Medical Dosimetry Certification Board Exam Revision:
Planning, QA, Mechanics, Physics, Radiobiology, Safety
questions with verified answers
- phantom set-up for PDD data table
- PDD dependencies Ans✓✓✓ 100 SSD, 10x10 f/s, 6MV, in water
phantom
dependent upon energy, field size, depth, SSD:
PDD will ↑ with ↑E, ↑SSD, ↑f/s, and ↓depth
- phantom set-up for TMR data table
- TMR dependencies Ans✓✓✓ 100 SAD, 10x10 f/s, 6MV, in water
phantom
dependent upon energy, field size, depth (independent of SSD): TMR
will ↑ with ↑E, ↑f/s, ↓depth
(4) Personnel Monitoring and how they work Ans✓✓✓ 1. film badges:
consists of film between metal filters. metal filters are used to
determine the quality of the radiation absorbed, and the film is read
using optical density proportional to the dose received.
2. TLD's: thermoluminscent dosimeter; uses lithium fluoride (LiF) crystal
that stores energy by trapping electrons, which can be read by heating
the crystals to emit light photons.
,3. OSL's: optically stimulating luminescence; uses aluminum oxide with
carbon that stores energy in the crystal, measured by shining a green
laser at crystal and measure intensity of blue light.
4. pocket ionization chambers: used by charging the device, and looking
through the eyepiece to note amount of charge lost upon irradiation.
5 R's of Radiobiology Ans✓✓✓ repair
reassortment
reoxygenation
repopulation
radiosensitivity
accelerated fractionation schedule Ans✓✓✓ 150-170 cGy/fx, 2 fx/day,
5-7 days/wk
pro: short overall treatment time, better control of rapidly growing
cancers, no significant increase in late effects
con: early responding normal tissue damage
accelerated hyperfractionation schedule Ans✓✓✓ 100-110 cGy/fx, 3
fx/day, 5-7 days/wk
pro: good local tumor control
con: increase in both early and late effects
alpha particle shielding Ans✓✓✓ low Z materials
,ATTENUATION:
1. Intensity (monoenergetic beams)
2. HVL
3. µ
4. mass attenuation Ans✓✓✓ 1. I(x) = I₀ · exp(-µx)
2. HVL = half value layer = ln(2)÷µ
3. µ = attenuation coefficient = (% reduction in intensity)÷(unit
thickness absorber)
4. mass attenuation = µ/ρ where ρ=density=mass/volume
autoradiography of encapsulated brachytherapy source Ans✓✓✓ a
test for gross nonuniformity of the radionuclide within a source capsule
BED concept Ans✓✓✓ BED = biological equivalent dose, the quantity
at which different fractionation schedules can be compared by
extrapolating tolerance dose in an infinite number of tiny factions or a
very low dose rate to cause same cell kill as a given schedule with
complete sublethal damage repair.
BED = E/α = nd(1 + d÷α/β) Gy, where
n = # fractions, d = dose/fx, nd = total dose (physical), and (1 + d÷α/β) =
relative effectiveness
, brachytherapy exposure rate and dose rate formulas Ans✓✓✓
exposure rate = Γ · A/d², where
A = activity, d = distance from the source
dose rate = Sk · ∧ · G(r,θ) · F(r) · g(θ), where
Sk = air kerma strength
∧ = dose-rate constant
G(r,θ) = geometry factor
F(r) = anisotropy factor
g(θ) = radial dose function
Brachytherapy Radiation Safety for Permanent Implants Ans✓✓✓ -
high activity patients (30mCi or greater) must be hospitalized until
decay levels decrease to <30mCi or 10 mR/hr.
- dose rate outside patient room needs to be ≤2 mrem/hr.
- expensive, requires trained staff to care for patient, and a shielded
room
bragg-gray cavity theory Ans✓✓✓ the interaction of photons with
some material with air inside, like a cavity, has a relationship that can
be made between the absorbed dose to the wall material and the air
cavity, as used in TG-21.
