2 Ch 12 Photon Beam Dosimetry

other terms for incident dose

dose to Dmax; given dose

ID

incident dose, aka dose to Dmax or given dose

incident dose (aka dose to Dmax or given dose) depends on ___ and ___

beam energy and field size

the primary photon beam also has some low energy ___ and ___ produced by the beam hitting the ___ which adds to dose

electrons and photons; collimators/filters/etc.

larger field sizes produce more or less scatter?

more

why does Dmax change a little bit with larger field sizes? how does the effect change at lower energies?

larger fields produce more scatter. scatter consists of lower energy photons and electrons that deposit their energy on the skin, which skews the ratio and effectively decreases the depth of Dmax.

less of a problem at lower energies

SAD

source to axis distance

D

dose at depth

CR

central ray

F.s.

field size

in higher energy beams, what is greater, the surface dose or the exist dose?

exit dose

a dose increment (usually daily)

fraction

number of fractions and time sequence in which the total dose is given

fractionation schedule

when 2 smaller fractions are given twice per day over a normal time frame, creating a higher total dose (each dose is a little more than just dividing the daily dose in half)

hyperfractionation

when 2 normal fractions are given twice per day, for the same total dose but protraction cut in half

accelerated fractionation

when the total dose is given over a shorter time frame (ex. Canadian breast study)

hypofractionation - i.e. lots of breast is now 5 or 16 fractions

BSF

backscatter factor

OF

output factor

DD or PDD

depth dose, or percentage depth dose

TAR

tissue-air ratio

TMR

tissue-maximum ratio

examples of equipment attenuation factors

block trays, table, pads

examples of patient attenuation factors

DD or PDD

TAR

TMR

radiation that is scattered back by the patient (or phantom)

backscatter factor (BSF)

BSF =

BSF = dose at Dmax in phantom / dose without backscatter (dose in air)

top number is always higher, because dose to Dmax (aka incident dose) is higher because of scatter, meaning BSF is always over 1

backscatter factor (BSF) is always over ___, up to about ___ for larger field sizes

1.0; 1.5

why is BSF not used in linac treatment calculations?

it’s highest for orthovoltage x-rays. less important for linacs with higher energy.

BSF varies with ___

beam energy

at lower energies, ___ and ___ don’t contribute much to surface dose (types of photon interactions)

coherent scatter and photoelectric effect

as beam energy increases, ____ (type of photon interaction) predominates at the skin surface. therefore, backscatter increases with energy up to a point.

Compton effect

scatter becomes more ___ as energy increases, and more ___ as field size increases

forward; prevalent

above 2 MeV, virtually all scatter is in the ___ direction and BSF would now be called ___ with a value of ___

forward; peak scatter factor (PSF); 1.0

PSF

peak scatter factor

replaces BSF above 2 MeV

output factor (OF) is measured in ___ for low energy photons and Co-60, and in ___ for high energy machines

R/min; cGy/MU

the larger the field size, the ___ the output factor (OF)

greater

OF =

dose at Dmax for f.s. (the one you’re using) / dose at Dmax for a standard f.s. (10 × 10)

the O.F for a 10 × 10 field will always be ___, for less than 10 × 10 field will be ____ than 1, and for greater than 10 × 10 field will be ___ than 1

1.0; less than 1; greater than 1

the reason the output factor changes is because of ___

scatter

ESF

equivalent square fields

three methods to find equivalent square fields

1. chart

2. ESF = 4 x A/P

3. 2 (WxL)/W + L

how is the equipment attenuation factor written in the MU calculation formula?

C_attn

C_attn

C_attn = dose with device in beam / dose without device in beam

what is the only thing that affects the equipment attenuation factor (C_attn)

beam energy

the equipment attenuation factor, C_{attn, is always ___}

below 1.0

if the factor is .97, that means the device attenuates 3% of the beam

patient attenuation factors depend on these 4 things

beam energy

field size

treatment depth

sometimes distance (SSD)

all patient attenuation factors (TAR, TMR, PDD) will ___ with increase in energy and field size (scatter) and ___ with increase in depth (attenuation)

increase; decrease

PDD =

PDD = dose at treatment depth / dose at Dmax

how does PDD change with depth, field size, energy, and SSD?

depth = indirect relationship (increase/decrease)

field size, energy, and SSD = direct relationship (increase/increase)

PDD is only used for what type of calculations?

