Transcript Organ Dose Assessment from Special Procedures Using the
Shielding Design for PET Clinics
Robert L. Metzger, Ph.D. Radiation Safety Engineering, Inc
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 18 F 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. Radiation Safety Engineering, Inc
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. Radiation Safety Engineering, Inc
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PET/CT Study
Malignant Melanoma
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Malignant Melanoma
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Quiet Rooms
PET Clinic Layout
Patient Bathroom Hot Lab PET/CT Control PET/CT Radiation Safety Engineering, Inc
PET Clinic Layout
This clinic layout utilizes distance rather than shielding to protect the technologist. The quiet areas are >20 feet distant from the tech at the PET/CT control.
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Example Clinic Layout
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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.
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Shielding Problems in PET
The 511 keV photons can easily penetrate shielding used for conventional Nuclear Medicine (140 keV) and diagnostic X-Ray (<120 kVp). PET patients can fog film stored in dry laser printers and dark rooms. Loaded cassettes are quite vulnerable.
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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).
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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 >5 rem in one month for a technologist and > 2 rem per year for adjacent tenants in some facilities.
These doses have also resulted in a regulatory crack-down in some states.
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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 18 F throughout the scanning process.
Uses conservative usage for the quiet rooms and scanner.
Uses NCRP 147 occupancy factors for surrounding areas.
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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.
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Source Term
AAPM Draft Guide uses a patient dose rate of 0.092 Sv m 2 /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.
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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 patient’s head with the remainder distributed in the body.
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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 manufacturer’s data for scanner attenuation.
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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.
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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.
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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 1/40 th occupancy factor.
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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.
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Shielding
The 511 keV photons from 18 F 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.
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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 99m Tc) has led to high doses to the Nuclear Medicine technologists.
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Hot Lab Shielding
NCRP 49
Don’t 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.
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Attenuation Curves
(from AAPM Draft Guide on PET Shielding) Radiation Safety Engineering, Inc 1.0000
0.1000
0.0100
0.0010
Lead Lead Monte Carlo Simulation (Broad Parallel Beam) Constant TVL 16.6 mm
0.0001
0 5 10 15 20 25 30 35 40 45 50 Figure 1.
1.0000
Attenuation Curves
Concrete Monte Carlo Simulation (Broad Parallel Beam) Constant TVL 17.6 cm
0.1000
0.0100
0.0010
0.0001
0 Radiation Safety Engineering, Inc 5 10 15 20 25 30
Thickness(cm)
35 40 45 50
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.
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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.
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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.
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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 (T=1) Weekly dose is 105.9 Sv (Eq 3).
Limit is 20 Sv, therefore required attenuation is 0.189.
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Determining Pb Thickness
Draft Guide: Table 4, 1.2 cm lead.
Point Kernel: I/I 0 =Be x = 1.79 cm -1 for Pb B = 1.35 at 1.2 cm I/I 0 = 0.16 – Good Agreement NCRP49: No Buildup – Off by about 30%.
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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).
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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.
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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, T=1.
D = (4.3 –1) + 0.5 = 3.8 meters.
Eq. 3: 117 Sv (one week) Radiation Safety Engineering, Inc
Determining Concrete x
Draft Guide Table 4: 17 cm concrete.
Point Kernel: I/I 0 =Be x = 0.204 cm -1 for Concrete B = 7.5 at 17 cm I/I 0 = 0.23 – Reasonable Agreement NCRP49: No Buildup – Gross overestimate of shielding provided by concrete deck.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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).
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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.
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Testing
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.
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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.
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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 18 F was equally split between the bladder and brain in the phantom.
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MCNP Model
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.
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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 phantom’s head with the remainder in the body.
The scanner was developed from data provided by GE Healthcare for their current PET/CT unit.
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MCNP Scanner Model
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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.
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Mercurad Scanner Model
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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.
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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.
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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 10 7 photons were run and the volume detector above the quiet room converged and passed all statistical tests. Radiation Safety Engineering, Inc
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).
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Moritz Source Verification
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Scanner Room Results
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Scanner Room Results
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Mercurad Scanner Results
0.030
0.025
0.020
0.015
0.010
0.005
0.000
0 50 100 150 Distance from Patient's Head (cm) Floor Above Scanner Room Floor Below Scanner Room 200 250 Radiation Safety Engineering, Inc
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.
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Measurements
Actual exposure rates were measured with an 1800 cc ion chamber while the rooms were occupied with dosed patients.
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Results (mR/wk)
Location Office Above QR 1 Office Above QR 2 MCNP 1.96
1.96
Mercurad 2.28
2.28
Observed 0.29
0.86
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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.
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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.
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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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