Brain Teaser #10
A) Question:
Beam data information is usually measured in a phantom that contains/made up of what kind of material?
A) Answer:
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.
B) Question:
Why is H2O or a material equivalent to H2O used to acquire beam data?
B) Answer:
H2O or equivalent is used because it has the approximate radiation absorption and scattering properties as muscle or soft tissue.
C) Question:
What is the name for the region that is located between the surface and the maximum dose of a given energy?
C) Answer:
The region between the surface and maximum dose is called the build up region.
Follow Up:
The clinical name for this build up region is called the skin sparing effect.
D) Question:
If the field size increases and all other variables stay the same, what happens to the dose at the central axis?
D) Answer:
When field size increases and everything else remains the same, the dose at the central axis will increase.
E) Question:
When field size increases and all other variables remain the same, what happens to the monitor units?
E) Answer:
If the field size increases and everything else remains constant, then the monitor units will decrease.
F) Question:
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?
F) Answer:
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.
G) Question:
What is the difference between tissue phantom ratio and tissue maximum ratio?
G) Answer:
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).
Follow Up:
In short, TPR is a reference depth that can be anything, where TMR is always d-max.
H) Question:
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?
H) Answer:
TPR is independent of distance where as PDD is dependent on distance.
I) Question:
As photon energy increases, what does the depth of d-max do?
I) Answer:
As photon energy increases depth of d-max also increases.
J) Question:
For a 15x photon beam, what is the approximate value for the depth of d-max?
J) Answer:
For a 15x photon beam, the approximate value for the depth of d-max is about 3.0 cm.
Follow Up:
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.
THE END
Author: Adam Buell
Editors: Mary Hare and Joan O'Brien
Date Created: September 2005
