooms are in use usage factors of
    0.65 are common.
  • Scanners are considered to be continuously
    occupied.

21
Occupancy Factors
  • Occupancy factors for surrounding areas are drawn
    from NCRP 147 (not NCRP 49).
  • NCRP 147 values are more realistic.
  • Caution must be used when choosing the 1/40th
    occupancy factor.

22
Draft Guide Limitations
  • The guide does not manage layered shields that
    typically comprise floor and ceiling shielding.
    Treating each layer individually and summing the
    attenuation causes overshielding as 511 keV
    photons are assumed to be incident on each layer.
  • Does not discuss the hot lab much.

23
Shielding
  • The 511 keV photons from 18F and the mobile
    nature of the source (patient) create some unique
    shielding design problems for a PET clinic.
  • New clinics are commonly sandwiched into existing
    imaging centers that are densely populated.
    Areas above and below the clinics are routinely
    occupied by other offices.

24
PET Clinic Shielding
  • Inadequate structural shielding in some
    facilities has led to high doses to
    non-occupationally exposed personnel both within
    the facility and adjacent to it.
  • Improper hot lab shielding (L block and syringe
    shields for 99mTc) has led to high doses to the
    Nuclear Medicine technologists.

25
Hot Lab Shielding
26
NCRP 49
  • Dont use it!
  • NCRP 49 HVLs are narrow beam attenuation values,
    while the PET patient represents a broad beam
    condition.
  • Buildup in concrete is high at 511 keV.

27
Attenuation Curves(from AAPM Draft Guide on PET
Shielding)
28
Attenuation Curves
Monte Carlo Simulation (Broad Parallel Beam)
Constant TVL 17.6 cm

29
Wall Shielding
  • Wall shielding is commonly required for the hot
    lab, quiet rooms, and scanner room.
  • Many designs use distance rather than shielding
    for the interior spaces as technologists dislike
    closing off their patients in the quiet rooms.
    Doors, when provided, are rarely closed.

30
Wall Shielding
  • Wall shielding can be easily calculated using
    point kernel techniques with buildup factors or
    from the AAPM Draft Guide on Pet Shielding
    Design.
  • Source terms and occupancy factors may be taken
    from the draft guide or from actual experience.

31
Wall Shielding
  • The height of the wall shielding is
    controversial. Some references say the shielding
    should extend to the floor above rather than the
    typical 7 foot height.
  • Not practical. AC, electrical, call button, CCTV
    hardware, intercom, etc. run in the interstitial
    space above the false ceiling.
  • Streaming is not significant at 450 KeV.

32
Example 1 Wall Shielding
  • See Page 9-10, Example 1 Quiet Room.
  • 15 mCi FDG, 40 pts/wk (one shift), uptake time
    one hour, 4 m to fully occupied uncontrolled area
    (T1)
  • Weekly dose is 105.9 ?Sv (Eq 3).
  • Limit is 20 ?Sv, therefore required attenuation
    is 0.189.

33
Determining Pb Thickness
  • Draft Guide Table 4, 1.2 cm lead.
  • Point Kernel I/I0Be-?x
  • ? 1.79 cm-1 for Pb
  • B 1.35 at 1.2 cm
  • I/I0 0.16 Good Agreement
  • NCRP49 No Buildup Off by about 30.

34
Ceiling and Floor Shielding
  • When occupied areas exist above and/or below the
    quiet rooms and the scanning room, it is
    sometimes necessary to add sheet lead to the
    concrete deck. The floor thickness alone may not
    be sufficient to meet the non-occupational limit
    of 1 mSv (100 mrem) per year (25 mrem in some
    European countries).

35
Detector Locations
  • The Draft Guide recommends that the dose limits
    be applied at 0.5 meter above the floor above
    (height of a low chair), and 1.7 meters (5.6 ft)
    above the floor below.
  • That is, low chair above, and tall standing
    person below.
  • Very conservative, even unrealistic.

36
Example 4
  • See Page 11 - 12.
  • Quiet room, 15 mCi FDG, 40 pts per week, uptake
    time 1 hr, 4.3 m floor to floor clear height, 10
    cm concrete deck, uncontrolled area above, T1.
  • D (4.3 1) 0.5 3.8 meters.
  • Eq. 3 117 ?Sv (one week)

37
Determining Concrete x
  • Draft Guide Table 4 17 cm concrete.
  • Point Kernel I/I0Be-?x
  • ? 0.204 cm-1 for Concrete
  • B 7.5 at 17 cm
  • I/I0 0.23 Reasonable Agreement
  • NCRP49 No Buildup Gross overestimate of
    shielding provided by concrete deck.

