Photon Beam Characteristics and Dosimetry

Question 1

Define Tissue Maximum Ratio (TMR)

Answer 1

Tissue Maximum Ratio (TMR) is the ratio of dose at a given point and depth in phantom to dose at the same point at the depth of maximum dose in phantom.

TMR is a special case of TPR with the reference depth taken as depth of maximum dose.

Question 2

Draw the Define Tissue Maximum Ratio (TMR) concept

Answer 2

Question 3

What is Tissue Phantom Ratio (TPR)

Answer 3

Tissue Phantom Ratio (TPR) is the ratio of dose at a given point and depth in phantom to dose at the same point at the reference depth in phantom.

Question 4

Draw the concept of TPR (Tissue Phantom Ratio)

Answer 4

Question 5

When using a TMR table, in the case of treatment areas that have significant open areas, the field size we reference should be what?

Answer 5

The field size we referecne should only be the area that is blocked by tissue.

Question 6

What are the 3 examples of using a TMR table in case of treatment areas that have significant open areas, the field size we reference should only be the area that is blocked by tissue?

Answer 6

Whole brain RT, breast tangents, and head/neck opposed laterals for Stage I squamous cell carcinoma of the true vocal cord.

This is because there is no tissue in these open regions to scatter dose back into the central axis and should be considered “blocked”

Question 7

When doing monitor unit calcuations (MUCs) by hand for a whole brain treatment, you notcie that the field size is significantly larger than the head being treated (often called treating with “flash”). How does this affect the equivalent square field that you should be using for TMR calculations?

Answer 7

The equivalent square that should be used for TMR tables is smaller than the current large field.