Test 1 Flashcards
Absorbed dose at depth as a percent of a dose at Dmax on central axis (CA) of the beam; percent of beam that’s left
What percent of dose occurs at certain depth
Percent depth dose (PDD)
PDD formula
PDD = Dd/Dmax or TD/GD or Rx/max dose
Dd = dose at depth TD = tumor dose GD = given dose Rx = prescription
Max dose occurs at Dmax (electronic equilibrium)
Given dose
Doses made at __________ because of flattening filter (lateral horns before ___ cm, forward peaked after ___ cm)
Central axis, 10cm
TD formula
TD = PDD(GD)
Finding an unknown data point between two known points
Interpolation
Increase beam energy = _______ dose at depth, less attenuation/________ PDD
Increase
Increase calculation depth (go through more tissue) = ________ PDD because of more tissue attenuation
Decrease
Increase field size = _______ time/MUs
Decrease
Increase SSD = __________ time
Increase
Scatter of a square field to scatter of a rectangular field
Equivalent square
What is the advantage of an isocentric technique?
Don’t have to set up patient every time
Mayneord factor (MF) formula
MF = (((New SSD + Dmax)^2)/((new SSD + depth)^2))/(((old SSD + Dmax)^2)/((old SSD + depth)^2))
New PDD formula
New PDD = old PDD x MF
POI
Point of interest = TD
Dmax ________ with increasing FS because more scatter and less penetrating beam; increase electron contamination that occurs when the collimator is open wider
Decreases
SSD MUs formula
MU = GD/(Dfs x PSF)
Dfs = collimator field size PSF = effective field size
Amount of time it takes to deliver 1 cGy to Dmax for 10x10 FS, 100 cm away
Monitor unit (MU)
Dependent on collimator size/scatter
Collimator field size
Dfs Collimator scatter (Sc)
Increase collimator size = ________ scatter = ________ time to deliver dose
Increase, decrease
MUs ______ with increased energy because of more penetrating beam
Decrease
Scatter on patient; SSD at surface, SAD at depth
Enhancement in dose going from “free space” to in phantom
Compares primary and total radiation
TAR at Dmax because most scatter occurs at Dmax
Peak scatter factor (PSF)
Effective field size
Scatter factor for low energy x-rays
Back scatter factor (BSF)
Area that’s coldest because it’s where the beam is attenuated the most
Prescription (Rx)
______ of wedge at thinner part of breast; this decreases apical dose and pushes dose to thicker part
Heel
Dose highest at ______ (about ___ cm deep) because of buildup and at apex of breast because it’s thinner
Surface, 3 cm
SSD setup depends on ______, SAD ________
PDD, independent
Ratio of dose in air versus dose in tissue after it’s been attenuated, more convenient for SAD treatments because it’s independent of SSD
Tissue-air ratio (TAR)
SAD MU equation
MU = TD/(Dfs x INV^2 x TAR)
SAD POI equation
POI = MU x Dfs x INV^2 x TARpoi
Output equation
Dfs x INV^2
Increase scatter = _______ time; increase energy = _________ time/MU
Decrease
Increase PSF = ________ treatment time
Decrease
Highest PSF
1.5 for large fields at orthovoltage energies (150-500 kV)
Increase SSD = ______ in TAR because they’re at same distance
No change
Decrease FS = ________ MU
Increase MU (less scatter so machine has to be on longer)
2 ways dose is lost
Inverse square law
Attenuation
Increase beam energy = _________ PDD/percent transmitted
Increase
Increase SSD = __________ PDD
Increase
Increase field size = __________ PSF because increasing field size increases scatter
Increase
Increase energy = ________ PSF
Decrease
Corrects for old and new SSD
Ratio of two PDDs
Mayneord factor (MF)
Ratio of the dose at a given point in phantom to the dose at the same point at a fixed reference depth, independent of SSD
Tissue-phantom ratio (TPR)
MUs and dose are _________ proportional
Directly
Typical breast bridge diameter
20-30 cm
Percent hotspot formula
GD/TD
Equivalent square formula
4A/P
A = area P = perimeter
Dmax of a Co-60, 4X, 6X, 10X, 15X and 18X beam
Co-60 = 0.5 cm 4X = 1 cm 6X = 1.5 cm 10X = 2.5 cm 15 X = 3 cm 18X = 3 cm
7 things dose distributions vary with (beam characteristics and patient)
Beam energy Calculation depth Field size Beam modifiers (wedges, MLCs) Distance from source Patient contour Tissue inhomogeneities (lung, bone, water, etc.)
2 contributions/components of beam
Primary
Scatter
Radiation that comes directly from the source without being scattered and usually contributes most dose
Primary radiation
Dose from photons that deposit energy after having undergone one or more scattering events; photons that have only done this once are most prominent
Difficult to directly compute to dose
Scatter radiation
Scatter ________ with increased volume of material irradiation (large field sizes); more matter is irradiated as field size increases, and therefore more radiation can be scattered to the observation point
Increases
Most accurate and best algorithm but takes a lot of computational power
Monte Carlo
When field size becomes very large, radiation scattered from central axis will be attenuated before it can reach central axis; in this case a further increase in field size will result in a __________ increase in dose
Negligible
Homogenous phantom that is tissue equivalent
Water
Dose hits surface and begins to rise to reach Dmax
Buildup region
Depth where dose reaches maximum value; scatter in equal to scatter out
Dmax
Electronic equilibrium
Radiation begins to attenuate and decrease as depth increases
Fall-off
Distance of DDs added to distance from source
Dose at depth (Dd)
Increase field size = _________ PDD because there’s more volume irradiated which contributes more scatter to the measurement point on central axis
Increase
