Electrons Powerpoint

Question 1

What modifications are needed for clinically useful electron beams?

Answer 1

Modifications must be made, including removing the ‘target’ and flattening filter.

Question 2

What is the purpose of the carousel in electron beam production?

Answer 2

The carousel rotates scattering foils into the path of the electron beam.

Question 3

How is the dose rate adjusted in electron beam production vs x ray production?

Answer 3

The ‘electron gun’ current is decreased to lower the dose rate.

Question 4

How can the beam be widened for clinical use?

Answer 4

The beam can be widened by using a scattering foil or by scanning the electron beam.

Question 5

What is the virtual source point?

Answer 5

The point from which the electron beam diverges.

Question 6

What happens to the overall energy of the beam when SSD is increased?

Answer 6

There is a decrease in overall energy of the beam.

Question 7

Why does the energy of the beam decrease with increased SSD?

Answer 7

It has more interactions with air molecules.

Question 8

How is e-behavior different from photons?

Answer 8

The rate of energy loss depends on the electron density of the medium. Unlike photons or X-rays, electrons have both mass and charge.

Question 9

What are the types of electron collisions?

Answer 9

There are two types of electron collisions: Elastic and Inelastic.

Question 10

When do elastic collisions occur?

Answer 10

Elastic collisions can occur with atomic electrons or nuclei.

Question 11

What are inelastic collisions?

Answer 11

Inelastic collisions can occur with atomic electrons or nuclei.

Question 12

What is an incident electron?

Answer 12

The interacting electron is known as the incident electron.

Question 13

What can incident electrons interact with?

Answer 13

Incident electrons may interact with the nucleus of an atom or the orbital electrons of an atom.

Question 14

What happens during elastic collisions with the nucleus?

Answer 14

Scattering of the electron occurs with no loss of energy.

The amount of scattering depends on the atomic number of the nucleus; higher atomic numbers cause more scattering.

Question 15

What occurs during elastic collisions with orbital electrons?

Answer 15

Scattering of the incident electron occurs with no transfer of energy.

Kinetic energy is not lost, although it may be redistributed among the particles.

Question 16

What is conserved in elastic collisions?

Answer 16

Momentum and kinetic energy are conserved.

Question 17

What happens during inelastic collisions with the nucleus?

Answer 17

Bremsstrahlung occurs as the electron slows down and changes direction, losing energy that is released as a photon.

Question 18

What occurs during inelastic collisions with orbital electrons?

Answer 18

There is a loss of energy from the incident electron to the orbital electron.

Question 19

What is conserved in inelastic collisions?

Answer 19

Momentum is conserved, but kinetic energy is not conserved.

Question 20

What processes occur during inelastic collisions?

Answer 20

Excitation and/or ionization occur.

Question 21

What is the desired effect of electrons in clinical usage?

Answer 21

To ionize atoms.

Question 22

What is not desirable in the clinical usage of electron beams?

Answer 22

Bremsstrahlung.

Question 23

What does the tail of the electron depth dose curve indicate?

Answer 23

It is related to photon contamination of therapeutic electron beams. (Represents bremsstrahlung)

Question 24

What is the useful energy range for electron beam therapy?

Answer 24

6 - 20 MeV.

Question 25

What types of tumors are treated with electron beam therapy?

Answer 25

Superficial tumors, <7 cm deep.

Question 26

What are some areas treated with electron beams?

Answer 26

Skin, boost for nodes (posterior cervical node chain), H & N cancers, chest wall.

Question 27

Why is ionization important in radiation therapy?

Answer 27

It is the primary means of electrons causing damage to the DNA molecule.

Question 28

What role does Bremsstrahlung play in radiation therapy?

Answer 28

It generates x-rays in an x-ray tube or linear accelerator and causes photon contamination.

Question 29

What is the significance of excitation in radiation therapy?

Answer 29

It explains characteristic x-rays.

Question 30

What is the purpose of accessory plugs on e-cones or consoles?

Answer 30

They automatically set the collimator size.

Question 31

Why must the collimator setting be larger than the cone size?

Answer 31

Because the electrons do not reach the edge of the field, requiring blocking back to have electrons at the edge.

Question 32

What are output factors measured for?

Answer 32

Each particular cone, which will automatically set a larger collimator setting.

Question 33

What is the energy of the beam when it leaves the waveguide?

Answer 33

It is the highest and is nearly monoenergetic.

Question 34

What happens to the beam energy after hitting the scattering foil?

Answer 34

The energy lowers on average and becomes a mix of energies.

Question 35

What is the energy of the beam at the surface of the patient called?

Answer 35

It is called the mean energy, Eo.

Question 36

What is the practical range of the electron?

