electrons

most common energies used for superficial tumors

6-20 MeV

inelastic collisions

with atomic e-=ionization and excitation (low Z)

with nuclei=bremsstrahlung (high Z)

elastic collisons

with atomic e-=electron electron scattering

with nuclei=coulomb scattering

KE redistributed

what does rate of energy loss depend on

e- density of medium it is travelling through

mass stopping power is greater for…

low Z material

high Z=more tightly bound e- and less e- oer gram

radiation losses (brems)

rate of energy loss is proportional to electron energy and Z²

probability of radiation loss increases iwth e- KE and Z

as KE and density increase, the chance of rad losses increase

XR produced more efficiently with higher energy e- and high atomic # absorbers

at what rate do electrons lose energy in soft tissue

2 MeV/cm in soft tissue when using > or = 1 MeV

mass stopping power

rate of energy loss

what is total energy lost made up of

energy lost to rad+energy lost to collisional

what force causes electrons to scatter

columb forces btwn incident e- and nuclei of medium cause electrons to scatter

most probable energy

(Ep)o= most probable energy which is defined at phantom surface

mean energy (E0)

related to depth at which dose is 50% of max dose (R50%)

energy at depth

most probable energy and mean energy decreases linearly with depth Z

mean energy at surface formula

E0=2.4 (meV/cm for water)x R50

output

absorbed dose per MU at Dmax

beam calibrated to

1 cGy/MU for reference applicator, Dmax, central axis, 100 SSD

depth dose distribution

can be determined by ion chambers, diode, films

where do electrons lsoe energy

at 2 MeV/cm of water

depth of 90% isodose

d90%=E/3.2

therapeutic range

D90% most useful txt depth

D80% formula

E/2.8

beyond max range of e-, the dose is contributed only by

XR contamination caused by Brems

why is XR contamination an issue for total skin e-

XR contam dose is increased by 6 times

what is XR contamination

dose contributed by Brems interactions with scattering foil

RELATIONSHIP BETWEEN DMAX AND ENERGY

DMAX INCREASES WITH ENERGY

lower energues-electrons scatter more easily so dose builds up more rapidly and over shorter distance

at what energies is there higher skin dose

higher energies, increase skin dose

what is the shape of low Energy electron isodose curves

they bulge out at all lebels

field flatness recommendation

ref plane perpendicular to central axis at 95% isodose depth

dose variation should be +- 5%

field symmetry recommendation

compares dose profile from one side to other

pair of points at same distance on each side should not differ by 2%

field size dependence

dose increases with FS due to more scatter from collimator and phantom

PDD increases with FS

deoth of Dmax increases with field size

does dmax shift towards or farther from the surface for smaller field sizes

it shift towards surface bc there is less lateral scatter, so reraches Dmax faster

when using a bolus, where does depth dose shift

shifts “upstream” equal to bolus thickness

boluses can..

fix surface irregularities

decrease electrons penetrating in certain areas

increase skin dose

density of bolus

1g/cm²

what do low atomic number material like acrylic do

reduces energy of electron beam

Dmax=

Dmax=Drx/%D

minimum thickness of lead needed to block

if using cerrobend?

electron energy/2

if using cerrobend, need 20% more material so 1.2

2 adjacent electron fields

create hotspot near skin surface

when electron and photon field abut, where does max dose/hotspot develop and where does cold spot develop

hotspot forms on photon section

coldspot develosp on electron side

bc electrons scatter into photon field

inhomogeneities formula

Deff=d-z (1-CET)

formula for tissue with diff kinds of inhomogeneities

Deff=(thickness of tissue x density)+ (thickness of tissue x density)

CET OF WATER

1

CET OF LUNG

0.3

CET OF SOFT BONE

1.1

CET OF HARD BONE

1.65