Beam Characteristics

Question 1

How does dose vary as a function of depth for photons?

Answer 1

Build up region where dose is increasing due to an increase in the number of secondary electrons

Dose increases to Dmax at a depth of dmax

The dose then falls off as a function of depth due to scatter, inverse square and attenuation

Question 2

What is TMR?

Answer 2

Tissue maximum ratio

The ratio between dose at a certain depth to the dose at dmax

Measured by changing the SSD (opposite to PDD or TPR, where the SSD is constant)

Question 3

In MV dosimetry, what does “Beam Quality” mean?

Answer 3

Beam quality is related to the energy of the radiation beam

Question 4

How does the MV Code of Practice say you can measure beam quality?

Answer 4

The code of practice defines it using Quality Index (TPR20:10). This is the relative dose at 20cm deep to 10cm deep in a water phantom where the isocentre is at the measurement point. (Also accept if stated as 20cm deep, 80cm SSD, and 10cm deep 90cm SSD), where a 10 x 10cm field is used

Question 5

What is the difference between the TMR and relative dose on a plot of field size against depth dose?

Answer 5

The inverse square law