CLRS342 Electrons

likelihood of interactions w/ matter is ____ w/ electrons

increased

electrons characteristics

• negative charge

• mass

• monoenergetic

• lose energy w/ depth

• MeV

how to get clinically useful electrons

remove target& flattening filter; introduce scattering foil

purpose of scattering foil

improve flatness and widen beam

why use electrons?

superficial tx, rapid fall off, increase surface dose with increasing energy

rules of thumb

• 90% line ~ 1/4 E

• 80% line ~ 1/3 E

• 50% line ~ 4 x E (mm)

• range ~ ½ E

electron beam isodoses

ballooning on sides, ballooning of lower % isodose lines

irregularly shaped fields

blocks <5% transmission; increase blocking, incerase MUs

internal blocking

lead shields (eye, tongue, oral mucosa); low Z attenuator (wax, plastic, aluminum)

partial bolus

should be avoided; area under block reduced dose; more sensitive to inhomogeneities than photons

tissue inhomogeneity

more air= electrons will go deeper in tissue

SSD dependence

• decrease depth dose w/ increasing SSD

• will deliver more MU to correct

balloon effect vs energy

increased surface penumbra w/ lower energy

deepest (cm) will go for electron tx

5-7cm

Rules of Thumb

• 90% ~ 1/4

• 80% ~ 1/3

• 50% ~ 4 x E

• range ~ 1/2 E

depth dose chareacteristics

surface dose, dmax, falloff, brems tail

clinical practice

enface & bolus

depth dose cont’d

• tissue, air, bone

• field size

• obliquity

interactions w/ matter

mass, negative charge, increased probability of interaction

energy dependence of electron interactions

2 MeV loss per cm tissue; 2 MeV loss per 6cm air

when to field match?

• area too large for 1 field

• need different energies

• sloping surfaces

with depth dose, ____ blocking and ____ output

decrease; decrease

extended SSD characteristics

• more than 100cm

• output & penumbra change

• no simple point source

• virtual point source method

virtual point source method corrects for…

small field size, low beam energy, large air gaps