C. dose calculation Flashcards
The beam energy that is used for treatment is determined by the
depth of the target volume, minimum target dose, and acceptable dose to organs at risk in path of beam
For dosimetric calculations, all rectangular treatment fields must be converted to
squares
What is equivalent square equation?
EQSQ= (2ab) / (a+B) or ( 4 x area)/ perimeter
_____fields are fields that use MLCs or blocks to change the shape of the treatment field
Blocked
A simpler approach is to take the square root of the open area minus the
blocked areaExample: the open area is 20 x 10 and the blocked area is 4x 6 First do the equation (20 x 10) — (4 x 6) Next, take the square root of the above answerClarkson integration can be used to find the area of irregularly shaped fields
The main cause of collimator scatter (sc) is from photons scattering within the
head of the gantry
Collimator scatter (Sc) Mostly occurs with the
collimating jaws and the flattening filter
Bigger field sizes creates ___ scatter
more
Smaller field size creates ___ scatter
less
Scatter that happens within the patient or phantom is called
phantom scatter (Sp)
phantom scatter (Sp) amount depends on the
volume of tissue within the treatment field
As the field size increases, the amount of phantom scatter ___
increases
Backscatter factor (BSF) and peak scatter factor (PSF) both compare the dose rate in free space to the dose rate within
a phantom at the dmax
Backscatter factor (BSF) is used for ___ beams
low-energy beams
Peak scatter factor (PSF) is used for ____ beams
high-energy beams
____ is the depth where electronic equilibrium is reached and 100 percent of the dose is delivered
dmax
The dmax for higher-energy beams is
deeper
What is dmax for 4MV beam
1 cm
What is dmax for 6 MV beam
1.5cm
What is dmax for 10MV beam
2.5 cm
What is dmax for 15MV beam
3 cm
What is dmax for 18MV beam
3.2cm
What is dmax for 20MV beam
3.5 cm
What is dmax for 25MV beam
5 cm
What is dmax for 6MeV beam
1.5cm
What is dmax for 9MeV beam
2.2cm
What is dmax for 12MeV beam
2.8cm
What is dmax for co-60
0.5cm
_____ is A ratio that compares the absorbed dose at a depth to the absorbed dose at a specific reference depth (dmax)
PDD= percent depth dose
What is equation for PDD?
[(Absorbed dose at depth) ÷ (absorbed dose at dmax)] x 100
PDD i only used to calculate MU in
SSD treatments
PDD changes depending on four components
energy, depth, field size, and SSD
An increase of ____ causes an increase of PDD because there is more scatter, which contributes to the dose
field size
As depth within the patient goes past Dmax then PDD
decreases
PDD and depth have a
inverse relationship
To determine the increase in PDD when there is a change in SSD, use
Mayneord F Factor
As field size increases PDD will
increase
As SSD increases PDD will
increase
As energy increases PDD will
increase
As Depth increases PDD will
decrease
_______ is the ratio of an absorbed dose at a depth within the phantom at a given distance compared with the absorbed dose in free space at the same distance
Tissue air ratio (TAR)
TAR is ____ of SSD
independent
TAR is dependent on what?
energy, field size, depth
As the energy increases, the beam becomes more penetrating and TAR ______
increases
_____is the ratio of an absorbed dose at a specified depth in a Phantom to the absorbed dose at dmax at the same distance from the radiation source
Tissue maximum ratio (TMR)
TMR is used to measure dose of
high energy beams
the value of TMR is usually
less than 1
SSD technique is when the ____is located on the skin surface
isocenter
SAD, or isocentric, technique is when the isocenter is located within the
target volume
____ Describes the correlation between the distance from the source of radiation and the intensity of the beam
Inverse square
The _____of the radiation beam is inversely proportional to the square of the distance from the source of radiation
intensity
What is the inverse square equation?
I1 / I2 = (D2 / D1 )^2
When the distance is increased, the same amount of radiation spreads to a larger area and the intensity is
reduced
if the distance is increased by two, the intensity of the beam is decreased by
one fourth of the original amount
Extended distances have SSDs that are greater than the
isocenter (100 cm)
____ are necessary when larger field sizes are required
Extended distances
_____change the shape of the isodose curve and dose distribution within patient
Wedges
Isodose curves are angled due to the unequal ____
absorption of a wedge
The “heel” of the wedge absorbs more of the beam than the
toe” of the wedge
The heel brings isodose curves up closer to the
skin surface
____wedge angles create a steeper isodose curve
Higher
Wedge angle formula
90-(hinge angle/2)
The angle of the wedge is not determined by the angle of the actual physical wedge, but by the angle between an isodose curve and a line perpendicular to the
central axis or at a specific depth
Field sizes cannot be larger than the
wedge itself
______ is added to the calculation of monitor units when a wedge is used
Wedge transmission factor, or wedge factor
______“harden” the beam, meaning the low-energy photons are absorbed by the wedge, so a lower number of photons are available for dose
Wedges
______ Calculate the dose rate at a point in the field away from the central axis
Off-axis calculations
A change in dose rate is expected toward the field edge due to the
penumbra region
___ can be used when the central axis is blocked by MLCs, blocks, or jaws for half-beam blocks
OARS
_____are a demonstration of the dose distribution within a field in directions parallel and perpendicular to the direction of the radiation beam
Isodose curves
Factors that can change the _____ are beam energy, beam type, SOUrce.to. surface distance, field weighting, beam modifiers, and field size
isodose distribution
A higher-energy beam’s isodose curves are more spread out than lower-energy beams because the higher-energy beams are
more penetrating
_____ can change the shape of the isodose curves
Wedges and patient contours
_____ show the doses that normal tissue and tumor will receive during the treatment
Dose-volume histograms (DVH)
When ____are needed for treatment, a tray made of some kind of plastic is used.
blocks
The tray attenuates some of the beam when used with blocks. Usually less than
5 percent
The _______, is a ratio of a dose with the tray in Place to the dose without the tray and js found by a physicist
tray factor (TF), or tray transmission factor
- Bring dose closer to surface - will attenuate some of the beam
bolus
Wedges and compensators are used to alter the isodose distribution based on the
patient’s skin contour
The transmission factor shows how much Of the beam is transmitted through the
wedge or compensator
The transmission factor will change with changing beam
energies
____ can change with tissue inhomogeneity
Dose distribution
Tissue inhomogeneities may cause _____that can affect dose to other organs nearby
scatter
What is lung correction factor
0.25 - 0.33 g/cm3
What is bone correction factor?
1.8 g/cm3
What is water correction factor?
1.0 g/cm3
Correction factors are applied to dose calculations to adjust for the
dose distribution in different tissues
The ___factor compares the dose rate in a known field size to the dose rate in a reference field size
output
The output factor for the reference field size of 10 x 10 cm is
1
