Flashcards in Electrons Deck (21):
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?
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 mateiral cuases a higher direcitional change
What are the main features of a PDD?
- build up region
- R90 (depth of 90% dose in the fall of region)
- bremmsstrahlung 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, electrson 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