MDCB Concepts Exam Questions and
Answers Graded A+
Hounsfield Units - Correct 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 - Correct answer-output factor
scatter factors (Sc, Sp, Scp)
inverse square factor
depth dose factor (PDD, TMR)
off-axis factor
transmission factor (tray, wedge)
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,MU = Rx Dose ÷ Final Output
Using Inverse Square Law - Correct 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 - Correct
answer-D1 = D2 x (TMR1/TMR2) x (SAD2/SAD1)²
Definition of a Wedge Angle - Correct 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 - Correct 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....
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,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 - Correct 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 - Correct 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
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, - pneumonitis (lungs) are the dose limiting factor
- dose validations are made using diodes, TLDs
Tissue Compensators - Correct 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 - Correct answer-Physical
Penumbra of MLCs are wider than both jaws and cerrobend blocks
CNS Planning: Cranial Field Orientation (Formula, given Spine Field Settings) -
Correct answer-θ(collimator) = arc tan (½ L₁/SSD)
θ(couch) = arc tan (½ L₂/SSD)
where
SSD is with regard to Spine Field
©COPYRIGHT 2025, ALL RIGHTS RESERVED 4
Answers Graded A+
Hounsfield Units - Correct 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 - Correct answer-output factor
scatter factors (Sc, Sp, Scp)
inverse square factor
depth dose factor (PDD, TMR)
off-axis factor
transmission factor (tray, wedge)
©COPYRIGHT 2025, ALL RIGHTS RESERVED 1
,MU = Rx Dose ÷ Final Output
Using Inverse Square Law - Correct 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 - Correct
answer-D1 = D2 x (TMR1/TMR2) x (SAD2/SAD1)²
Definition of a Wedge Angle - Correct 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 - Correct 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....
©COPYRIGHT 2025, ALL RIGHTS RESERVED 2
,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 - Correct 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 - Correct 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
©COPYRIGHT 2025, ALL RIGHTS RESERVED 3
, - pneumonitis (lungs) are the dose limiting factor
- dose validations are made using diodes, TLDs
Tissue Compensators - Correct 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 - Correct answer-Physical
Penumbra of MLCs are wider than both jaws and cerrobend blocks
CNS Planning: Cranial Field Orientation (Formula, given Spine Field Settings) -
Correct answer-θ(collimator) = arc tan (½ L₁/SSD)
θ(couch) = arc tan (½ L₂/SSD)
where
SSD is with regard to Spine Field
©COPYRIGHT 2025, ALL RIGHTS RESERVED 4