38
Example 4
  • Guide calls for 0.65 cm (slightly more than ¼
    inch) of lead to be applied to the ceiling above.
  • Sums attenuation provided by layered lead and
    concrete.
  • Approach commonly leads to ½ inch lead
    requirements.
  • Inaccurate. Better approach described later.

39
Ceiling and Floor Shielding
  • The ceiling and floor shield consists of lead
    suspended under the support trusses of the
    concrete deck, forming a layered shield.
  • Much of the shielding cost of a PET clinic is
    driven by these layered shields as they
    frequently require structural reinforcement to
    support the weight.

40
Imaging Room
  • Correctly assumes about ½ of the delivered dose
    remains in patient.
  • Assumes a 15 reduction in dose by scanner
    gantry, but does not use this in example
    calculations.
  • The guide was developed before the advent of
    PET/CT units with multi-slice CT scanners.
  • Actual dual gantries of the modern PET/CT units
    provide substantial attenuation.

41
Example 2
  • See page 11.
  • 15 mCi FDG original dose, 1 hr uptake, imaging
    time 30 min, 40 pts per week, 3 m to uncontrolled
    area with occupancy of 1.0.
  • Calculates 59.7 ?Sv and calls for 0.8 cm Pb (1/2
    inch Pb practically).
  • Very conservative.

42
AAPM Guide Assumptions
  • Occupancy of the quiet room(s) with a dosed
    patient is 1.0. This is physically impossible.
    The patient must have a blood sugar test, be made
    comfortable, have an IV started, have the
    procedure described, and only then given the
    dose.

43
AAPM Guide Assumptions
  • Single Quiet Room. Virtually all facilities now
    have 2 to 3 quiet rooms. The objective is to
    keep the scanner scanning. With the resting
    period of 45 minutes and a scanning time of 30
    minutes for new units, multiple quiet rooms are
    necessary for efficient utilization of the
    scanner.

44
AAPM Guide Assumptions
  • 511 KeV. The guide correctly considers patient
    self-attenuation. But, since the
    self-attenuation is due to Compton Scattering,
    the energy is also reduced. The shielding curves
    are for 511 KeV photons incident on a shield wall
    under broad beam conditions. MCNP models
    indicate the actual energy is 350 450 keV
    incident on the shielding.

45
AAPM Guide Assumptions
  • Limited to No Scanner Attenuation. The double
    gantries of the modern PET/CT units provide
    significant attenuation to the sides, top, and
    bottom. The guide describes an attenuation
    factor of 15, but then does not use it for the
    example calculation of the wall shielding.

46
CT
  • Where distances are large (e.g. control booth),
    the shielding needed for the CT may dominate.
    Common for new PET/CT rooms as the space
    requirements are large for the dual gantry
    scanners.
  • Use NCRP 147 to calculate CT shielding
    requirements.
  • Look at both PET and CT shielding requirements
    and pick.

47
Testing
  • NCRP 147 and many regulatory agencies ask for
    tests to ensure that the erected shielding is
    adequate.
  • May be performed with pressurized ion chambers
    (e.g. Victoreen 451P) or large volume ion
    chambers (e.g. 1800 cc Radcal chamber)
  • Alternately, monitor badges may be used (very
    cheap).

48
Testing
LLD is an issue. 100 mrad/year is 0.05 mrad/hour
for full occupancy. The survey instrument must
be able to accurately measure at 0.05 mR/hr or
integrate to achieve this LLD.
49
Monitor Badges
  • Place away from areas frequented by patients.
  • Factor for area occupancy and shift change (if
    any).
  • May stop after compliance is determined.
  • Some regulatory agencies have been erecting their
    own monitor badges where shielding is suspect.

50
Ceiling and Floor Shielding
  • Point Kernel methods that calculate the
    attenuation provided by each layer and then sum
    them to obtain the total attenuation, tend to
    overestimate the shielding requirements when the
    layers are thick (in mfp). They assume 511 keV
    photons are incident on each layer.
  • This can dramatically increase the shielding
    cost, particularly when the second course of lead
    requires structural reinforcement of the ceiling.

