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According to TG-50, what is the maximum allowable MLC transmission if the MLC is to replace an upper or lower jaw?
0.5%, same as a standard collimator
According to TG-50, what is the maximum allowable MLC transmission if the MLC is to be a tertiary collimator?
5%
A dose of radiation X is achieved at distance d1 from a point source. What is the dose at distance d2?
X2 = X1(d1/d2)^2
What three factors go into producing a PDD curve?
Inverse Square Law, Attenuation, and Buildup
As SSD increases, what happens to PDD beyond dmax?
PDD increases with increasing SSD
As field size increases, what happens to PDD?
PDD increases with increasing field size due to increased scatter, both before and after dmax.
Produce the formula for equivalent square of an irregular field
a = 4*(A/P)

  • a = side of a square field
  • A = area of irregular field
  • P = perimeter of irregular field 
Where is flatness generally defined, and what is the tolerance?
80% of the field size at a depth of 10cm. Recommended tolerances vary but are generally around +/- 2.5-3%
Where is symmetry generally defined, and what is the tolerance?
80% of the field size at 10cm depth, compared to the point an equal distance from midline in the opposite direction. Recommended tolerances vary but are generally around +/- 2-3%
As energy increases in a megavoltage beam, what happens to:
  • The depth of dmax
  • Skin sparing
  • Hot spots
  • Uniformity 
  • Increases
  • Increases
  • Decreases
  • Increases
As energy increases, does heterogeneity become more or less important?
Less.
What is the rule of thumb for comparing the density of lung tissue to water?
ρlung ~ 0.33*ρh20 = 0.33g/cm3
What is the rule of thumb for comparing the density of bone tissue to water?
ρbone ~ 1.8*ρh20 = 1.8g/cm3
What is the rule of thumb for comparing the density of air to water?
ρair ~ 0.0012*ρh20 = 0.0012g/cm3
How do you determine the correction time for shutter effect (aka end effect)?
τ = [(Rn-R1)t]/(nR1-Rn)

  • τ = correction time
  • R1 = Measurement taken with sufficiently large exposure time
  • Rn = Measurement taken from a series of exposures of equal time, summing to an equivalent itme as R1
  • t = total exposure time
Define and compute a Field Size Correction Factor
FSCF = Stot = Sc*Sp

  • Stot = total scatter factor
  • Sc = Collimator scatter factor
  • Sp = Phantom scatter factor
Where is the wedge angle defined?
The angle through with an isodose curve croses the central axis at a depth of 10cm.
What is the relationship between wedge angle and hinge angle? Define both.
θ = 90° - Φ/2

  • θ = wedge angle
  • Φ = hinge angle
Define the Wedge Transmission Factor
The ratio of doses with and without a wedge along central axis at any depth beyond dmax.
How does a universal wedge work, and how does it differ from a dynamic wedge?
A single 60° wedge is housed in the gantry head, and used in conjunction with open fields. The ratio of open to wedged fields alters the overall beam profile angle.
List the four components of cerrobend.
Bismuth, lead, tin, and cadmium.
What is the approximate HVL for cerrobend for both 6MV and 18MV photon beams?
1.5cm for 6MV
1.8-2.0cm for 18MV
Compare the density of cerrobend to that of lead.
~80%, ρcerro = 9.4g/cm3, ρPb = 11.34g/cm3
What five maladies can be treated with TBI?
  • Multiple myelomas
  • Limited immunity deficiencies
  • Limited genetic disorders (e.g. diamond blackfan anemia)
  • Bone palliation
  • Leukemia
What does MIP stand for?
Maximum Intensity Projection
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