Radiation Shielding Information and Guidance

Radiation Shielding Information and
Guidance
Don Parry, CHP
District Health Physicist
Michigan Department of Consumer &
Industry Services
Radiation Safety Section
[email protected]
September 18, 2016
1
Regulations and Standards Regarding
Plan Reviews
Michigan’s first rules were published in February of
1958; Shielding requirements were listed in an
appendix based on NBS Handbook 76
Rules were modified in 1965 and more clearly
indicated that: Shielding shall be approved by the
Michigan Department of Health
Rule 331(2) of current Ionizing Radiation Rules
(Promulgated in 1975) states that shielding is subject
to design approval by the department
Plan reviews have been done routinely by the State
Radiation Control Program since 1958
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NCRP and SSRCR Recommendations
Page 2 of NCRP report 49 states that it is
highly desirable that final shielding plans be
reviewed by the pertinent federal, state or
local agency before construction is begun
The Suggested State Regulations for the
Control of Radiation published by the CRCPD
also require that plans be submitted to the
state for approval prior to construction
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Current Basis for Shielding Design
Criteria
Current NCRP reports including NCRP
49, 51, 79 and 116
New shielding reports will be reviewed
when published
Current criteria still uses NCRP 49’s
leakage model and occupancy factors
and NCRP 116’s dose limits
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NCRP 116 (page 47)
the NCRP reaffirms its previous recommendations
(NCRP, 1984b) that whenever the potential exists to
exceed 25 percent of the annual effective dose limit
as a result of irradiation attributable to a single site,
the site operator should ensure that the annual
exposure of the maximally exposed individual,
from all man made exposures (excepting that
individual’s medical exposure), does not exceed 1
mSv on a continuing basis. Alternatively, if such an
assessment is not conducted, no single source or
set of sources under one control should result in
an individual being exposed to more than 0.25 mSv
annually.
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Rule 201(2)
In addition to occupational and general
public dose limits, Rule 201(2) also
requires that doses to uncontrolled
areas be as as far below the limits as
reasonably practicable.
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What changed?
Public dose limit went down by a factor
of five, and yet;
Around 1998 we started finding more
and more new facilities that failed to
submit shielding plans and failed to
shield secondary barriers.
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Example 1 of inadequate shielding
for new diagnostic x-ray room
Designer assumed:
-100 mA-min/wk
-Ignored leakage
-Specified wood door
and drywall
These walls and this
door were not leaded
-No plan submitted
-Facility had to add
shielding after
construction
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Example 2 of inadequate shielding
for new diagnostic x-ray room
Designer assumed:
- P = 0.2 mSv/wk
- T = 0.25 for hall
- Used attenuation
of the bucky
-Specified drywall for
one wall and 1/16th
behind bucky
This wall not shielded
-Plan submitted and
changes made prior to
construction
calculations were claimed to be based on AAPM TG-9, 1989, NCRP 116 and updated NCRP 49
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Example 3 of inadequate shielding
for mobile CT scanner
No shielding plan submitted
No shielding information available at time of
survey
Maximum exposure rate at 4 feet above
ground at surface of vehicle was 380 mR/hr,
most areas were ~ 2 mR/hr
Rate several yards away ~ 10 mR/hr
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Example 3 of inadequate shielding
for mobile CT scanner
~2 mR/hr
~ 10 mR/hr
~380 mR/hr
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Policy on Submission of Plan Reviews
and Delay of Equipment
Registration
All medical facilities should submit shielding plans for
review for any new construction or shielding
modifications that involve radiation producing
machines
Failure to do so can can cause expensive
modifications after the project is completed
Equipment that requires a Certificate of Need (CON)
will not be registered until plans have been reviewed
and approved by the Radiation Safety Section
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Actions taken to communicate need
to submit shielding plans
Letters sent with registration applications starting April 2001
indicate the need to submit shielding plans
Certificate of need letters to CON applicants state that plans
need to be submitted to the Radiation Safety Section (RSS)
Letters from the Health Facilities Evaluation Section state that
radiation shielding plans need to be submitted to RSS
Additional letters are sent by RSS for all equipment requiring a
CON and now include the statement: Review and approval of
the proposed shielding should be obtained from RSS prior to
registration and prior to placing the equipment into use.
Copies of the CON letters are now sent to the facility’s RPS in
addition to the CON contact
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Communication of Plan Review
Requirements
CON letters state

The applicant is required to contact the Michigan
Department of Consumer and Industry Services,
Bureau of Health Systems, Division of Health
Facilities and Services at (517) 241-1989 to obtain
radiation shielding approval when constructing
new radiation facilities or installing new or
replacement x-ray equipment ........"
Engineering Plan Review Section letter states

Provisions for radiation shielding protection of xray and gamma ray installations are subject to
review by the Division of Health Facilities and
Services, Radiation Safety Section.."
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Additional Shielding Information
Form BHS/HFS 852 – Information Required
for a Radiation Shielding Plan Review

