PET/CT:
Radiation Safety and
Protection
GN Chimon PhD
CEO Singapore Radiopharmaceuticals: FORTIS HEALTHCARE
Educational Objectives
• Radiation regulatory limits
• PET exposure/dose calculations
• Radiation safety
Outline
• Introduction to principles of radioactive materials
• Radiation regulatory limits
• PET exposure/dose calculations
• Radiation safety principles
• PET isotope shielding
• Typical exposure to staff
Types of Radiation
• Ionisation: electron in the inner orbit receives sufficient energy
to escape from the nucleus, forming a positive and negative ion
• Ionisation radiation is emitted when an electron from an outer
shell falls in to take the place of the electron that was ejected
• Ionisng radiations cause ionisation when they pass through
matter
• Different types of ionising radiation have different energies: particles can be stopped by a sheet of paper. -particles can be
stopped by 1 cm of plastic. -rays and x-rays need lead for
shielding, the thickness depending on the energy of the
radiation
Activity of Radioactive Materials
• The activity of radionuclide is a measure of the radioactivity of the
substance. It is determined by the number of disintegrations per second
(Bq). The activity of a radioactive material varies with time
exponentially
• A = A0 e-t
• A = current activity at time t
• A0 = original or initial activity
• e = base natural log
• t = amount of time elaspsed from A to A0
• λ = Radioactive decay constant
• Relationship between half life and radioactive decay constant:
• T½ = In2/
Radionuclide Half-Life
• The half-life is a characteristic of the radionuclide. It is not
related to the atomic number or mass number of the material
• After 2 half- lives the activity is one quarter (1/2)2 its initial value
• After 3 half-lives the activity is one eighth (1/2)3 its initial value
• After 10 half-lives, the activity is (1/2)10 = (1/1024) i.e. about
one-thousandth its initial value
• In general, the activity after n half-lives is (1/2)n its initial value
Equivalent Dose
• Equal absorbed doses of different radiations do not necessarily
produce biological effects of the same magnitude
• one unit of absorbed dose to a tissue from alpha radiation is much
more harmful than one unit of absorbed dose from beta radiation
• The absorbed dose of each type of radiation, must be multiplied
by a radiation weighting factor WR which reflects the ability of the
particular type of radiation to cause damage
• The quantity obtained after such multiplication is known as the
equivalent dose i.e.
• Equivalent Dose = Absorbed Dose x Radiation Weighting Factor
Weighting Factors
Type of ionising radiation and energy
range
Photons, all energies
Radiation weighting factor
1
Electrons and muons, all energies
1
Neutrons, energy, 10 keV
5
10 keV to 100 keV
10
>100keVto 2 MeV
20
>2 MeV to 20 MeV
10
>20 MeV
5
Protons, other than recoil protons,
energy >2MeV
5
Alpha particles, fission fragments,
heavy nuclei
20
Unit Equivalent Dose: mSv/Sv
• UED is the sievert (Sv). Represents doses received by human
beings
• Radiation dose depends on the activity (Becquerel, Bq) of a
radioactive source, the distance from the source, whether there is
any shielding, and the exposure time. The old unit for the
equivalent dose is expressed in rem
• 1 Sv
=
100 rem
• 1 mSv
=
100 mrem
• 1 Sv
=
0.1 mrem
• The sievert expresses biological effect on the human body. In
radiation protection it is the biological effect of radiation which is
of interest
Regulatory Requirements
• The Federal Code of Regulations 10 CFR20 establishes the
