Jo Had Erson Pshielding petet Shielding

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AAPM Summer School, 2007 [email protected] 1

PET Shielding andPET Shielding andRadiationRadiation

ProtectionProtection

Jon A. Anderson, PhDJon A. Anderson, PhDDepartment of RadiologyDepartment of Radiology

The University of Texas Southwestern Medical Center at DallasThe University of Texas Southwestern Medical Center at Dallas

July 2007July 2007American Association ofAmerican Association ofPhysicists in MedicinePhysicists in Medicine

Summer SchoolSummer School


What Is Different When You HavePET/CT in Your Facility?

1) 511 keV energy-increases exposure rate from doses,patients-greatly increases thickness of requiredshielding

2)Requirements for patient handlingduring injection and uptake phase

3) Combined modality scanners(PET/CT) require consideration of bothgamma-ray and x-ray hazard


The 18F-Injected Patient as a Source(average of different investigators, 2003 )

0.103 ( Sv/hr)/MBq0.383 (mrem/hr)/mCi




all at 1 m from surface ofbody, average value fromseveral investigators

Anterior

Inferior

Lateral

Superior

compare this to0.014 ( Sv/hr)/MBq or0.05 (mrem/hr)/mCi

for99m

Tc:18

F valuesfactor of 8 larger!

not asanisotropic as itmight seem


A Revealing Comparison of LeadRequirements: X-Ray vs PET

Lead Thickness Requiredmm (in, to next 1/16)

#HVL's

46.0 (1 13 /16)1.366 (< 1/16)1032.5 (1 5/16)0.718 (< 1/16)8

19.0 (3/4)0.278 (< 1/16)49.9 (7/16)0.103 (< 1/16)25.3 (1/4)0.044 (< 1/16)1

PET 2X-ray 1

(average primary forrad room)

1. NCRP 147: Structural Shielding Design for Medical X-Ray Imaging Facilities2. Simpkin, 2004, developed for AAPM Task Group on PET Facility Shielding

Even asingle half-value layerfor PET isanexpensiveproposition!


A Revealing Comparison of LeadRequirements: X-Ray vs PET

Lead Thickness Requiredmm (in, to next 1/16)

#HVL's

46.0 (1 13/16)1.366 (< 1/16)1032.5 (1 5/16)0.718 (< 1/16)8

19.0 (3/4)0.278 (< 1/16)49.9 (7/16)0.103 (< 1/16)25.3 (1/4)0.044 (< 1/16)1

PET 2X-ray 1

(average primary for

rad room)

1. NCRP 147: Structural Shielding Design for Medical X-Ray Imaging Facilities2. Simpkin, 2004, developed for AAPM Task Group on PET Facility Shielding

Even asingle half-value layerfor PET isanexpensiveproposition!

Remember this morning's bottom line: for DX,we can apply 3 models from NCRP 147 and findthat 1/16" is (usually) the answer, with some tospare. We usually calculate the "closest" point.

Not true in PET. As we will see, it is true that

normally we need 1-3 HVL's of shielding. Wetend to put just what we need, due to $$$.

Implication: At every protection point, we needto include all sources that can be contributing tothe dose at that point (i.e. multiple injectionrooms, scan rooms, etc.). Corresponds toadjusting protection limit downward.


Workflow at the PET Center(FDG Whole Body Scans)

Receive dosesArrival of patient

Injection of Pt

Uptake of pharmaceutical

Assay of dose

Transport Pt to scanner

Position Pt

Scan

QA Check of Scan

Pt instruction and prep

Release Pt

Read study

Distribute to PACS or Media

Have Pt empty bladder

30-60 min

10-30 min

5-10 min

**

*

* steps withhighesttechnologistexposure

*

*

8-20 mCi




Technologist Doses

0.000

0.050

0.100

0.150

Reference

D o s e

/ A c

t i v i t y H a n

d l e d

S v

/ M B q

( m r e m

/ m C i )

SI Units 0.018 0.012 0.023 0.019 0.019 0.011 0.009 0.029 0.021 0.018

Conventional Units 0.067 0.044 0.085 0.069 0.069 0.041 0.032 0.107 0.077 0.066

B en et ar Ch is ea Ch is ea M cEl ro y UT SW Ro be rt s Gu il le t B ir an Ye st er Ave ra ge

Magnitude of TechnologistExposure Consistent with conventional nuclear

medicine practice, most of technologistdose comes from positioning, transport,and injection.

average dose/procedure< 10 Sv (1 mrem)


More on Technologist Exposure

1) Technologist dose will probably drop as experienceincreases

Over a two year period with the same technologists, we saw a40% decrease in radiation dose per unit activity handled.

