Wk06 Electron Beam Therapy

Question 1

For electron beam, the beam is made to strike on a ___________________.

Answer 1

Scattering foil

Question 2

What is the use of a scattering foil?

Answer 2

To spread the beam to get a uniform electron fluence across the treatment field.

Question 3

What is the scattering foil made of?

Answer 3

Low Z material eg. Aluminium
To prevent production of bremstrahlung x rays

Question 4

What are the major characteristics of an electron central axis depth dose curve?

Answer 4

1. A region of uniform dose —> can treat lesions with 1 single beam
2. Rapid falloff of dose —> minimise dose to structures beyond the target
3. High surface dose —> use to treat superficial lesions

Question 5

Why is there a bremsstrahlung tail in electron depth dose curve?

Answer 5

It is caused by interaction between the electron beam and the machine components in its paths
Eg. Waveguide exit window, scattering foil, transmission ionization chamber

Question 6

The bremsstrahlung tail is more prone to _______ energy electron beams.

Answer 6

High

Question 7

How to estimate the treatment depth of election beam?

Answer 7

90% depth dose = E/3.2 cm
80% depth dose = E/2.8cm
Where E = most probable energy in MeV at the surface

Question 8

For electron beams, energy ________________, surface dose ______________.

Answer 8

Increases ; increases

Question 9

What are the two problems of electron dose distribution?

Answer 9

1. Lateral constriction —> the corners might not receive enough dose
2. Bulging out for low level isodose line

Both problems are more prone to high energy beams

Question 10

How is penumbra measured in electron beam?

Answer 10

p80-20: distance from 80% to 20% isodose line at the specified depth R85/2.

Question 11

The output and depth dose curve are both ___________ dependent.

Answer 11

Field size

Question 12

The effect of the field size on the output and depth dose curve is significant when_______?

Answer 12

When the distance the field edge and central axis is smaller than the range of laterally scattered electron. i.e. more significant in smaller field size

Question 13

Electron beam appears to originate from a point in space called __________. Which locate behind the __________.

Answer 13

Virtual source ; scattering foil

Question 14

What is virtual source distance?

Answer 14

The distance from the virtual source to the surface

Question 15

Electron beam is almost ______________ when produced.

Answer 15

Monoenergetic

Question 16

What is Epk(0)?

Answer 16

The most probable energy of electron beam on the phantom surface

Question 17

What is \Ek(0)?

Answer 17

The mean energy of the electron beam on the phantom surface.

Question 18

What is R50?

Answer 18

Half-value depth on the percentage depth dose curve of the electron beam

Question 19

What is Rp?

Answer 19

Practical range of the electron beam

Question 20

What is Eko?

Answer 20

The mean energy of the electron beam at the exit window
- very narrow
- almost monoenergetic

Question 21

What ionization chamber is recommended for electron dosimetric measurement

Answer 21

Plane-parallel chambers

Question 22

Where is the reference point for plane-parallel chambers and cylindrical chambers respectively?

Answer 22

Plane-parallel chambers: inner surface of the entrance window, at the centre of the window

Cylindrical chambers: 0.5r deeper, where r is the radius of the air cavity

Question 23

What parameter is used to specified electron beam quality

Answer 23

R50 = the depth in water at which the absorbed dose is 50% of he maximum dose

Question 24

What is the ionisation chamber is actually measuring?

Answer 24

The quantity measured is the depth-ionization distribution. Conversion is needed

Question 25

What is reference depth Zref?

Answer 25

The position where the chamber should be placed. We need to place our chamber at different depths for different energy measurements.

Question 26

The choice of energy for electron treatment planning depends on:

Answer 26

1. The depth of target volume
2. Minimum target dose required
3. Dose to a critical organ

Question 27

Dose beyond the inhomogeneity can be corrected by ___________________________ method

Answer 27

Coefficient of equivalent thickness (CET)

Question 28

What is the use of bolus

Answer 28

1. Flatten out an irregular surface
2. Reduce the penetration of electrons in part of the field
3. Increase the surface dose

Question 29

What can be used for electron field shaping

Answer 29

Low melting point alloy eg. Cerrobend, which allow less than 5% of transmission

Question 30

How to calculate the lead thickness needed for shielding?

Answer 30

Energy in MEV / 2 + 1mm safety margin

If the lead is too thin, the dose may even be enhanced

Question 31

What lesions required internal shielding?

Answer 31

Lip, buccal mucosa, eyelid lesions

Question 32

What are the benefits of using internal shielding?

Answer 32

1. Protect the structures beyond the target volumes
2. Electron backscatter from lead will enhance the dose upstream up to 30-70%
   => hence, a suitable thickness of bolus or wax is placed between the lea and preceding tissue surface to reduce back scatter dose.