Beam data information is usually measured in a phantom that contains/made up of what kind of material?
H2O is usually the material a phantom is made up of or, in some cases the phantom is made up of a material that is water equivalent.
Why is H2O or a material equivalent to H2O used to acquire beam data?
H2O or equivalent is used because it has the approximate radiation absorption and scattering properties as muscle or soft tissue.
What is the name for the region that is located between the surface and the maximum dose of a given energy?
The region between the surface and maximum dose is called the build up region.
The clinical name for this build up region is called the skin sparing effect.
If the field size increases and all other variables stay the same, what happens to the dose at the central axis?
When field size increases and everything else remains the same, the dose at the central axis will increase.
When field size increases and all other variables remain the same, what happens to the monitor units?
If the field size increases and everything else remains constant, then the monitor units will decrease.
If you compare a 15x15 to a 30x10, which field will have more dose at central axis (or will the dose be the same for both) if all other variables remain the constant?
The 15x15 will have more dose at central axis then the 30x10. This is because the 15x15 will have more scatter contributing to central axis then 30x10.
What is the difference between tissue phantom ratio and tissue maximum ratio?
A tissue phantom ratio is the ratio of dose at a given point in the phantom to the dose at the same point at a fixed reference depth. This reference depth can be any depth. Tissue maximum ratio is a special tissue phantom ratio where your reference depth is your maximum dose (d-max).
In short, TPR is a reference depth that can be anything, where TMR is always d-max.
When comparing percent depth dose to tissue maximum ratio, what is the variable that percent depth dose is dependent of, and tissue maximum ratio is independent of?
TPR is independent of distance where as PDD is dependent on distance.
As photon energy increases, what does the depth of d-max do?
As photon energy increases depth of d-max also increases.
For a 15x photon beam, what is the approximate value for the depth of d-max?
For a 15x photon beam, the approximate value for the depth of d-max is about 3.0 cm.
Please keep in mind this value may change by 1 or 2 mm depending on what machine you are using, and the values that the physicist has measured at your clinic.
Author: Adam Buell
Editors: Mary Hare and Joan O'Brien
Date Created: September 2005