Properties of Electrons
• Have a finite range
• Rapid dose falloff
• Do not deposit dose at depth, reduces normal tissue dose
• Provide a high surface dose compared to photons
• Have a wide penumbra which increases at depth
• A range of electron energies to choose from, 6,9,12,15,18 depending on the
energies commissioned
Why use Electrons?
• Provide a uniform dose from the surface to approx 6cm depending on electron
energy
• Useful in treating skin, nose, ears, chestwall, eyelids, scalp, limbs whilst sparing
normal tissue
Percentage Depth Dose
• The shape of the depth dose curve
is fairly uniform, followed by a rapid
drop off
- Dose is usually prescribed to the 90%
- The depth in cms @ 90% is approx one third of the electron energy
- The treatment depth and therefore electron energy to provide this depth must be carefully selected
• Reference point must be placed on
the CAX @ the required depth
%DD (approx values for 10 x 10 field) (6MV)
100% - 1.5 cm
90% - 4 cm
50% - 15 cm
%DD (approx values for 10 x 10 field) (10MV)
100% - 2.5 cm
90% - 5.5 cm
50% - 18 cm
Advantages of Electrons
• Sharp dose fall off below
the surface
• Less absorption in bone
and cartilage
• Good cosmetic results
Disadvantages of Electrons
- Expensive linac reqd
- Greater radiation protection compared to superficial treatments
- Field size limitation
- %DD less accurate under 4cm
- Dose inhomogeneity on curved surfaces
• Eyes; shields can cause scatter, bowing of the isocurves treats a larger area under the surface than
at the surface
Disadvantages regarding Dose
- Dose distribution is significantly affected by heterogeneities such as air, bone
- The dose within these heterogeneities can be difficult to measure, the dose beyond them is measurable
- Electron beams are difficult to model, and look-up table type algorithms do not easily predict dose for oblique incidences or tissue interfaces
The Use of Bolus
Bolus, made of a tissue equivalent material, is often used in electron beam treatments for the following purposes:
●To increase the surface dose;
●To flatten out irregular surfaces;
●To reduce the electron beam penetration in some parts of the treatment field
Impact of Electrons
Electrons start interacting and depositing dose as soon as the skin surface is reached
- Surface dose for electron beams increases with increasing energy, i.e.77-95% compared with 20 % photons
- This is opposite to photon beams where the surface dose decreases with increasing energy
- Best contact is used to minimise changes in SSD over the entire field
At extended SSD:
• The lower % lines get wider
• The higher % lines (80-100%) get narrower and lose depth
Mycosis Fungoides
Mycosis fungoides, is the most common form of lymphoma.
It generally affects the skin, but may progress internally over time.
Is a class of non-Hodgkin’s lymphoma, which is a type of cancer of the immune system.
Total Skin Electron Therapy
- For superficial lesions covering large areas like mycosis fungoides
- Different methods are possible to expose the whole body. Scatter place is closer to body
- Traditionally, patient is on stretcher. Modified as standing or rotating.
- Modified Stanford Technique
Types of Electron Set Up
- Best Contact/Skin Apposition
- Set Angles
Best Contact/Skin Apposition
• The field is setup using skin apposition
• This may mean variable gantry, collimator and floor angles each day
• Used for patients where
immobilisation/positioning may vary
• Used for patients where there is no concern of OAR or overlap of fields
Set Angles
• Used in regions where dose to OAR
require consideration
• Near previously irradiated fields or matching/close to current fields
• Requires accurate stable reproducible immobilisation
• Gantry, collimator and floor rotation are fixed as per the plan
What happens if best contact is not setup?
A setup that is not best contact may compromise coverage at a depth
