Planning 1

Question 1

What are the 2 aspects of dosimetry

Answer 1

Absolute absorbed dose
Relative spatial distribution of dose also known as percentage depth dose.

Question 2

What is absolute absorbed dose

Answer 2

Amount of energy imparting by ionising particles to a unit mass of irraomaterial.
Grays

Question 3

What is percentage depth dose?

Answer 3

refers to the percentage of the maximum dose delivered by a radiation beam at a specific depth in tissue,

Question 4

What is the central axis on a percentage depth dose?

Answer 4

Dose distribution on central axis
It has 3 regions
Build up - lack of electron equilibrium
Attentuation - loss of beam intenstity
Depth of max dose - electron equilibrium

Question 5

What is beam profile in radiotherapy?

Answer 5

Measure of dose at diff positions in a cross sectional plane.

Question 6

What are the characteristics of an isodose curve

Answer 6

Lines joining points of equal PDD
Shows variation in dose as a function of depth
Curves relate to reference points in central axis

Question 7

Isodose charts characteristics

Answer 7

Set of isodose curves
Shows increase of PDD
Typically max to 10%

Question 8

What is a build up region

Answer 8

Skin sparing
It’s at MV energies
Rapid increase in dose and max at specific depth

Question 9

What is the difference between %DD and Tissue max ratio

Answer 9

%DD is measured in a water tank and has fixed FSD and chamber moves so measuring points moves

TMR is used for isocentric treatment
No measuring point
Measures absorption

Question 10

What factors affect %DD

Answer 10

Beam type and energy and quality
Field size and shape
Shielding

Question 11

What happens when the beam energy increases

Answer 11

Increases %DD
increases primary photon penetration depth
Exit dose increases

Question 12

What is field size and what affect does it have on %DD

Answer 12

due to scattered radiation.
As field size increases so does scattered radiation on central axis.