Khan Outline: Ch 14- Electron Beam Therapy

Front 1

What are the four types of electron interactions?

Back 1

Inelastic collisions with atomic electrons

Inelastic collisions with atomic nuclei

Elastic collisions with atomic nuclei

Elastic collisions with atomic electrons

Front 2

Compare the collisional and radiation loss interactions between water and lead.
Why are they the way they are? See graph on page 265.

Back 2

In low-atomic-number media such as water or tissues, electrons lose energy predominantly through ionizationg events with atomic electrons.

In higher-atomic-number materials, such as lead, bremsstrahlung production is more important.

There are two reasons for this:

1. High Z materials have vewer electrons per gram than low-Z materials

2. High-Z material have more tightly bound electrons, which are not as available for this type of interaction.

Front 3

Describe electron scattering.

Back 3

When a beam of electrons passes through a medium, the electrons suffer multiple scattering due to coulomb force interaction between the incident electrons and, predominantly, the nuclei of the medium. As a result, the electrons acquire velocity components ands displacements transverse to the original direction of motion

Front 4

What is the tail of an electron PDD curve caused by?

Back 4

Photon contamination

Front 5

What are the rules of thumb for electrons:
a. 90%
b. 80%
c. 50%
d. Range

Back 5

a. E / 4
b. E / 3.2
c. E / 2.8
d. E / 2

Front 6

How is film dosimetry used in electron measurements?

Back 6

Film dosimetry offers a convenient and rapid method of obtaining depth dose distribution or a complete set of isodose curves in the plane of the film.

Front 7

How is absorbed dose determined for electron beams?

Back 7

Calorimerty and Fricke dosimetry are the most basic methods for measuring dose in a phantom, but calibrated ion chambers are more practical and used in the clinical setting.

Front 8

Which gives the highest surface dose, high or low energy electrons?

Back 8

High energy electrons

Front 9

Describe the characteristics of electron isodose curves.

Back 9

-The low -level isodose curves (<50%) bulge out beyond the field borders

-The higher level isodose curves (>80%) tend to show lateral constriction, which becomes worse with increasing energy and depth.

Front 10

What happens to electron penumbra as you increase your SSD?

Back 10

The penumbra is decreased.

Front 11

How much energy does and electron lose per centimeter?

Back 11

2 MeV / cm

Front 12

What is meant by electron flatness and where is recommended to be defined?

Back 12

Field flatness of an electron beam should be specified in a reference plane perpendicular to the central axis, at the depth of the 95% dose beyond the depth of dose maximum.

Front 13

What is the flatness tolerance?

Back 13

The lateral dose variation relative to the dose at central axis should not exceed +- 5% over an area confined within lines 2 cm inside the geometric edge of fields of size equal to or larger than 10 X 10 cm2.

Front 14

What s meant by symmetry and what is the tolerance for electrons?

Back 14

Beam symmetry compares a dose profile on one side of the central axis to that on the other.

The cross-beam profile in the reference plane should not differ more than 2% at any pair of points located symmetrically on opposite sides of the central axis.

Front 15

What happens to electron output when you increase the field size?

Back 15

As the field size increases

Front 16

What is the beam path for an electron beam?

Back 16

***needs answer***

Front 17

What is the virtual source?

Back 17

An electron beam appears to diverge from a point, called the virtual source.

Defined as an intersection point of the back projections along the most probable directions of electron motion at the patient surface.

Front 18

How is the electron energy and field size selected?

Back 18

The beam energy may be set so that the target volume lies entirely within the 90% isodose curve.

In the treatment of the breast, the energy is often chosen so that the depth dose at the chest wall-lung interface is 80%.

The choice of field size in electron beam therapy should be strictly based on the isodose coverage of the target volume.

Front 19

How does beam obliquity and air gaps affect electron beams?

Back 19

As the beam obliquity increases:

- The magnitude of the maximum dose (peak dose) is increased.

- The depth dose cure is shifted toward the surface. Consequently, the therapeutic range (e.g., d90) of the beam is reduced.

As the air gap increases:

- The isodose curves become increasing shallower laterally.

Front 20

What is the formula for calculating effective depths for electrons?

Back 20

deff = d - z (1 - pe)

d = actual depth
z = the overlying thickness of inhomogeneity
pe = the electron density of inhomogeneity relative to that of water

Front 21

What is the purpose of a bolus in electron therapy?

Back 21

The purpose of a bolus in electron therapy is to:

a. flatten out an irregular surface

b. reduce the penetration (energy) of the electrons

c. build up the surface dose (buildup bolus)

Front 22

What are problems with adjacent electron fields?

Back 22

When two adjacent electron fields are abutting on the surface, there is a danger of delivering excessively high doses in the overlap region at depth.

Front 23

How are lead cut outs used with electrons?

Back 23

Lead or Cerrobend cutouts are often used to give shape to the treatment field and to protect the surrounding normal tissues ora critical organ.

These cutouts are placed eiher directly on the skin surface or at the lower end of the treatment applicator.

Front 24

What is the effect of blocking in electron beams on dose rate?

Back 24

The output factor and the depth dose distribution at a point in the blocked field are not affected if the field radius drawn from the point of interest in any direction is larger than the minimum radius required for lateral scatter equilibrium.

If the blocked field radius in any direction is less than Req, the output factor and depth dose distribution would differ from those of the unblocked field. The output factor must be measured.

Front 25

When is internal shielding needed and what does it consist of?

Back 25

Internal shield are used to protect normal structures beyond the target volume for treatments such as lip, buccal mucosa, and eyelid lesions.

Internal shields consist of varying thickness of lead, encased in a low atomic material which helps to dissipate the effect of electron backscatter.

The thickness of lead required to block the electrons is approximately E/2

Front 26

What are the benefits of external surface shielding when treating with electrons?

Back 26

External shielding help defines the field edge and blocks primary electrons.

Front 27

When is electron arc therapy used?

Back 27

Electron arch therapy is most suited for treating superficial volumes that follow curved surfaces such as the chest wall, ribs, and entire limbs.

Front 28

What are the elements of tx planning with electrons?

Back 28

The elements of TX planning with electrons are choice of:

- Beam energy
- Field size
- Isocenter
- Field Shaping
- Isodose distribution

Front 29

Explain total skin electron therapy.

Back 29

Electrons in the energy range of 2 to 9 MeV have been found useful for treating superficial lesions covering large areas of the body, such as mycosis fungoides and other cutaneous lymphomas.

Superficial skin lesion extending to about 1 cm depth can be effectively treated without exceeding bone marrow tolerance.

Front 30

What algorithm is used in electron beams? What are the benefits?

Back 30

Pencil beam algorithms based on multiple scattering theories are most commonly used for electron beam treatment planning. This method allows for pixel-by-pixel calculation of heterogeneity correction.