2) For 0.018 S v/(MBq injected)370 MBq (10 mCi) injected/pt

10 pt/day

Yearly: 16.6 mSv (1660 mrem) (<5 rem, >ALARA trigger)9 Months: 12.5 mSv (1250 mrem) (> declared pregnancy limit)


Operating Suggestions to MinimizeTechnologist DoseMinimize handling time. Use unit doses.

Use tungsten syringe shields, employ syringe carriers,transport carts, etc. to minimize handling exposure.

Instruct patient before injection. Minimize contactafterward.

Establish IV access with butterfly infusion set.

Use other personnel for hot patient transport.AAPM Summer School, 2007 [email protected] 10

ScanRoom 1

West Corridor

Office 3

Future CameraRoom

Office 2

Office 1

Injection#2

BreakRoom

Injection#1

Storage

HotToilet

Hot Lab

Dock LaundryScanUtility

ScanRoom 2

SouthCo rridor

Reception &Waiting

BusinessOffice

Control

East Corridor

ReadingRoom

PET Facility Tour: University of TexasSouthwestern Medical Center at Dallas(2001-2007)Radioactive

Materials

Patients


ScanRoom 1

West Corridor

Office 3

Future CameraRoom

Office 2

Office 1

Injection#2

BreakRoom

Injection#1

Storage

HotToilet

Hot Lab

Dock Laundry

ScanUtility

ScanRoom 2

SouthCo rridor

Reception &Waiting

BusinessOffice

Control

East Corridor

ReadingRoom

PET Facility Tour: University of TexasSouthwestern Medical Center at Dallas(2001-2007)Radioactive

Materials

Patients

If you can get involved in thearchitectural planning, you can savesome money, by using distance insteadof lead.


Hot Lab Details: Dose StorageArea

Notes:1) Floorprotection(containersweigh > 66 lbs)

2) Space neededdepends on howoften deliveriesare made; mayhave >100 mCihere at a time,even for onescanner3) Extrashielding maybe required

alternative Wtransport carrier






Site Evaluation for PET ShieldingUses of adjacent spaces (including above and below) andoccupancy factors for them

# patients/weekisotopes to be used, activity/pttypes of PET studies to be performed (brains, WB, cardiac)uptake time and scan time for this equipment/study/center

dose delivery schedule (once a day?, multiples?); maximumactivity on hand

CT technique factors (kVp, mAs/scan [depends of # beds])# scans per patient (additional diagnostic scans?)amount of "non-PET" CT workload expected


Radiation Sources to Include in theShielding PlanDoses (pre-injection) in Hot Lab

Calibration sources for scanner

Patient (post injection, in uptake rm)

Patient (in scanner, hot toilet)

TX Sources in scanner (PET)

CT x-ray source (for PET/CT)

require isotopicworkloadparameters:pts/wk, mCi/ptuptake,scan timesisotope type anddelivery scheds

require CT x-rayworkload factors,techniques,include non-PETCT work


P: Radiation Protection TargetsLimitations ALARA

per 10CFR20 Action LimitRadiation workers 50 mSv/yr 5 mSv/yr

(5000 mrem/yr) (500 mrem/yr)Pregnant worker's fetus 5 mSv/9 mo

(500 mrem/9 mo)

Members of public 1 mSv/yr(from each licensed operation) (100 mrem/yr)in any hour, not to exceed .02 mSv

(2 mrem)