effective dose equivalent limits (NCRP116)
• Occupational = 50 mSv/yr = 5000 mrem/yr
• Public = 1 mSv/yr = 100 mrem /yr
• Shielding design limits (NCRP 147)
• Occupational = 5 = 500 mrem/yr = 10 mrem/wk
• Public = 1 mSv/yr = mrem/yr = 2 mrem/wk
Radiation Dose from PET
Radiopharmaceuticals
A
D
Average Activity
• Activity decays exponentially during any substantial period of
exposure to radiation
• PET radionuclides have short half-lives
Shielding Basics
• Radiation dose design limits
• Radiation safety principles
• Time, Distance, Shielding
• The ALARA principle
• As Low as Reasonably Achievable
• Consider surrounding (all six directions)
Radiation Safety Principles
• Time
• Exposure varies linearly with time
• reducing exposure time by half reduces absorbed dose by half
• Minimize time personnel must spend in high exposure rate areas
• Uptake rooms
• Scanner rooms
• Hot patient proximity
Radiation Safety Principles
• Distance
• Exposure follows inverse-square law (1/D2)
• Increase distance by 2 decrease the exposure by 4
• Within a meter, 2/D may be more accurate
Radiation Safety Principles: HVL
HVL
1
2
4
8
Transmission
0.50
0.25
0.0625
0.0039
Material
Concrete
Iron
Lead
HVL @ 511 keV
~8 cm
~ 5 cm
~2 mm
Occupancy Factor
• Allows for partial occupancy with T the occupancy factor. P represents design
dose per Wk
• T is the weekly (40 hr) fraction of the time the location is occupied
• Shielding barrier is acceptable if it decreases the dose to P/T. - if T<1, the “fulltime” dose will be P/T; any shielding requirements unlikely
Barrier Thickness: How Much Shielding?
• Typically Physicist given an exposure level and shielding goal
• Calculate the attenuation factor and corresponding # of HVL to achieve it
• Xbarrier = Target Exposure / Actual Exposure
• NHVL = -In(Xbarrier) / In(2)
• Barrier thickness depends on barrier material (attenuation)
• Tbarrier = NHVL x HVLbarrier
Sample Calculation
• NM Tech exposure during PET scan
• Assume 6.5 mCi of F-18 FDG located 3 m away for 30 minutes
• Activity @ start = 6.5 mCi; @ end = 5.4 mCi
•
•
•
•
•
Average activity, A = 5.95 mCi
D = 3 m, T = 0.5hr/40hr = 0.0125
Dose/patient = 0.43 µSv/wk
Dose for 80 patients/wk = 34 µSv/wk
Sample Calculation
•
•
•
•
•
•
NM Tech exposure during PET injection
Assume 15 mCi of F-18 FDG located 1 m away for 5 minutes
Average activity, A = 15
D = 1 m, T = 0.083 hr/40 = 0.0021
Dose/patient = 0.0017 mrem/wk
Dose for 80 patients/wk = 0.14 mrem/wk
Sample Calculation
• Public area adjacent to radiopharmacy
• Assume unshielded exposure 10 mrem/wk
• Shielding design limits: 2 mrem/wk
• Office: Occupancy = 1
• Attenuation factor
• Xbarrier = 2 mrem / 10 mrem = 0.2
• NHVL = –In(0.2)/In(2) = 2.32
• TLEAD = NHVL x HVLLEAD = 2.32 x 5 mm = 11.6 mm
Sample Calculation
• Public area adjacent to radiopharmacy
• Assume unshielded exposure 10 mrem/wk
• Shielding design limits: 2 mrem/wk
• Waiting area: Occupancy 1/20
• Attenuation factor
• Xbarrier = (20) * 2 mrem / 10 mrem = 4
• Xbarrier = P/T = < 1 implies no shielding required
PET/CT facilities represents unique
challenges for shielding
• Annihilation radiation is energetic & highly penetrating
• Eγ = 511 keV; HVL ~ 5 mm Pb (broad-beam)
• Movement of radioactive patients in the clinic results in radiation
sources that are stationary in neither space nor time
• e.g., T1/2 of F-18 ~ 110 min
• Space is a premium – making distance hard to exploit
• Adjacent location within PET facilities are usually occupied
• Increasing demand for PET-CT examinations
• Scanners operate continuously for 8-10 hours/day
Thank You Be Safe