Part 4: Electron Beam Isodose Curves

Which dosimeter is a convenient and rapid method of obtaining isodose summation

film dosimetry

True or False: Film dosimetry for electron beams is accurate enough for a spot check and for beam calibrations.

False: only accurate for spot check

What is the shape of the isodose curves for electron therapy?

bell shape

fyi: it strikes the patient’s skin surface, and under the surface it bells out

in order to adequately cover a surface area, you must make the field size (smaller/bigger) than it appears from the field light by a margin of ___ between the field light and the target volume

bigger, 1 cm

What 2 things must you remember when treating using matching electron fields?

1. bell shaped curves produce hot spots under the skin surface

2. use a plastic wedge, field feathering, or e- arc to fix the problem of abutting e- fields

Electron beam calcs:

True or False: You can use the Equivalent Square formula to figure out electron calcs

False

True or False: You can use a strict application of the inverse square corrections to calculate electron beam calcs

false

What defines the field size of an electron beam?

Cones

Electron beam __ varies significantly with different collimator settings

dose rate

def. The intersection point of the back-projections along the most probable directions of electron motion at the patient surface

term. Virtual Source

Define Virtual Source using your own words

the point where the electron beam starts scattering before reaching the scattering foil

True or False: field arrangements for electron beams are always single field

True, they don’t go deep enough to meet at the same isocenter in the patients body (to form shapes like with POP fields)

What electron field arrangements are considered not to be single field (even tho they techincally are since they don’t meet at an isocenter)?

Stanford technique = 6 field technique

but they are still 6 individual fields

True or False: It is your goal to make sure that the electron beam is as perpendicular to the skin surface as possible

True

True or False: If you need to move the gantry angle a bit to make the beam as perpendicular to the surface as possible, you must do so

True

Bolus for electron beam therapy can be used for the following reasons:

1. as a tissue compensator

2. to increase surface dose (for beams less than 20 MeV)

3. Decrease electron dose deeper in tissue

FYI: for #3, For someone with a very thin chest wall that is 1 cm deep, even using a 6MeV energy, 6 MeV is zero at 3 cm. Not good cuz you’d radiate the lung. So use 1 cm bolus so by the time it gets to the lung the dose is 0.

def.. A plate of low x# material placed between the machine and the patient to reduce (degrade) the energy of the electron beam..

Beam decelerator (degrader)

What material is the beam decelerator made out of?

Low z# material: Lucite or polystyrene

What technique is a beam decelerator usually associated with?

TSI: Total Skin Irradiation to degrade a 6 MeV beam to 4 MeV

How much tissue equivalent material would be required to degrade a beam by 2 MeV?

1 cm (Mass Stopping Power)