GUIDANCE: ALARA -- As Low As Reasonably Achievable

Targets

controlled areas:100 Sv/wk or10 mrem/wk

uncontrolled areas:20 Sv/wk or2 mrem/wk


PET Facility Throughput Example:90 Minute Uptake, 30 Minute Scan

7:00 8:00 9:00 10:00 11:00 12:00 13:0014:0015:00 16:00 17:00

1

3

5

7

9

11

13

15

P a

t i e n

t N u m

b e r

Time of Day

Phase

Uptake

Scanning

N: Maximum Workload Estimation

#pts/day = (T work - Tuptake )/T scan_rm # uptake areas = T uptake /T scan_rm

This facility needsthree uptake rooms


18F: A Plethora of Dose RateConstants for Point Sources (TG-108)

(mrem/hr) m 2/mCi0.6956( Sv/hr) m 2/MBq0.188Maximum Dose (ANS-1977)

(mrem/hr) m 2/mCi0.6771( Sv/hr) m 2/MBq0.183Deep Dose Equivalent(ANS-1977)

(mrem/hr) m 2/mCi0.5476( Sv/hr) m 2/MBq0.148Tissue Dose Constant

(mrem/hr) m 2/mCi0.5291( Sv/hr) m 2/MBq0.143Effective DoseEquivalent (ANS-1991)

(mrem/hr) m 2/mCi0.4958( Sv/hr) m 2/MBq0.134 Air Kerma RateConstant

(mR/hr) m 2/mCi0.5735( R/hr) m 2/MBq15.5Exposure RateConstant

Conventional UnitsSI Units18 F Rate Constants

TG-108 recommends0.143 ( Sv/hr)/MBq0.53 (mrem/hr)/mCifor F-18 bare source


Dose Rate from 18F Injected Patient at 1 m

0.000

0.100

0.200

0.300

0.400

0.500

0.600

Source

D o s e

R a

t e

[ ( S v

/ h r ) / M B q

]

[ ( m r e m

/ h r ) / m

C i ]

S IUn its 0.075 0.055 0.100 0.150 0.137 0.08866 0.097 0.100

Conventional Units 0.279 0.203 0.370 0.553 0.508 0.328 0.359 0.372

K e ar f o tt 19 92 C h i se a 1 99 7 C r o ni n 1 9 99 B e n et a r 2 0 00 W h it e 2 00 0Yester * 2005(attenuation)

Massoth 2003(unpubl.)

Average

: The 18F-Injected Patient asa Source (retained activity)

sources of variation:delay time to measurement,micturationstatus, exposure-to-dose conversion, etc.

about 20% of dose will be in bladder after 1-2 hours;TG108 uses 15%

TG-108 recommends(0.092 Sv/hr)/MBq(0.34 mrem/hr)/mCi




Correction for Decay During DoseIntegration Period: Reduction Factor

Correction for Decay of F-18

0

0.2

0.4

0.6

0.8

1

1.2

0 20 40 60 80 100 120 140 160 180 200

Time [minutes]

R e

d u c

t i o n

F a c

t o r

R t

R 0

t

TA0

e

T

T half

ln 2( )

−⌠

⌡d

0

t

TA0

⌠

⌡d

= 1.443 * (T 1/2/t) * (1 - exp(-0.693 * t/T 1/2)


Effect of CorrectionsCumulative Dose 1 m from Patient, 370 MBq F-18

0

20

40

60

80

100

120

140

160

180

0 20 40 60 80 100 120 140 160 180 200

Time [minutes]

D o s e

[ m i c r o

S v

]

Uncorrected

Corrected for Physical Decay

Corrected for Physical Decay,Self-ShieldingCorrected for Physical Decay,Self-Shielding, Voiding

Effects of Adding Corrections toDose Calculation

Inject @ 5 minutesVoid @ 65 minutes

A factor of 2= 1/4" of lead


Simplified Task Group Formalism:Uptake Room

B, the required barrier transmission factor, will be calculated as

B =P * d 2

Γ * T * N w * (A0 * F tot * t * R t)

B =10.9 * P[ μSv] * d[m] 2

T * N w * (A0[MBq] * tU[hr] * R tU)

10.9 is 1/ in (hr/ Sv)(MBq/m 2);F tot = 1 (no physical decay prior to injection, no

elimination)R tU = reduction factor for uptake time t U

UptakeRoom:


Simplified Task Group Formalism:Scan Room

B, the required barrier transmission factor, will be calculated as

ScanBay:

