The AAPM PET/CT Task Group Recommendations - Radiation ...

AAPM Task Group 108PET/CT ShieldingiDouglas J. Simpkin, Ph.D.Aurora St. Luke’s Medical CenterMilwaukee, Wisconsin, USAdsimpkin@wi.rr.comwww. http://geocities.com/djsimpkin1

Positron Emission Tomography• Well established modality in physiologicresearch• PET in clinical facilities– In USA, federal Medicare reimbursement for18FDG available 1998, other insurancecompanies follow– Led to establishment of a distribution system oflocally-produced 18 FDG (…don’t need your owncylcotron)– 18 FDG is the accepted methodology forquantitative tumor imaging2

AAPM Task Group 108 on PETand PET/CT Shielding• Formed fall of 2002• Membership p( (from Nuclear MedicineCommittee of American Assoc of Physicistsin Medicine)– Mark T Madsen, Chair– Jon A Anderson, James R Halama, Jeff Kleck,Douglas JSi Simpkin, John RV Votaw, Richard dEWendt III, Lawrence E Williams, Michael VYester• Published in Med Phys 33(1);4-15(2006).3

Facility Design Considerations• A clinical i l PET facility will include a– “Hot Lab” for receiving, storing, and preparing18 FDG for injection– Injection / “Uptake” Room•often an old closet– Scanner room4

Physics Considerations• 18 F decays (T 1/2 = 110 min = 1.83 h) by positronemission; the positron travels a short distance andthen annihilates, forming 2 × 511 keV photons.• For the same activity, the kerma/exposure ratefrom 18 F is ~6x that of 99m Tc!• These 511 keV photons are very penetrating• Distance = shielding. In a new facility, try to– Build a BIG department– Isolate the PET department, especially from nuclearmedicine devices5

Sources of Radiation Exposure inClinical PET and PET/CT• PET radiopharmaceutical in preparation andinjection• Patient injected with the 18 FDG• X rays from CT scan• Sealed calibration source in PET scanner6

Sources of Exposure in ClinicalPET• PET radiopharmaceutical in preparation andinjection– Typically receiving and administering “unitdoses” so handling is minimal– Thick Pb shielding must be available in the“Hot Lab”• Pb block “cave” for source storage• L blocks for handling• Special syringe shields7

Sources of Exposure in ClinicalPET• The thickness (& weight) of PET shields is ~ anorder of magnitude greater than for Tc-99m– “L” blocks– Special syringe shields for injection• 1.3 cm Pb-equiv W• 1.9 cm Pb-glass window• W disk at end protects hands8

Sources of Exposure in ClinicalPET• Sealed pin or rod calibration source(s) inPET scanner– Sealed ~180 MBq (5 mCi) Ge-68 sourcerobotically removed from its Pb shield toexposed patient for transmission scan– Nearly all photons are absorbed by patient ordetector assembly– Not a concern for structural shielding9

Sources of Exposure in ClinicalPET – The Patient• Rdi Radiopharmaceutical lij injected tdit into patientt• Typical F-18 activity administered ~550 MBq(~15 mCi, but may vary widely; 10-30 mCi)• The injected patient is the primary source ofexposure in a PET facility– Despite regulations that permit the patient to bereleased from the clinical setting, the facility stillhas responsibility to assure that radiation doses tostaff and others is ALARA10

Sources of Exposure:“Point” Sources* of F-18F-18 Rate Constant, Γ Value UnitsExposure rate constant 15.4 µR m 2 /MBq hAir Kerma rate constant 0.134 µSv m 2 /MBq hEffective DoseEquivalent (ANS-1991)0.143 µSv m 2 /MBq hTissue dose constant 0.148 µSv m 2 /MBq hDeep Dose Equivalent(ANS-1977) 0.183 µSv/ m 2 MBq hMaximum Dose (ANS-1977) 0.188 µSv m 2 /MBq h* Syringe of activity11

Sources of Exposure:F-18 in Patients• Patient ≠ Point Source (patient attenuation, & scatterin patient degrades the energy of photons emanatingfrom patient)• Task Group reviewed literature, recommends arealistic Effective Dose Rate Constant Γ for F-18 inpatient:Γ = 0.092 µSv m 2 MBq - 1 h -1• But, to be conservative, assume all photons emittedfrom patient are 511 keV12

Sources of Exposure:F-18 in Patients• Then unshielded Dose Rate D at distance r(m) from activity A (MBq) is 0D=Γ A0r2•The Dose, D, accumulated over time t is0.693⎛ − × t⎞= × × × ⎜ −T1/2D 1.44DT e ⎟0 1/ 21⎜ ⎟⎝⎠13