Available on Web site at:
 www.michigan.gov/bhs (click on Mammography and
X-ray Machines – Radiation Shielding Guidance)
Web Site includes guidance on shielding:





Radiographic or RF rooms
Mammography Rooms
Computed Tomography Rooms
Linear Accelerator Vaults
Additional Guidance
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Table of Recommended Shielding
(Rad /Fluoro Rooms)
High Workload
(e.g., rooms in
hospitals and in
radiology and
orthopedic offices)
Up to 1000 mAmin/week
Moderate Workload
Low Workload
(e.g., rooms in medical
(e.g., rooms in podiatry
and chiropractic
and veterinary offices)
offices)
Up to 250 mAmin/week
Up to 15 mA-min/week
Lead
(inches)
Concrete
(inches)
Lead
(inches)
Concrete
(inches)
Lead
(inches)
Concrete
(inches)
1/16 – 1/8
5-9
1/16
5
1/16
5
Walls(U=0.25)
D=2.1 m
1/8
7-8
1/8
5-7
1/16
5
Floors(U=1)
D=3.0 m
1/8
8-9
1/8
5- 8
1/16
5
Walls
1/16
4-6
1/16
3-4
1/32
2
Doors
1/16
-------
1/16
-------
1/32
-------
Floors
1/16
4-5
1/16
3-4
1/32
2
Ceilings
1/16
4-5
1/16
3-4
1/32
2
Operator
Shields
Primary
Beams
Secondary
Radiation
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Recommended Shielding
(CT Rooms)
Computed tomography rooms typically have high
workloads and high kilovoltage technique settings. As
a result, at least 1/16 inch lead shielding or
equivalent is required for the walls, doors, floors,
ceilings, and operator's barrier. CT rooms with high
workloads and with fully occupied uncontrolled
space directly adjacent to the scanner may need
shielding that is thicker than 1/16 inch lead to
meet the 25 millirem per year dose constraint for
uncontrolled areas. Facilities with CT x-ray
equipment may also want to enlist the services of a
qualified medical physicist for shielding advice.
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Guidance on Concrete Floor Thickness
Floors
3"
4.5 "
Need to use
minimum thickness
of concrete floor in
shielding
calculations
Need to know
density of concrete
(147 lbs/ft3
standard)
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Radiation Shielding Implications of CT Scatter
Exposure to the Floor (Langer and Gray)
(Health Physics Vol 75, Number 2, August 1998 )
“Care may be needed to assure adequate
shielding for floor areas near CT
scanners specializing in either head or
body work. Also, while increased
throughput with helical scanners is
currently x-ray tube limited, shielding
plans should allow for enhanced heat
capacity (and generator throughput) in
future generation scanners.”
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CT Shielding Methodologies
AAPM Report number 39

Uses isodose contour lines and one set of
coefficients for the Archer Equation to determine
transmission curve
BIR/IPEM(British Institute of Radiology)

uses similar method with a slightly different set of
coefficients
NCRP 49 re-write

Uses scatter fraction determined by CTDI;
 9x10-5 for head and 3x10-4 for body

Uses slightly different set of coefficients
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CT Shielding Coefficients for the
Archer Equation
From NCRP draft rewrite
Alpha
Beta
kVp
120
140
120
lead
2.246
2.009
5.73
concrete
0.0383
0.0336
0.0142
From AAPM report 39
Alpha
kVp
120
lead
2.74
concrete
0.039
From BIR/IPEM
Alpha
kVp
125
lead
2.23
concrete
0.0351
September 18, 2016
Beta
140
2.77
0.034
120
13.23
0.062
Beta
150
1.791
0.0324
125
7.89
0.066
Gamma
140
120
3.99
0.547
0.0122
0.658
140
0.342
0.519
Gamma
140
120
10.86
0.943
0.046
1.18
140
1.03
1.17
Gamma
150
125
5.48
0.73
0.078
0.783
150
0.568
1.566
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Operator Booth Shielding Guidance
(Suggested State Regulations)



