MDCB KEY EXAM QUESTIONS AND CORRECT
ANSWERS
Hounsfield Units - CORRECT ANSWERS 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 ANSWERS 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 - CORRECT ANSWERS 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 ANSWERS D1
= D2 x (TMR1/TMR2) x (SAD2/SAD1)²
Definition of a Wedge Angle - CORRECT ANSWERS 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 ANSWERS 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 - CORRECT ANSWERS - 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 ANSWERS - 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 - CORRECT ANSWERS 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 ANSWERS Physical
Penumbra of MLCs are wider than both jaws and cerrobend blocks
CNS Planning: Cranial Field Orientation (Formula, given Spine Field Settings) - CORRECT
ANSWERS 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 - CORRECT ANSWERS 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
ANSWERS
Hounsfield Units - CORRECT ANSWERS 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 ANSWERS 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 - CORRECT ANSWERS 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 ANSWERS D1
= D2 x (TMR1/TMR2) x (SAD2/SAD1)²
Definition of a Wedge Angle - CORRECT ANSWERS 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 ANSWERS 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 - CORRECT ANSWERS - 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 ANSWERS - 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 - CORRECT ANSWERS 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 ANSWERS Physical
Penumbra of MLCs are wider than both jaws and cerrobend blocks
CNS Planning: Cranial Field Orientation (Formula, given Spine Field Settings) - CORRECT
ANSWERS 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 - CORRECT ANSWERS 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
