Electrons Quiz

How are electrons commonly used clinically?

• 6-20 MeV

• superficial tumors: skin, lip, boosting lymph nodes

What is an inelastic collision?

• Kinetic energy is converted from one form to another

• with electrons: ionization and excitation (low Z material)

• with nuclei: Bremsstrahlung (high z material)

What is an elastic collision?

• with electrons: electron scattering

• with nuclei: Coulomb scattering

• KE not lost, redistributed

What is linear energy transfer?

• rate of energy loss per unit path length in collisions

• energy is locally absorbed, not carried away by secondary electrons

What is electron scattering?

• Columb forces between electrons and nuclei of the medium cause electrons to scatter

• Z²/KE²

What type of material is used to construct scattering foil?

• High Z materials to spread out beam that emerges from accelerator tube

What is the electron beam kept thin?

• to minimize X ray contamination

• minimize energy degrading

Beam journey

Linac elements

How are electron beams named?

• by most probable energy at the body surface

• beam starts monoenergetic but gets degraded by the time it reaches phatom surface

What is (E_p)o?

most probable energy, defined at the phantom surface

What is mean energy (E₀)?

• related to the depth at which dose is 50% of max dose

• 2.4(MeV/cm for water) x R₅₀

What is energy at depth?

The most probable energy and mean energy decreases linearly w/ depth Z

How can depth dose distribution be determined?

• ion chamber

• diode

• film

What is an ionization chamber?

• determines absorbed dose

What are the advantages of silicon diodes?

• no corrections needed

• small

• high sensitivity

What are the disadvantages of silicon diodes?

• suffer from energy and temperature dependence

• damaged by radiation

• measurements must be backed up with ionization chamber (2x more work)

• cannot be used for absolute dosimetry

What are the pros and cons of films?

• Pros: useful for determining the electron beam dose distributions, practical range, isodose curves, and beam flatness

• cons: cannot be used for absolute dosimetry

What are phantoms typically made of?

• water or water equivalent density

• need to make correction if not water

At what rate do electrons lose energy?

2MeV/cm of water

What is the formula for depth of the 90% isodose?

• D₉₀=E/3.2

• D₉₀ is the most useful treatment depth

• also called therapeutic range (R_t)

What is the formula for depth of 80% isodose?

• E/2.8

What contributes to dose beyond the max range of electrons?

x-ray contamination

What is x-ray contamination?

• dose contributed by bremsstrahlung interactions w/ scattering foil

In 6-12 MeV, how much dose is due to x-ray contamination?

0.5-1%

In a 12-15 MeV beam, how much dose is due to x-ray contamination?

1-2%

In a 15-20 MeV beam, how much dose is due to x-ray contamination?

2-5%

What is x-ray contamination a concern?

• total skin electron therapy b/c entire body is irradiated from 6 directions, so contamination increased 6x

How is dmax related to energy?

• dmax increases with energy

• at lower energies, electrons will scatter more easily and through larger angles

• so dose builds up more rapidly over a shorter distance

How is percent surface dose related to energy?

increases w/ energy

Why is there a more rapid dose buildup in lower energy electron beams?

• in low energy, more scatter through larger angles, bigger difference between surface dose and max dose

How are isodose curves shaped?

• electron scattering causes low E curves to bulge out

• In high E electrons, low dose levels bulge out

• high E electrons show lateral constriction

What is beam flatness and symmetry?

dose at any point should not exceed 103% of central axis dose

What is the AAPM recommendation for field flatness?

• perpendicular to CAX at 95% isodose depth

• dose variation should be ±5%

What is the AAPM recommendation for field symmetry?

• evaluated from measured cross beam profile

• comapres teh dose profile on one side of CAX to the other

• pair of points same distance on either side of the CAX should not differ by 2%

How is dose related to field size?

• dose increases with field size

• more scatter from collimator and phantom

How is PDD related to field size?

• increases w/ field size until it exceeds lateral range of electrons

• after, PDD is almost consistent w/ field size

How is depth of dmax related to field size?

• increases w/ field size until lateral range is reached

• dmax shifts toward surface for smaller field sized b/c less scatter, so Dmax is reached faster

How is output impacted by small FS?

• decreased output, more MUs

What is the electron source?

• electrons do not follow inverse square law

• there is no target, so there is no “source”

• we use imaginary source = virtual source point

• virtual source point: intersection point of back-projections along the most probable directions of electron motion

How can virtual source point be determined?

• geometric method: take films at different SSDs, measure reduction in FS, project back to 0 FS

• effective SSD method: dose measured in phantom at Dmax w/ phantom in contact w/ cone, then at various distances

Do electron cases typically use SSD or SAD setup?

• SSD set up

• isocenter maybe center of the lesion or scar

• may or may not use bolus

What is an SSD setup?

• 100cm to skin surface

• typically single field b/c have to move the patient between fields

• iso is on area we are treating

How are electrons set up at MSH?

• typically for primary or metastasis, a template created during sim and then used daily for treatment

What is the electron set up method at MSW?

• breast boosts

• sim in decubitis if patient is prone

What is the electron setup method at MS chelsea

• keloid

• skin lesions

• sarcomas

How are electron cases planned?

• typically single field

• if surface is not flat, isodose charts are more complex

• TPS accounts for this

How is energy chosen in electron cases?

• chosen by depth of target, minimum target dose required, and dose to OARs

• If no OARs, typically treat so PTV is within 90-95% isodose line

What isodose line is used when treating chest wall?

80%

What is the purpose of a bolus?

• shifts dose upstream

• shift is equal to thickness of bolus

• ex: 1cm thick bolus, dmax shifts up 1 cm

Why is bolus used with electrons?

• fix surface irregularities, flatten

• decrease electrons penetrating certain areas

• increase skin dose

What is the density of bolus?

• should be that of tissue

• 1g/cm²

• superflab, wax

What are decelerators?

• low Z material to reduce energy of electron beam

• should not be placed close to patient surface

How to relate dose to a point that is not dmax

Ddmax = D_rx/%D

What is beam obliquity?

when you don’t have a flat surface or the beam is not en face

How does beam obliquity change depth dose?

• increase max dose (hotter hotspot)

• increases side scatter at dmax

• shifts dmax towards surface

• D₉₀ is reduced

• x ray contamination is increased

How do surface irregularities impact dose?

• can create hot or cold spots in different areas

• can compensate with bolus

• taper edges of bolus

How is the beam shaped?

• lead blocks on skin

• need to be thick enough to reduce beam to <5% transmission

• place on skin b/c penumbra

How to calculate the minimum thickness of lead for blocking

• energy/2

• for cerrobend x1.2

How do 2 electron fields impact skin surface?

hotspot near skin

What happens if a photon and electron field abut?

electrons scatter laterally, so cold spot will be on electron side, and hotspot on photon side

How to account for inhomogeneities

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