51
Monte Carlo Model of the Quiet Room
  • We developed an MCNP model of the PET quiet room
    consisting of a MIRD phantom in a reclining
    position centered in a quiet room where the room
    and ceiling dimensions are taken from facility
    plans.
  • The 0.55 GBq (15 mCi) of 18F was equally split
    between the bladder and brain in the phantom.

52
MCNP Model
53
Mercurad Model
  • Mercurad is a deterministic code developed by CEA
    specifically for layered shielding problems.
  • We developed a second model for this code using
    the same room materials and dimensions but a
    water sphere for the source term as human
    phantoms have not been ported to this code.

54
MCNP Model of Scanner
  • A second model was developed for the scanner room
    using a MIRD phantom in a double gantry of a
    PET/CT scanner.
  • The phantom was loaded with 7.5 mCi of FDG. 30
    of the dose was in the phantoms head with the
    remainder in the body.
  • The scanner was developed from data provided by
    GE Healthcare for their current PET/CT unit.

55
MCNP Scanner Model
56
Mercurad Scanner Model
  • A second model of the scanner room was developed
    for Mercurad using water filled spheres and
    cylinders to simulate the patient in the scanner.
  • The source term was 7.5 mCi, with 30 in the head
    and the rest distributed uniformly in the body.
  • Arrays of detectors were placed above and below
    the scanner room.

57
Mercurad Scanner Model
58
Example
  • A clinic design was chosen where the point kernel
    method indicated that two layers of ¼ inch lead
    would be needed to protect the office above the
    quiet rooms.
  • The actual room dimensions and concrete floor
    deck thickness was used and a volume detector was
    set 61 cm (chair height) above the second floor
    deck. The actual floor deck was corrugated, but
    only the thinner section was used in the
    calculation.

59
Scanner Example
  • A scanner room with a fully occupied OT clinic
    above and a cafeteria below was used to test the
    scanner model.
  • This room had a 4 inch ceiling and six inch
    concrete floor deck.
  • The areas above and below were uncontrolled and
    fully occupied.

60
MCNP Runs
  • Moritz visualization was used to ensure that all
    of the source points were within the brain and
    bladder and that the volumes were adequately
    sampled.
  • After verification, 5 x 107 photons were run and
    the volume detector above the quiet room
    converged and passed all statistical tests.

61
Mercurad Run
  • A point detector set to read in exposure rate
    was set at the same location for the Mercurad
    model. An array of detectors were used for the
    scanner room as the maximum exposure is difficult
    to predict given the complex geometry.
  • The code converged in less than a minute.
  • Results are expected to be higher than MCNP for
    the quiet room as no patient self attenuation was
    considered in this model (source term was a ten
    cm sphere).

62
Moritz Source Verification
63
Scanner Room Results
64
Scanner Room Results
65
Mercurad Scanner Results
66
Testing
  • Once construction was complete, the exposure
    rates in the adjacent areas were measured before
    the facility went into full operation.
  • Exposure rates on the second floor were measured
    with a Radcal 1800 cc ion chamber.

67
Measurements
  • Actual exposure rates were measured with an 1800
    cc ion chamber while the rooms were occupied with
    dosed patients.

68
Results (mR/wk)
Location MCNP Mercurad Observed
Office Above QR 1 1.96 2.28 0.29
Office Above QR 2 1.96 2.28 0.86
69
Discussion
  • The actual measurements were lower than predicted
    by either code.
  • The floor trusses, lead shielding support,
    electrical , plumbing, and lighting fixtures in
    the false ceiling all provided some attenuation
    and reduced the actual dose above the rooms.

70
Discussion
  • The Mercurad model (as expected) predicted
    slightly higher doses than the MCNP model as
    patient self attenuation was not considered.
  • Mercurad results should be factored for patient
    self attenuation.
  • Mercurad model development and execution was
    remarkably easy.

71
Conclusion
  • The AAPM Draft Guide provides a conservative and
    simple shielding guide for PET clinics.
  • The guide overshields clinics with multiple quiet
    rooms, layered ceiling and floor shielding, and
    scanner rooms where modern PET/CT units are
    employed.
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