MDCB CONCEPTS-MEDICAL
DOSIMETRY CERTIFICATION BOARD
EXAM REVISION: PLANNING, QA,
MECHANICS, PHYSICS,
RADIOBIOLOGY, SAFETY
Hounsfield Units - ANSWER-A measure of the beam attenuation capability of a specific
structure. Also called pixel values, density numbers or CT numbers.
HU = 1000 · [µ(tissue) - µ(water)] ÷ µ(water)
bone ∼ 1000
tissue ∼ 0
air ∼ -1000
Factors in Calculating Monitor Units - ANSWER-output factor
scatter factors (Sc, Sp, Scp)
inverse square factor
depth dose factor (PDD, TMR)
off-axis factor
transmission factor (tray, wedge)
MU = Rx Dose ÷ Final Output
Using Inverse Square Law - ANSWER-Relationship between intensity and the distance
from the source: d₁² x I₁ = d₂² x I₂
ex: Output at 1cm = 160 cGy
Output at 100cm = ?
(1cm)²(160cGy) = (100cm)²I₂
I₂ = 0.016cGy
SAD Set Up: Finding Dose at 2nd point of calculation using TMRs - ANSWER-D1 = D2
x (TMR1/TMR2) x (SAD2/SAD1)²
Definition of a Wedge Angle - ANSWER-the angle between an isodose curve at
specified depth and a line perpendicular to the CAX at the same specified depth
Dose Computation Algorithms - ANSWER-Correction-Based: based on measured data
in a phantom and applied correction factors (attenuation, scatter, f/s, shape, geometry,
inverse square law, etc)
Model-Based: based on physical model simluating actual transport of scattered photons
and electrons: ie. Convolution-Superposition and Monte Carlo....
,Convolution-Superposition: simulates primary photon track and calculates separately
scatter photons/electrons
Monte Carlo: simulates transport of millions of photons and particles through matter
using fundamental laws of physics (requiring long computation time and power)
Proton Radiation Therapy - ANSWER-- High LET means minimal dose dumping along
path until the velocity decreases enough to dump the rest of the dose in a small area
(narrow bragg peak). Brag peak can be spread out (SOBP) across target volume by
adjusting beam energy.
- protons increase cell kill over xrays and has superior dose distribution and critical
structure sparing.
- protons produced in cyclotron.
- stopping power is inversely proportional to velocity squared.
TBI Treatment - ANSWER-- used to condition the body for bone marrow
transplantations for diseases like leukemia.
- use photons to achieve ±10% uniformity of dose using spoiler and TBI filters,
prescribed to midplane at the umbilicus.
- 6 fractions BID is per usual
- pneumonitis (lungs) are the dose limiting factor
- dose validations are made using diodes, TLDs
Tissue Compensators - ANSWER-sometimes surface irregularities gives rise to
unacceptable non-uniformity in target or excessive dose in critical structures, requiring
the use of tissue compensators like:
- wedges
- multiple fields
- bolus: to even irregular patient contours
- compensators: preserves skin sparing effect whereas bolus does not
- blocked for some fractions
Physical Penumbra of MLCs vs Cerrobend vs Jaws - ANSWER-Physical Penumbra of
MLCs are wider than both jaws and cerrobend blocks
CNS Planning: Cranial Field Orientation (Formula, given Spine Field Settings) -
ANSWER-θ(collimator) = arc tan (½ L₁/SSD)
θ(couch) = arc tan (½ L₂/SSD)
where
SSD is with regard to Spine Field
L₁ is length of PA Spine Field
L₂ is length of Cranial Field (sup to inf)
TAR, TPR, TMR, SAR - ANSWER-TAR = tissue air ratio = D(d)/D(air)
TPR = tissue phantom ratio = D(d)/D(ref)
TMR = tissue maximum ratio = D(d)/D(dmax)
SAR = scatter air ratio = TAR(d,r) - TAR(d,0) where r is field size
, (4) Personnel Monitoring and how they work - ANSWER-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.
Pinnacle Acronyms: ODM, SVD, TPB - ANSWER-ODM: opening density matrix - use
for very small fields or machines with MLCs <5mm width.
TPB: Todd's/Tradiational Pencil Beam - a method of beam optimization
SVD: single value decomposition - a method of beam optimization similar but superior
to TPB
V vs eV - ANSWER-V: polyenergetic energy spectrum
eV: monoenergetic energy
Components of a Linac - ANSWER-magnetron: microwave generator
klystron: microwave amplifier
modulator: modulates high frequency pulses
circulator: prevents reflected microwaves from reaching the magnetron
power supply: powers modulator (direct current)
waveguide: conducts microwaves to electron gun
electron gun: source of electrons
accelerator tube: accelerates electrons to treatment head using high voltages
bending magnet: creates homogeneous energy electrons
target: produce x-rays (bremstrahlung)
flattening filter: flattens x-ray beams for uniformity
scattering foil: scatters electron beams for uniformity
ion chamber: verifies beam output and flatness
collimation/jaws: shapes beam
Retrospective vs Prospective Planning - ANSWER-Retrospective: forward planning
Prospective: inverse/IMRT planning
DICOM RT Objects (5) - ANSWER-- RT Image: DRRs, portals, CT/sim
- RT Plan (RTP): geometric and dosimetric data - linked to structure
- RT Structure set (RTSS): anatomy with respect to isocenters and markers
- RT Dose: dose distribution, DVH
- RT Tx Record: data from treatment sessions
DOSIMETRY CERTIFICATION BOARD
EXAM REVISION: PLANNING, QA,
MECHANICS, PHYSICS,
RADIOBIOLOGY, SAFETY
Hounsfield Units - ANSWER-A measure of the beam attenuation capability of a specific
structure. Also called pixel values, density numbers or CT numbers.
