Week 1 Lec 1

Question 1

Inverse square law

Answer 1

Intensity of radiation from a point source is inversely proportional to the distance from the source.
I = 1/D^2

Question 2

Percentage Depth Dose

Answer 2

Percentage depth dose is a way of expressing the dose at a particular depth. It is defined as the ratio of absorbed dose at a depth (d) to the absorbed dose at a reference depth (dr ) along the beam central axis.

Question 3

Why does beam intensity reduce as it pass through the body?

Answer 3

• Attenuation processes
  
  • Beam quality or energy, depth, field size and shape, SSD
    The inverse square law

Question 4

Skin Sparing effect

Answer 4

The region between the patients surface and the point of maximum dose is called the dose buildup region. Gives rise to skin sparing effect

Question 5

Isodose Curve

Answer 5

Lines passing through points of equal dose

Drawn at regular intervals and expressed as a % of the dose at the reference point

Question 6

Dmax

Answer 6

Depth of maximum equilibrium for single field photon beams

Question 7

Define Field Size and maximum, minimum F.S.

Answer 7

• Defined as the lateral distance between the 50% isodose lines at a reference depth, this coincides with the light field (symmetrical field only)
  Maximum = 40 x 40 cm field Minimum = 4 x 4 cm field

Question 8

Beam effects

Answer 8

• Greater lateral scatter with low-energy beams causing the isocurves to bulge out
• Orthovoltage beams have increased scattered dose to tissue outside the treatment region
• Megavoltage beams, the scatter is mainly forward

Question 9

What is penumbra

Answer 9

• Penumbra is the area at the edge of the radiation beam where the doserate changes rapidly.
  
  • A radioactive source size can have a greater effect on the degree of penumbra, larger source size=larger penumbra
  • In an image penumbra is seen as the area of unsharpness at the edge of the beam
    Penumbra is defined as the area between the 20% and 90% isodose line at the depth of D max

Question 10

Factors impacting penumbra

Answer 10

§ Source size (e.g. 2cm Cs source)
§ SSD (Source to Skin Distance)
§ Position of the collimators, the closer the collimator is to the patient
the smaller the penumbra, composition material
§ Geometric and transmission penumbra
Effects of scatter in the patient

Question 11

Transmission penumbra

Answer 11

occurs as the radiation passes through the edge of the primary collimators, therefore secondary collimators are added to reduce this effect

Question 12

Geometric penumbra

Answer 12

this is where the lack of sharpness or “fuzziness” occurs at the edge of the field

Question 13

Exit Dose

Answer 13

the amount of radiation at the surface of the body opposite that to which the radiation is directed

Question 14

Treatment Planning Process

Answer 14

1. Patient positioning & immobilisation
2. Image Acquisition and input-CT/MRI/PET
3. Anatomy Definition-target & OAR
4. Beam Technique
5. Dose Calculations
6. Plan Evaluation
7. Plan Implementation
8. Plan Review & QA
9. Treat the patient

Question 15

Decisions to be made for planning

Answer 15

-Using fields of appropriate size
- Increasing the number of fields
- Selecting appropriate beam angles
- Adjusting beam weights (contribution from each field)
- Selecting appropriate beam energy
Utilising beam modifiers

Question 16

Uniformity depends on

Answer 16

 Patient size
 Tumour position
 Beam energy
 Different tissue densities

Question 17

Why should multifield plans contribute different dose

Answer 17

 To improve dose uniformity
 The PTV may not be centrally located
 There are different tissue densities in the volume
 Reduce dose to organs at risk

Question 18

Normalisation

Answer 18

This is just the process where the dose distribution is normalised to the reference dose or PTV dose or max dose- usually so that the dose can be easily recorded and reported and we can see by looking at it what it represents

Question 19

What factors affect choice of beam arrangements

Answer 19

PTV location, size and shape
Structures in entrance and exit beam
OAR location and dose limits, beam energy
External contour shape

Question 20

What factors affect isocentre placement

Answer 20

PTV location

PTV size and shape

Question 21

Where do we define the field size

Answer 21

at 100cm. 100cm can be placed on skin surface (SSD) or within patient (SAD)