Title: Shielding Design for PET Clinics
1 Shielding Design for PET Clinics
2Introduction
- 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.
3A 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.
4PET/CT Study
5Malignant Melanoma
6Malignant Melanoma
7PET Clinic Layout
Patient Bathroom
Quiet Rooms
Hot Lab
PET/CT Control
PET/CT
8PET 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.
9Example Clinic Layout
10Shielding 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.
11Shielding 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.
12Shielding 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).
13Shielding 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.
14AAPM 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.
15Design 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.
16Source 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.
17Scanner 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.
18Scanner 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.
19Usage
- 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.
20Usage
- 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.
21Occupancy 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.
22Draft 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.
23Shielding
- 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.
24PET 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.
25Hot Lab Shielding
26NCRP 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.
27Attenuation Curves(from AAPM Draft Guide on PET
Shielding)
28Attenuation Curves
Monte Carlo Simulation (Broad Parallel Beam)
Constant TVL 17.6 cm
29Wall 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.
30Wall 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.
31Wall 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.
32Example 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.
33Determining 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.
34Ceiling 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).
35Detector 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.
36Example 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)
37Determining 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.
38Example 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.
39Ceiling 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.
40Imaging 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.
41Example 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.
42AAPM 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.
43AAPM 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.
44AAPM 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.
45AAPM 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.
46CT
- 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.
47Testing
- 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).
48Testing
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.
49Monitor 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.
50Ceiling 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.
51Monte 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.
52MCNP Model
53Mercurad 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.
54MCNP 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.
55MCNP Scanner Model
56Mercurad 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.
57Mercurad Scanner Model
58Example
- 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.
59Scanner 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.
60MCNP 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.
61Mercurad 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).
62Moritz Source Verification
63Scanner Room Results
64Scanner Room Results
65Mercurad Scanner Results
66Testing
- 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.
67Measurements
- Actual exposure rates were measured with an 1800
cc ion chamber while the rooms were occupied with
dosed patients.
68Results (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
69Discussion
- 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.
70Discussion
- 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.
71Conclusion
- 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.