Answer 36

The depth at which their average energy reaches zero.

Question 37

How much energy does the electron beam lose in soft tissue?

Answer 37

The electron beam loses approximately ~ 2 MeV per 1 cm of travel through soft tissue.

Question 38

What is the energy of a 12 MeV electron beam at 1 cm depth?

Answer 38

10 MeV at 1 cm depth.

Question 39

What is the energy of a 12 MeV electron beam at 2 cm depth?

Answer 39

8 MeV at a depth of 2 cm.

Question 40

What is the rule for penetration at E/2?

Answer 40

R » practical range; ~2 MeV/cm lost.

Question 41

What is the depth for 50% isodose line?

Answer 41

E/2.33.

Question 42

What is the depth for 80% isodose line?

Answer 42

E/3.

Question 43

What is the depth for 90% isodose line?

Answer 43

E/4

Question 44

What is the depth for 100% Dmax?

Answer 44

E/5

Question 45

What dose this variable represent?

Answer 45

Practical range: MeV/2

Question 46

What does this variable represent?

Answer 46

50% isodose line: E/2.33

Question 47

What is the formula for average energy using 50% isodose information?

Answer 47

Question 48

What happens to skin dose with increasing energy of electron beams?

Answer 48

The skin dose increases with increasing energy of electron beams.

Question 49

What does the exact value of the surface dose depend on?

Answer 49

The exact value of the surface dose depends on machine and collimation.

Question 50

What factors affect electron dose uniformity?

Answer 50

Electron dose uniformity depends on field size, beam energy, beam collimation, and SSD.

Question 51

What is the shielding dependence on electron beam energy?

Answer 51

Shielding dependence on electron beam energy can be achieved using lead strips, Cerrobend cutouts, or masks placed directly on the patient's skin or at the end of the treatment cone or collimator.

Question 52

What is the rule of thumb for shielding thickness?

Answer 52

The shielding thickness rule of thumb is MeV/2 = Shield thickness in millimeters of lead.

Question 53

How can the thickness of Lipowitz alloy be determined?

Answer 53

The thickness of Lipowitz alloy required may be obtained by multiplying the thickness of lead indicated by 1.2.

Question 54

What do isodose curves depend on?

Answer 54

Isodose curves depend on beam energy & field size.

Question 55

How do low energy beams affect isodose curves?

Answer 55

Isodose curves always bulge laterally below the surface at nearly all depths with low energy beams.

Question 56

How do high energy beams affect isodose curves?

Answer 56

Isodose curves bulge laterally only at deeper levels with high energy beams.

Question 57

What is the difference in bulging between low and high energy isodose curves?

Answer 57

Low energy beams bulge laterally at all depths, while higher energies only bulge at lower isodose levels.

Question 58

What is the relationship between field flatness and beam energy?

Answer 58

Field flatness is inversely related to beam energy.

Question 59

What is the relationship between field flatness and field size?

Answer 59

Field flatness is directly related to field size.

Question 60

What causes large fluctuations in dose in tissue?

Answer 60

Variations in tissue geometry or an inhomogeneous material cause large fluctuations in dose.

Question 61

What is the correction factor for compact bone?

Answer 61

The correction factor for bone is 1.65.

Question 62

What is the correction factor for air cavity?

Answer 62

The correction factor for air cavity is 0.5.

Question 63

What is the purpose of bolus in an electron field?

Answer 63

Bolus is used to increase skin dose, fill in for missing tissue, flatten out an irregular surface, and decrease electron penetration.

Question 64

What should bolus material be equivalent to?

Answer 64

Bolus material should be equivalent to tissue in stopping power and scattering power.

Question 65

What happens if only a portion of the field is covered by the bolus?

Answer 65

The edge of the bolus that lies within the treatment area may disrupt the dose homogeneity.

Question 66

What is Total Skin Electron Beam Therapy?

Answer 66

It treats the patient's entire skin surface.

Question 67

What condition is treated with Total Skin Electron Beam Therapy?

Answer 67

Cutaneous T-Cell Lymphoma (CTCL) ## Footnote Example: Mycosis Fungoides

Question 68

How often is Total Skin Electron Beam Therapy administered?

Answer 68

3-4 days a week.

Question 69

What is the typical duration of Total Skin Electron Beam Therapy?

Answer 69

3-9 weeks.

Question 70

What is the dosage range for Total Skin Electron Beam Therapy?

Answer 70

12-36 Gy.

Question 71

What is the MU formula for electrons?

Answer 71

Question 72

What is the minimum isodose line acceptable for electrons?

Answer 72

80%

Question 73

What is the formula for Depth dose?

Answer 73

Depth dose = mu x cGy/mu X area factor X PDD