The lead glass viewing window should have an
area of at least 1 square foot
At least 1 square foot of the viewing window
should be centered no less than 2 feet from the
open edge of the booth and be centered 5 feet
from the floor.
The operator should be allotted not less than 7.5
square feet of unobstructed floor space in the
booth
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Radiation Shielding Plan Reviews
Summary
Plans should be submitted prior to construction
Rule 331(2) of the Ionizing Radiation Rules states
that shielding is subject to design approval by the
department
Most medical x-ray rooms should normally be
provided with 1/8 inch thick lead for primary barriers
(including floors) and 1/16 inch thick lead for
secondary barriers (including doors)
Construction of an x-ray room without an approved
plan may cause shielding modifications to be done
after the room is constructed
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GLAAPM Shielding Statement and
Various Shielding Assumptions
The Qualified Medical Physicist
Design Dose Constraints
Models for Tube Housing Leakage
Workload Assumptions
Occupancy Factors
Use Factors
Shielding Credit for Imaging Hardware
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The Qualified Medical Physicist
Currently only ~20 % of the submitted
plans involve a AAPM QMP
Others include HPs, CHPs, Architects,
Engineers, RSOs, Physicians, X-ray
Equipment installers
Current NCRP and Current draft do not
list QMP requirements similar to
GLAAPM
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Design Dose Constraints
NCRP 116 - if such an assessment is not conducted,
no single source or set of sources under one
control should result in an individual being exposed
to more than 0.25 mSv annually.
GLAAPM - If such an assessment is not conducted…
no single source(of radiation)…should result in an
individual being exposed to more than 0.25 mSv
annually. The GLAAPM declares that if shielding
designs are performed by a Qualified Radiation
Physicist they are ensuring the exposure of a
maximally exposed individual from a single source
of radiation, will not be greater than 1 mSv annually
and therefore the lower dose limit is not required.
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Tube Housing Leakage
150 kVp is the LEAST conservative
leakage kVp to assume
Many MDs and DCs use lower rated
tube housings
Amount of leakage calculated is a
strong function of assumed kVp
spectrum
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Leakage Technique Factors
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Workload Assumptions
NCRP 49 philosophy was to assume a fixed
kVp and busy workload for design purposes
(e.g. 100kVp,1000 mAmin/wk)
The draft of the NCRP re-write of Report 49
uses mean workloads based on survey of
hospital facilities
Since workloads change over time (and have
increased with digital) we advocate using
conservative workload assumptions
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Workload Assumptions
From Simpkin, Med. Phys. 23(4), April 1996

Given the broad distributions of the workload per
patient, a reasonable concern arises as to the use
of the mean normalized workload per patient for
estimating room workloads…. A more conservative
approach would be to assume an upper percentile
workload as the per patient value, since this would
only be exceeded in a minority of cases…. The
90th percentile workload would be especially
conservative. From the PEMNET data, the 90th
percentile workload is 2 to 3 times the mean.
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Workload Assumptions
Use of kVp spectrums
kVp Distribution for 22 cm AP lumbar spine
number of exposures
1200
1000
800
600
400
200
0
50
55
60
65
70 75
80
85
kVp
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90
95 100 105 110 115 120
The use of kVp
distributions are
advocated based on the
premise that “The
relative distribution of
the workload over kVps
is set by the
radiographic contrast
required for imaging”
However, we find kVp
varies considerably by
facility, even for the
same sized patient
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Occupancy factors
The occupancy factor for radiation workers
should be unity (1) throughout controlled
areas (NCRP49 page 6)
The occupancy factor in generally accessible
areas for non radiation workers should not be
less than the ratio of the uncontrolled design
dose limit to the controlled area design dose
limit
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Radiation Shielding for Diagnostic X-rays –
(Copyright 2000, The British Institute of Radiology, ISBN 0-905749-44-8)
IRR99 explicitly states that areas in which exposure
can be greater than 6mSv per year should be
controlled. Any design criteria and associated
assumptions on occupancy must reflect this fact.
Therefore consequent to the design criterion of 0.3
mSv per annum must be the adoption of 5% as the
lowest occupancy factor. Use of a lower occupancy
factor with a dose constraint of 0.3 mSv implies that
the area outside of the room would have to be
designated as a controlled area.
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Use factors
NCRP 49 assumes walls are ¼ and floor
is 1 and ceiling 0. New draft uses new
model (NT/Pd2) which “hardwires” in
uses factors for walls that sum to unity
We believe barrier use factor should be
selected as the reasonable maximum
percentage of the time that the barrier
might intercept primary radiation.
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Shielding credit for image receptor
Should not be counted unless;



The primary beam is permanently and positively
restricted to the image receptor
The image receptor is designed to fully intercept
the primary beam regardless of collimator
settings, image receptor size, SID, user controlled
alignment, alignment overrides, etc.
X-ray room will not be used in the future with
conventional general purpose x-ray equipment
unless shielding is redesigned
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Conclusion
Submit plans for approval prior to
construction
We recommend utilizing the shielding
guidance on our web site at:

www.michigan.gov/bhs (click on Mammography
and X-ray Machines – Radiation Shielding
Guidance)
The new NCRP shielding report will be
reviewed once the report is made final and
we may revise some policies and guidance
after the review
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Contact Information
Headquarters:Radiation Safety Section
Michigan Department of Consumer & Industry Services
P.O. Box 30664
Lansing, Michigan 48909 Phone: (517) 241-1989
Fax: (517) 241-1981
Visitor/delivery address:5th Floor, G. Mennen Williams
Building
525 W. Ottawa Street
Lansing, Michigan 48933
District Office:Our district office, formerly in Pontiac, has
moved to: Radiation Safety Section
Michigan Department of Consumer & Industry Services
38600 Van Dyke, Suite 375
Sterling Heights, Michigan 48312
Phone: (586) 446-0200
Fax: (586) 446-0227
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