Radiation Therapy

Question 1

What is Percentage Depth Dose (%DD)?

Answer 1

The fraction (percentage) of the dose absorbed at a depth
(d), compared to the depth where maximum dose occurs (dmax)
Ddepth/Dmax * 100

Question 2

%DD and graphs

Answer 2

The greater energies (18MeV) - greater percent of dose at deeper levels
- Roughly the same as the other energies

Question 3

Relationship between attenuation coefficient and max dose

Answer 3

The attenuation coefficient decreases, the more penetrating the beam is and thus the higher the depth of maximum dose

Question 4

Name the factors that affect %DD

Answer 4

Other factors affecting %DD
➢ Radiation field size
➢ Depth of tissue
➢ Source-surface distance

Question 5

Why does field size decrease %DD?

Answer 5

Increasing the field size increases the opportunity for photons to scatter off the
collimators and flattening filter, i.e. more photons to the beam that reaches the patient
* Higher photon energy will reduce Compton
scatter (more pair production)
* So, field size changes affect %DD less for
higher energy photon beams

Question 6

Describe the relationship between source and distance and %DD

Answer 6

%DD increases with SSD due to the
inverse square law
➢Actual dose rate decreases but %DD
relative to dose at a reference point
increases

Question 7

How is dose described in tissue across the
whole radiation field as opposed to just along the central axis (for %DD)?

Answer 7

Through the use of isodose curves or beam profiles

Question 8

What is an isodose curve?

Answer 8

➢A line passing through a points of equal dose
➢Can be expressed as a percentage of a
reference dose or in absolute dose terms

Question 9

What is a beam profile?

Answer 9

➢A line showing the dose variation across the field
at a specific depth

Question 10

What factors affect radiation dose?

Answer 10

Absorbed dose also varies with:
➢Radiation energy used
➢Field size
➢Distance from the source
➢Any beam altering devices
added to the primary radiation beam