SSD

(because with SAD/isocentric, the whole point is that dose is 100% at the depth we’re treating)

on a PDD chart (relating depth and beam energy to find PDD), the PDD for each energy is always 100% at

Dmax

MU calculation formula for SSD treatment

• N_mu =  ______TD_______

           C_cal  x C_fs  x  C_attn  x PDD

TD = tumor dose (divided by number of fields)

C_cal = calibration factor of machine; 1.0 unless told otherwise

C_fs = OF field size correction factor (look up on table)

C_attn = equipment attenuation factors (not always used)

PDD = look up on table

what would the MU calculation equation look like for a single field, 180 cGy, with blocks (TF = .97), Cfs = 1.03, PDD = 88%

N_mu = _______180_______

               1.03 x .97 x .88   

for an extended SSD MU calculation, what factors would you have to add to the equation?

ISL correction factor (in denominator) and Mayneord factor (multiply MF by PDD to correct the PDD for the problem)

ISL cf = (SSD1 + Dmax)₂ / (SSD2 + Dmax)²

ID =

ID = TD / DD

(incident dose = total dose / depth dose or PDD)

so if total dose is 204.5 cGy, but the tumor is getting 88% of the dose, it gets 180 cGy

ratio of dose in phantom to dose in air

TAR = dose at depth in phantom at SAD / dose without phantom at SAD (dose in air)

tissue-air ratio (TAR) is NOT dependent on ___, and is used for ___ calculations at low energy

SSD; SAD

TAR = BSF at ___

Dmax

backscatter factor (BSF) can only be used for the depth of ___, whereas TAR can be used for any depth

Dmax

TAR was invented for ___ calculations, so we won’t be using them

TAR

the most commonly used patient attenuation factor in dose calculations

tissue maximum ratio (TMR)

because it’s used for SAD/isocentric technique, which is most modern treatments

TMR

tissue maximum ratio

TMR is only used for what kind of calculations?

SAD/isocentric technique

is TMR dependent on SSD?

no

TMR =

TMR = dose at depth / dose at Dmax (ratio of dose at tumor to maximum dose)

equation that relates BSF, TAR, and TMR

BSF = TAR/TMR

equation for MU calculations for SAD/isocentric technique

• MU =  ______TD___________

           C_cal  x  C_fs   x C_att  x TMR  x  SAD_cf 

  TD = tumor dose (needs to be divided by number of fields)

  C_cal = calibration factor, 1 unless told otherwise

  C_fs = OF field size correction factor (look up on table)

  C_att = equipment attenuation factors

  TMR = will be told

  SAD_cf = SAD factor, a constant for each energy. (SAD = Dmax)² / (SAD)²

is SADcf a constant?

yes, a constant for each energy

6 MV = (100+1.5)² / 100² = 1.03

10 MV = 1.05

what would the MU calculation look like for an SAD treatment, single field, 200 cGy, 10 × 10 cm fs, no blocks, 6 MV (=1.03 SADcf), TMR = 0.8 (look up in appendix 5)

200 / 0.8 × 1.03

isodose curves are lines that represent areas of equal ___

depth dose, or percent depth dose

when looking at isodose curves, lower energies have greater ___ and the size of each isodose area is ___

penumbra; larger

constancy of intensity across the beam (80% of the beam width, because of penumbra)

flatness

intensity difference between opposite sides of the beam

symmetry

the field width/beam edge is defined at the ___% intensity line

50%

the smaller the field, the ___ the flatness

poorer (because of less scatter)

on a dose profile, higher intensity regions beneath the thin edge of the flattening filter

horns / ears

graph that shows flatness and symmetry of the beam

dose profile