Planning, QA, Mechanics, Physics, Radiobiology, Safety
questions with verified answers
- phantom set-up for PDD data table
- PDD dependencies Ans✓✓✓ 100 SSD, 10x10 f/s, 6MV, in water
phantom
dependent upon energy, field size, depth, SSD:
PDD will ↑ with ↑E, ↑SSD, ↑f/s, and ↓depth
- phantom set-up for TMR data table
- TMR dependencies Ans✓✓✓ 100 SAD, 10x10 f/s, 6MV, in water
phantom
dependent upon energy, field size, depth (independent of SSD): TMR
will ↑ with ↑E, ↑f/s, ↓depth
(4) Personnel Monitoring and how they work Ans✓✓✓ 1. film badges:
consists of film between metal filters. metal filters are used to
determine the quality of the radiation absorbed, and the film is read
using optical density proportional to the dose received.
2. TLD's: thermoluminscent dosimeter; uses lithium fluoride (LiF) crystal
that stores energy by trapping electrons, which can be read by heating
the crystals to emit light photons.
,3. OSL's: optically stimulating luminescence; uses aluminum oxide with
carbon that stores energy in the crystal, measured by shining a green
laser at crystal and measure intensity of blue light.
4. pocket ionization chambers: used by charging the device, and looking
through the eyepiece to note amount of charge lost upon irradiation.
5 R's of Radiobiology Ans✓✓✓ repair
reassortment
reoxygenation
repopulation
radiosensitivity
accelerated fractionation schedule Ans✓✓✓ 150-170 cGy/fx, 2 fx/day,
5-7 days/wk
pro: short overall treatment time, better control of rapidly growing
cancers, no significant increase in late effects
con: early responding normal tissue damage
accelerated hyperfractionation schedule Ans✓✓✓ 100-110 cGy/fx, 3
fx/day, 5-7 days/wk
pro: good local tumor control
con: increase in both early and late effects
alpha particle shielding Ans✓✓✓ low Z materials
,ATTENUATION:
1. Intensity (monoenergetic beams)
2. HVL
3. µ
4. mass attenuation Ans✓✓✓ 1. I(x) = I₀ · exp(-µx)
2. HVL = half value layer = ln(2)÷µ
3. µ = attenuation coefficient = (% reduction in intensity)÷(unit
thickness absorber)
4. mass attenuation = µ/ρ where ρ=density=mass/volume
autoradiography of encapsulated brachytherapy source Ans✓✓✓ a
test for gross nonuniformity of the radionuclide within a source capsule
BED concept Ans✓✓✓ BED = biological equivalent dose, the quantity
at which different fractionation schedules can be compared by
extrapolating tolerance dose in an infinite number of tiny factions or a
very low dose rate to cause same cell kill as a given schedule with
complete sublethal damage repair.
BED = E/α = nd(1 + d÷α/β) Gy, where
n = # fractions, d = dose/fx, nd = total dose (physical), and (1 + d÷α/β) =
relative effectiveness
, brachytherapy exposure rate and dose rate formulas Ans✓✓✓
exposure rate = Γ · A/d², where
A = activity, d = distance from the source
dose rate = Sk · ∧ · G(r,θ) · F(r) · g(θ), where
Sk = air kerma strength
∧ = dose-rate constant
G(r,θ) = geometry factor
F(r) = anisotropy factor
g(θ) = radial dose function
Brachytherapy Radiation Safety for Permanent Implants Ans✓✓✓ -
high activity patients (30mCi or greater) must be hospitalized until
decay levels decrease to <30mCi or 10 mR/hr.
- dose rate outside patient room needs to be ≤2 mrem/hr.
- expensive, requires trained staff to care for patient, and a shielded
room
bragg-gray cavity theory Ans✓✓✓ the interaction of photons with
some material with air inside, like a cavity, has a relationship that can
be made between the absorbed dose to the wall material and the air
cavity, as used in TG-21.
