4. Beam Geometry Part 2

Question 1

Inverse square law definition

Answer 1

The inverse square law states that the intensity of the beam (from point source) varies inversely with the square of the distance from that source

Referring to intensity not energy

Question 2

Fluence

Answer 2

Number of photons per cross section area

Question 3

Inverse square law derivation

Answer 3

The intensity is emitted evenly in all directions

Number of photons does not change

Question 4

How does the inverse square law apply to RT ?

Answer 4

• dose (rate is directly related to the intensity of the beam (dose/rate is inversely proportional to the square distance)
• small change in distance can have a large effect on dose rate

Question 5

Subclinical malignant disease is contoured as part of the..

Answer 5

Subclinical- can’t be detected clinically (CTV)

Question 6

A distance metastasis is initially contoured as part of the

Answer 6

GTV

Question 7

The IM can be added to accommodate all of the following treatment variables except:

Answer 7

Movement of the gantry

IM= internal Marin

Question 8

Lymphadenopathy associated with thoracic tumour would be contoured initially as part of the

Answer 8

GTV

Question 9

Penumbra

Answer 9

A partial shadow, between regions of complete shadow

The greyish outer part of a sunspot

Area at the edge of the radiation beam at which the dose rates changes rapidly as a function of distance from the beam axis

Area of fuzziness at the edge of the beam not a steep drop off in dose

Question 10

Different causes of penumbra

Answer 10

Geometric

Transmission

Scatter within patient

Question 11

1. Geometric penumbra

Answer 11

Caused by the size of the radiation source

This would be the reason for fuzziness because it’s not a straight line

Question 12

2. Transmission penumbra

Answer 12

Region irradiated by photons that are transmitted through the edge of the collimator block

• should not more that 5% transmission

More important in linear accelerators because Co already has larger penumbra

Question 13

3. Penumbra-scatter within patient

Answer 13

Photons which have been deflected from their original direction by interacting with matter in the path of the beam

Question 14

What are some ways to reduce penumbra

Answer 14

Geometric:
Smaller source
Distance of the source
Source to diaphragm distance

Transmission:
Divergent diaphragms