12.8 includes the value of and the effects of voiding 15% of theinjected activity before imaging;

FU=exp(-0.693t U /T 1/2), the physical decay of the isotope before thepatient enters scan bay

B =P * d 2

Γ * T * N w * (A0 * F tot * t * R t)

B =12.8 * P[ μSv] * d[m] 2

T * N w * (A0[MBq] * F U * t I[hr] * R tI)


Lead

0.0001

0.0010

0.0100

0.1000

1.0000

0 5 10 15 20 25 30 35 40 45 50

Thickness(mm)

T r a n s m

i s s

i o n

Going from Barrier Transmissionto Shield Thickness

Monte Carlocalculations byDouglasSimpkin (2004)Monte Carlo Simulation

(Broad Parallel Beam)

Constant TVL 16.6 mm

x B( ) 1α γ ln

B γ− β

α

+

1

βα

+:=

Curves and fitting parameters for ironand concrete arealso found in thereport


Fitting Parameters for DifferentMaterials per TG108

γ

[cm -1] [cm -1]Lead 1.543 -0.4408 2.136

Concrete 0.1539 -0.1161 2.0752Iron 0.5704 -0.3063 0.6326

Arch er Parameters

B x( ) 1β

α+

eα γ x β

α−

1−γ

x B( )1

α γln

B γ− β

α+

1β

α+






Grid Calculation: Shielded

Y image

DoseToTargetRatio

No shielding in walls in excess of 5/16" Pb; did require ceiling, floorshielding. Was not necessary to run "box" to ceiling.


Grid Calculation: No Shielding

Sources: Injection room, HL, HWC, Scanner, CT, Cal Source4 pts/day, 1 hr in uptake, 2 hrs in scan room

Scan InjCHT

DoseToTargetRatio

Ratio of calculated dose to t arget dose,adjusted for occupancy

Office/Lab

Reading

Corridor C or r i d or

R e a d i n

g

Of f i c e / L a b

E l e v a t or

E l e v a t or

L o b b y

OOPS's happen with complicatedschemes: Both floor and ceiling

needed lead, installed as lead sheetbonded to plywood panels and heldin brackets fastened to structuralweb, but different thickness aboveand below.

The contractor switched them inspite of drawings and labeled leadpallets.


Example: University of TexasSouthwestern Medical Center 2007

STORAGE2.490

INJECTION42.480F

5 4 "

5 4 "

MANAGER2.591H

CORRIDOR2.481

CORRIDOR2.5

READING22.591D

READING12.591E

SCANNERBAY32.591C

SCANNERBAY22.591B

PHYSICIAN22.440

CORRIDOR2.482

CLINICALSTORES2.480HINJECTION3

2.480EINJECTION2

2.480D'HOT'TOILET

2.480A

CONTROL/OBSERVATION2.591

NURSE2.481B

FILEROOM

2.591G

PHYSICIAN12.450

LAB2.591F

SUB-WAITING2.481A

FUTURESCANNERBAY1

2.591A

SCANUTILITY

2.592

'HOT' TOILET2.480G

INJECTION12.480B

'HOT'LAB2.480C

Overalldesign: Nolead in excessof 3/8". Northwall ofinjectionrooms, hot labshielded with16" of dry-laid, full-densityconcrete block


Look Up, Down, and Sideways

Floor Plan Relative Dose Map on Floor Above

Mechanical Space

Exterior

ElectricalElectrical Elevator,Lobby

Corridor Corridor

M e c h a n i c a l S p a c e

Glass Wash

Duct penetrations in ceilingrequired separate shielding.


Dry-Laid Concrete Block as aFlexible Alternative

Use full density concreteblocks (not standarditems!).

Lay to offset seams.

Provide cosmetic stud-wall w gypsum board toprevent tampering ifarea is not controlled.


An Alternative: Shadow Shielding

from JA Anderson, RJ Massoth, and LL Windedahl, 2003 AAPM




Acknowledgements

• Michael Viguet, CNMT• Dana Mathews, MD• Thomas Lane, PhD• Richard Massoth, PhD• Larry Windedahl• Doug Simpkin, PhD• Mark Madsen, PhD

[email protected]


TheEnd

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Jo Had Erson Pshielding petet Shielding