Wk06 Electron Beam Therapy Flashcards
For electron beam, the beam is made to strike on a ___________________.
Scattering foil
What is the use of a scattering foil?
To spread the beam to get a uniform electron fluence across the treatment field.
What is the scattering foil made of?
Low Z material eg. Aluminium
To prevent production of bremstrahlung x rays
What are the major characteristics of an electron central axis depth dose curve?
- A region of uniform dose —> can treat lesions with 1 single beam
- Rapid falloff of dose —> minimise dose to structures beyond the target
- High surface dose —> use to treat superficial lesions
Why is there a bremsstrahlung tail in electron depth dose curve?
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
The bremsstrahlung tail is more prone to _______ energy electron beams.
High
How to estimate the treatment depth of election beam?
90% depth dose = E/3.2 cm
80% depth dose = E/2.8cm
Where E = most probable energy in MeV at the surface
For electron beams, energy ________________, surface dose ______________.
Increases ; increases
What are the two problems of electron dose distribution?
- Lateral constriction —> the corners might not receive enough dose
- Bulging out for low level isodose line
Both problems are more prone to high energy beams
How is penumbra measured in electron beam?
p80-20: distance from 80% to 20% isodose line at the specified depth R85/2.
The output and depth dose curve are both ___________ dependent.
Field size
The effect of the field size on the output and depth dose curve is significant when_______?
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
Electron beam appears to originate from a point in space called __________. Which locate behind the __________.
Virtual source ; scattering foil
What is virtual source distance?
The distance from the virtual source to the surface
Electron beam is almost ______________ when produced.
Monoenergetic
What is Epk(0)?
The most probable energy of electron beam on the phantom surface
What is \Ek(0)?
The mean energy of the electron beam on the phantom surface.
What is R50?
Half-value depth on the percentage depth dose curve of the electron beam
What is Rp?
Practical range of the electron beam
What is Eko?
The mean energy of the electron beam at the exit window
- very narrow
- almost monoenergetic
What ionization chamber is recommended for electron dosimetric measurement
Plane-parallel chambers
Where is the reference point for plane-parallel chambers and cylindrical chambers respectively?
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
What parameter is used to specified electron beam quality
R50 = the depth in water at which the absorbed dose is 50% of he maximum dose
What is the ionisation chamber is actually measuring?
The quantity measured is the depth-ionization distribution. Conversion is needed
What is reference depth Zref?
The position where the chamber should be placed. We need to place our chamber at different depths for different energy measurements.
The choice of energy for electron treatment planning depends on:
- The depth of target volume
- Minimum target dose required
- Dose to a critical organ
Dose beyond the inhomogeneity can be corrected by ___________________________ method
Coefficient of equivalent thickness (CET)
What is the use of bolus
- Flatten out an irregular surface
- Reduce the penetration of electrons in part of the field
- Increase the surface dose
What can be used for electron field shaping
Low melting point alloy eg. Cerrobend, which allow less than 5% of transmission
How to calculate the lead thickness needed for shielding?
Energy in MEV / 2 + 1mm safety margin
If the lead is too thin, the dose may even be enhanced
What lesions required internal shielding?
Lip, buccal mucosa, eyelid lesions
What are the benefits of using internal shielding?
- Protect the structures beyond the target volumes
- 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.
