Electrons Flashcards
What are some features of electrons?
- have a finite range
- rapid dose fall odd
- do not deposite dose at depth and thus reduces normal tissue dose
- provide high surface dose
- have wide penumbra which increases at depth
Why do we use electrons?
- provide a uniform dose from the surface to approx 6cm depending on energy
- useful in treating skin, nose, ears, chestwall, eyelids, scalp, limbs
What is the minimum field size?
- 4x4 is smallest applicator but can be determined by lead cut out
- if we use smaller there is a loss of lateral electrons and thus the dose is not accurate
What is the electron depth dose?
- the shape of the dpeth dose curve is fairly uniform followed by a rapid drop off
Where is dose prescribed for electrons?
- 80 or 90%
What is the depth in cm at 80%?
- approximately 1/3 of the electron energy
Where is the RP placed?
- on the central axis at the required depth
What happens to the surface dose as the electron energy increases?
- it increases
What happens to the dose as the SSD is extended?
- the lower % lines get wider
- the higher % lines (80-100%) get narrower and lose depth
- more MU are required
What is the distance bwteeen the end of applicator to the patient when treating electrons at 100cm SSD?
- 5cm
What occurs with oblique incidence?
the skin dose increases dramatically as there are more oblique electrons
What are advantages of electrons compared to superficial?
- sharp dose fall off below the surface
- less absorption in bone and cartilage
- good cosmetic results
What are some disadvantages of electrons compared to superficial?
- expensive linac required
- greater raditaion protection compared to superficial treatments
- field size limitations
- %DD less accurate under 4cm
- dose inhomogentiy on curved surfaces
- eyes: sheilds can cause scatter, bowing of the isocurves treats a larger area under the surface than at the surface
What are some general disadvantages of electrons?
- dose distribution is signicantly affected by heterogeneities such as air and bone
- dose within these heterogeneities can be difficult to measure
- electron beams are difficult to model
What is TSET?
- total skin electron therapy
- for superifical lesions covering large areas like mycosis fungoides
- different methods are possible to expose the whole body
- modified stanford technique
Why is bolus used?
- increase surface dose
- flatten out irregular surfaces
- reduce the electron beam penetration in some parts of the treatment field
What happens to electrons as they travel to greater depths?
- their paths become more oblique
- higher atomic number material causes a higher directional change
What are the main features of a PDD?
- build up region
- R100
- R90 (depth of 90% dose in the fall of region)
- R50
- Rp
- bremsstrahlung tail
What causes the dose in the build up region?
- knock on electron
- increasingly oblique electron paths (have a longer path then vertical travel and thus more dose)
Why is the depth of dmax closer to the dose at surface for higher energy electron beams?
- at higher energy beams, electrons are deflected less as they travel so the depth of maximum dose is deeper
- since higher energy electrons tend to be deflected less in general their paths tend to be straighter and less oblique at the depth of maximum dose
What causes the bremsstrahlung tail?
- due to photon contamination
- varies with energy, 1% in low energy, 5% in higher energy