HU = 1000 · [µ(tissue) - µ(water)] ÷ µ(water)
bone ∼ 1000
tissue ∼ 0
air ∼ -1000
Factors in Calculating Monitor Units - ANSWER-output factor
scatter factors (Sc, Sp, Scp)
inverse square factor
depth dose factor (PDD, TMR)
off-axis factor
transmission factor (tray, wedge)
MU = Rx Dose ÷ Final Output
Using Inverse Square Law - ANSWER-Relationship between intensity and the distance
from the source: d₁² x I₁ = d₂² x I₂
ex: Output at 1cm = 160 cGy
Output at 100cm = ?
(1cm)²(160cGy) = (100cm)²I₂
I₂ = 0.016cGy
SAD Set Up: Finding Dose at 2nd point of calculation using TMRs - ANSWER-D1 = D2
x (TMR1/TMR2) x (SAD2/SAD1)²
Definition of a Wedge Angle - ANSWER-the angle between an isodose curve at
specified depth and a line perpendicular to the CAX at the same specified depth
Dose Computation Algorithms - ANSWER-Correction-Based: based on measured data
in a phantom and applied correction factors (attenuation, scatter, f/s, shape, geometry,
inverse square law, etc)
Model-Based: based on physical model simluating actual transport of scattered photons
and electrons: ie. Convolution-Superposition and Monte Carlo....
,Convolution-Superposition: simulates primary photon track and calculates separately
scatter photons/electrons
Monte Carlo: simulates transport of millions of photons and particles through matter
using fundamental laws of physics (requiring long computation time and power)
Proton Radiation Therapy - ANSWER-- High LET means minimal dose dumping along
path until the velocity decreases enough to dump the rest of the dose in a small area
(narrow bragg peak). Brag peak can be spread out (SOBP) across target volume by
adjusting beam energy.
- protons increase cell kill over xrays and has superior dose distribution and critical
structure sparing.
- protons produced in cyclotron.
- stopping power is inversely proportional to velocity squared.
TBI Treatment - ANSWER-- used to condition the body for bone marrow
transplantations for diseases like leukemia.
- use photons to achieve ±10% uniformity of dose using spoiler and TBI filters,
prescribed to midplane at the umbilicus.
- 6 fractions BID is per usual
- pneumonitis (lungs) are the dose limiting factor
- dose validations are made using diodes, TLDs
Tissue Compensators - ANSWER-sometimes surface irregularities gives rise to
unacceptable non-uniformity in target or excessive dose in critical structures, requiring
the use of tissue compensators like:
- wedges
- multiple fields
- bolus: to even irregular patient contours
- compensators: preserves skin sparing effect whereas bolus does not
- blocked for some fractions
Physical Penumbra of MLCs vs Cerrobend vs Jaws - ANSWER-Physical Penumbra of
MLCs are wider than both jaws and cerrobend blocks
CNS Planning: Cranial Field Orientation (Formula, given Spine Field Settings) -
ANSWER-θ(collimator) = arc tan (½ L₁/SSD)
θ(couch) = arc tan (½ L₂/SSD)
where
SSD is with regard to Spine Field
L₁ is length of PA Spine Field
L₂ is length of Cranial Field (sup to inf)
TAR, TPR, TMR, SAR - ANSWER-TAR = tissue air ratio = D(d)/D(air)
TPR = tissue phantom ratio = D(d)/D(ref)
TMR = tissue maximum ratio = D(d)/D(dmax)
SAR = scatter air ratio = TAR(d,r) - TAR(d,0) where r is field size
, (4) Personnel Monitoring and how they work - ANSWER-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.
Pinnacle Acronyms: ODM, SVD, TPB - ANSWER-ODM: opening density matrix - use
for very small fields or machines with MLCs <5mm width.
TPB: Todd's/Tradiational Pencil Beam - a method of beam optimization
SVD: single value decomposition - a method of beam optimization similar but superior
to TPB
V vs eV - ANSWER-V: polyenergetic energy spectrum
eV: monoenergetic energy
Components of a Linac - ANSWER-magnetron: microwave generator
klystron: microwave amplifier
modulator: modulates high frequency pulses
circulator: prevents reflected microwaves from reaching the magnetron
power supply: powers modulator (direct current)
waveguide: conducts microwaves to electron gun
electron gun: source of electrons
accelerator tube: accelerates electrons to treatment head using high voltages
bending magnet: creates homogeneous energy electrons
target: produce x-rays (bremstrahlung)
flattening filter: flattens x-ray beams for uniformity
scattering foil: scatters electron beams for uniformity
ion chamber: verifies beam output and flatness
collimation/jaws: shapes beam
Retrospective vs Prospective Planning - ANSWER-Retrospective: forward planning
Prospective: inverse/IMRT planning
DICOM RT Objects (5) - ANSWER-- RT Image: DRRs, portals, CT/sim
- RT Plan (RTP): geometric and dosimetric data - linked to structure
- RT Structure set (RTSS): anatomy with respect to isocenters and markers
- RT Dose: dose distribution, DVH
- RT Tx Record: data from treatment sessions
