RT Clinical Radiation Beams Flashcards
1
Q
What is meant by the build-up effect and why is the surface dose non-zero?
A
- the build-up effect is generated by the overlapping paths of secondary electrons generated in the tissue
- once charged particle equilibrium is reached then the d_max is reached
- the electrons travel mainly in the forward direction
- this is due to the attenuation of the beam with depth
- the surface dose is non-zero due to contributions from back-scatter as well as electron contamination and the extended source of the flattening filter
2
Q
On a PDD what happens at the exit of the patient and why?
A
- at the exit there is a drop in the amount of dose
- this is due to the loss of the backscatter contribution
3
Q
How and why are PDD and TRP distributions different?
A
TPR tails off more gradually and this is due to no effect of the ISL as the distance from the source to detector is kept the same while the amount of tissue above the detector or measurement point is varied.
4
Q
Why is the beam penumbra important and what is it usually defined as? What are the three sources which generate it?
A
- it defines the useful extent of the beam
- it is usually defined as the width of the 80-20 region
- the sources are geometric, transmission and dosimetric
5
Q
Consider a beam profile, how does the shape of a beam profile vary with depth?
A
- at 10 cm depth or whatever the reference conditions are the beam should be flat
- at shallow depths the beam will be horny
- at greater depth due to hardening of the beam, the profile will be more rounded
6
Q
Where is a monitor chamber located and what is it used for?
A
- they are normally just behind the jaws
- used to monitor how much dose has been delivered and terminates the beam once the desired number of MUs/dose has been reached
- used to link the charge collected in the chamber which is equal to a certain number of MU to dose at the reference point, normally calibrated so that 100 MU is 1 Gy under reference conditions