Sources of Exposure in ClinicalPET• The patient is the source– FDG Patient in “incubation” (inanquiet,isolatedisolatedroom) for ~30-60 minutes. Shielding for thisroom must be considered.– Patient is then put into the scanner for another30-60 minutes. Shielding for scanner room mustbe considered.14

What needs to be shielded fromPET sources?• Nuclear Medicine Equipment– Scintillation detectors (e.g. thyroid uptakeprobes)– Gamma cameras– Bone-mineral densitometers• “A nuclear medicine department is theworst place to put a PET facility!” MarkGroch, Ph.D. 2002.15

Effect of F-18 on NearbyGamma Cameras• Measured count rate in 15% Tc-99mwindow from F-18 in phantom– Jaszczak phantom (21.6 x 18.6 cm cylinder)– Siemens Orbiter camera with LEAP collimator– Vary angle16

Effect of F-18 on NearbyGamma Camerasθ cpm m 2 / MBq0º 1.4 E+0445° 4.0 E+0390° 9.9 E+02180° 2.6 E+03intrinsic 6.9 E+04cpm = counts per minuteJaszczakphantomwith F-18Siemens Orbiter15% 99M Tc windowθLEAPCollimator17

Shielding Gamma Cameras:Sample Calculation• Have gamma camera (with no collimator)7.3 m from patient in PET scanner with 418MBq on board at start of scan• Ct rate=6 6.9E+4 × 418 / 7.3 2 = 5.4E+5 cpmUnshieldeddct rate5.4E+5cpmLimit =1%increase inRequired#HVL HVLsAssumedRequired15 kcps Transmiss required HVL shielding9E+3 cpm 0.017 5.9 0.54 cm Pb 3.2 cm Pb18

Protecting Nuclear MedicineEquipment from PET Sources• Distance• As Pb barriers tend to be thick (>2 cm),localize barriers (to minimize weight andcost)0.7 cm PbPETScannerto protectpeople incorridor2.5 cm Pbto protectgammacamera2 headedgamma camera19

Shielding People from PETSources•In uncontrolled areas folks are notmonitored for radiation dose.•In USA– Must restrict dose in any one hour to< 0.02 mSv and– Must restrict annual dose to

Shielding People from PETSources•In controlled areas radiation workers aremonitored for occupational radiation dose– (USA: Must restrict annual dose to < regulatedlimit 50 mSv y -1 with ALARA considerations)– USA: For pregnant workers, must restrictannual occupational dose to 5 mSv y -1 21

Shielding People from PETSources• The design goal is the permitted kerma inthe shielded area, P, divided by theoccupancy factor, T.• T is the fraction of the time someone is inthe shielded area and requires protection.22

NCRP-147 Recommended TOffices, labs, pharmacies, receptionist areas, attendedwaiting rooms, kids’ play areas, x-ray rooms, filmreading areas, nursing stations, x-ray control roomsPti Patient t exam &treatment t trooms½Corridors, patient rooms, employee lounges, staff restrooms11/5Corridor doors 1/8Public toilets, vending areas, storage rooms, outdoorareas w/ seating, unattended waiting rooms, patientholdingOutdoors, unattended parking lots, attics, stairways,unattended elevators, janitor’s closets1/201/4023

Kerma Transmission of511 keV Photons• For a barrier of thickness x, transmission, B, isB =K xshielded( )Kunshielded• Determined from EGS4 Monte Carlo simulation ofbroad parallel beams of 511 keV photons in–Pb– Iron– Concrete of standard density = 2.4 g cm -3 ; Much modernconstruction ti is with “light weight” concrete with density~75% that of the standard concrete)24

Kerma Transmission of511 keV Photons• Will be conservative since the photonsemanating from the patient will have beenscattered to lower energies25

Iron28

Fit Transmission Data• Fit transmission data B(x) to eqn of Archeret al. (Health Phys 44:507-517; 1983)1−⎡γ⎛ β ⎞ α γ β ⎤= ⎢⎜1 +xB ⎟ e −α α⎥⎣⎝⎠ ⎦• Which can be inverted29

Transmission Fitting Data• For thickness x in cm,ShieldingMaterial α(cm -1 ) β(cm -1 ) γLead 1.543 -0.4408 2.136Concrete 0.1539 -0.1161 2.0752Iron 0.5704 -0.3063 0.632630

Where in the Occupied Area do youcalculate the dose?0.5 mTo the closestsensitive ii organ!Injectedpatient0.3 m1.7 m31

Conclusions• Thick Pb caves, L blocks, and special syringeshields are needed in the “Hot Lab”• The injected patient is the primary source ofradiation exposure in the PET facility• For 18 FDG, the uptake room and scanner willprobably require lead shielding• This shielding will often be 2-10× thicker thatwhat’s typically in a diagnostic i x-ray room32