Principles test 2 Electrons

How do electrons compare in size to protons?

Electrons are about 2,000 times smaller than protons

What is mass stopping power?

The rate at which a particle loses energy per unit length as it travels through matter

What is a delta ray?

A scattered (knocked out) electron ejected from an atom that can cause additional ionization (aka free electron)

What is restricted mass collisional stopping power?

A measure of absorbed dose that accounts for local energy deposition while excluding energy carried away by delta rays

What organization creates protocols for radiation detecting and testing devices?

AAPM (American Association of Physicists in Medicine)

What is true about low Z materials?

Low Z materials have a high electron density

Why do low Z atoms have high electron density?

They contain many small atoms packed into a volume, resulting in many electrons per unit volume

What interaction mainly causes electrons to lose energy?

Collisional interactions

What does energy dependence mean for electrons?

Electrons continuously lose energy as they travel through tissue

What is the approximate energy loss of electrons in tissue?

About 2 MeV of energy is lost for every 1 cm of tissue

How do electron dose rates compare to photon beams?

Electrons have lower dose rates than photon beams

How is an electron pencil beam widened?

Using a primary scattering foil or a scanning beam

What is the purpose of the primary scattering foil?

To widen the electron beam

What is the purpose of the secondary scattering foil?

To flatten the electron beam

What is the electron isodose level at the skin?

Approximately 85%

What happens to photon contamination as electron energy increases?

The Bremsstrahlung tail increases

What is the Bremsstrahlung tail?

Photon contamination produced when electrons interact with material

At what isodose line is electron dose usually prescribed?

The 90% isodose line

What is the 50% isodose electron energy formula?

Mean electron energy (MeV) = 2.4 × depth (cm) to the 50% isodose

How do you calculate electron energy if the 50% isodose depth is 5 cm?

2.4 × 5 cm = 12 MeV electron beam

Why should electron field size not be smaller than the practical range?

A field smaller than the practical range causes poor lateral scatter and bad dose distribution

What is the practical range of a 12 MeV electron beam?

Approximately 6 cm

What is the minimum field size for a 12 MeV electron beam?

At least 6 × 6 cm

What happens if electron field size is smaller than the practical range?

The dose distribution becomes distorted

What are the three functions of bolus in electron therapy?

Increase surface dose,

Remove irregular surfaces,

Shape isodose contours at depth

IRS

What is the edge effect?

A perpendicular dose drop from the bolus to the skin that disrupts dose distribution

How is the edge effect prevented?

By beveling bolus edges at a 45-degree angle

What happens when electron fields are abutted?

A teardrop-shaped dose distribution forms under the skin

Why are abutting electron fields problematic?

They create hotspots

How are hotspots from abutting electron fields corrected?

By feathering the fields

What is the rule for lead shielding in electron therapy?

2 mm of lead per 1 MeV of electron energy

How much lead is needed for a 10 MeV electron beam?

5 mm of lead

What units are used for practical range and lead shielding?

Practical range is in centimeters; lead shielding is in millimeters

How much of an electron beam can lead shielding block before replanning is required?

Up to 5%

What are internal electron shields used for?

Protecting sensitive facial structures such as eyes and nose

Where are internal electron shields most commonly used?

Head and neck treatments, especially the face

What is a disadvantage of internal electron shields?

They can cause scatter instead of reducing dose

By how much can internal electron shields increase dose?

Approximately 30–70%

How does electron dose rate and output factor depend on field size?

They are highly dependent on field size compared to photon beams

Why is electron dose rate strongly affected by field size?

Because electron dose is largely produced by scatter

What happens to scatter as electron field size increases?

Scatter increases, leading to higher dose rate and output

Where does absorbed dose in electron therapy primarily come from?

Scatter

How does scatter differ between electrons and photons?

Electrons scatter more, while photons scatter less overall

What is lateral electronic equilibrium?

A condition where the number of electrons entering a volume equals the number leaving it

Why is it called lateral equilibrium?

Because the balance of electrons occurs side-to-side (laterally), not along the beam direction (i fucking guess…)

Why is lateral equilibrium important?

It ensures the dose delivered is stable and accurate

How is lateral equilibrium achieved in electron therapy?

The field size must be larger than the practical range

What happens if lateral equilibrium is not achieved?

There will be a drop in dose

What happens if more than 25% of an electron field is blocked?

The output must be recalculated

What happens to output when electron energy increases?

Output increases

What happens to dose when a lesion is next to bone?

Lateral scatter from bone increases dose to nearby tissue

What happens to dose beyond an air cavity in electron therapy?

Dose increases due to lack of attenuation

How much overdose can occur beyond an air cavity?

Approximately 15–30%

What happens to dose when a lesion is behind bone?

Tissues receive an underdose due to bone attenuation

How much underdose occurs behind bone?

Approximately 20%

What happens when electron fields are gapped or abutted?

Teardrop-shaped hotspots are created

What is a delta ray?

A free or scattered electron

What are collisional losses?

Energy loss due to ionization and excitation

What are radiation losses?

Energy loss due to Bremsstrahlung production

How is electron dose distribution measured?

Using in vitro dosimetry (IVD) or silicon diodes

What is a characteristic of electron dose falloff?

Electrons have a rapid dose falloff

Why are electrons good for sparing contralateral tissues?

Because of their limited penetration and rapid dose falloff

What causes X-ray contamination in electron treatments?

The Bremsstrahlung tail

What does increased obliquity do to electron dose distribution?

It distorts the dose distribution

What happens if an electron beam is not enface and oliquity is increased

Increased obliquity throws off dose distribution

What is the Stanford technique?

A whole-body electron therapy technique

What disease is commonly treated with the Stanford technique?

Mycosis fungoides

How is the Stanford technique delivered?

Six fields spaced every 60 degrees

What is a shrinking or reduced field?

A smaller field used because the tumor has shrunk or the dose needs to be decreased

What does a boost field encompass?

The gross tumor volume (GTV) only

What is a simultaneous integrated boost (SIB)?

A boost dose delivered within another treatment field

What is a vertex field?

A field delivered from the top of the head

Why is a vertex field considered non-coplanar?

It is not delivered in a straight plane and requires a floor kick, think non-coplanar=not straight