Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

Physics for FRCR CO1 > 4.5 Physics of teleptherapy beams > Flashcards

4.5 Physics of teleptherapy beams Flashcards

You may prefer our related Brainscape-certified flashcards:
Physics 101 flashcards AP® Physics flashcards
1
Q

What is an Electron Volt?

A

Energy gained by 1 electron accelerated through an electrical potential of 1 volt

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

How man electron volts in 1 keV?

A

1000

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

How many electron volts in 1 MeV?

A

1,000,000

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What does mV stand for?

A

Milivolts

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

What is the rule of calculating Dmax (cm) of energy (MV)?

A

Divide by 4

e.g. 6MV Dmax is 1.5cm

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What is the energy range for Superficial RT? How deep can they penetrate? What are they used for?

A

50-160kV
<5mm treatment depth
e.g. skin

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What is the energy range for Orthovoltage? How deep can they penetrate? What can they treat?

A

160-300kV
<6cm treatment depth

e.g. skin, ribs

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What is the energy range for megavoltage? How deep can they penetrate? What are they used for?

A

> 1MV
10cm treatment depth

e.g. deep tumours

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What is the isocentre?

A

Small point in space (usually the tumour) around the machine rotates

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What is the beam axis?

A

The axis that the collimator rotates on

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What is the radiation field?

A

Plane section through the beam (perpendicular to the axis)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What is the principle plane?

A

The plane containing the axis, parallel to the face (not the length) of the collimator

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

What does SAD stand for?

A

Source to Axis Distance (where the chamber is)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

What does CAX stand for?

A

Central AXis (where the PDD is measured)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What does FSD stand for?

A

Focus to Skin Distance

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

What does SSD stand for?

A

Source to Surface Distance

17
Q

What is Dmax?

A

Depth of maximum dose

18
Q

Whate are the types of scatter within the radiotherapy beam?

A
  • Primary photons
  • Head of linac scatter photons
  • Phantom scatter photons
19
Q

What is the Scatter Factor?

A

Ratio of total absorbed dose at a point to the primary dose at that point

20
Q

Peak Scatter Factor (PSF) = Backscatter Factor

A
  • Ratio of dose at Dmax : dose without scatter
21
Q

What is the Back Scatter Factor of <45kV?

A

Photoelectric effect dominant + minimal surface dose

(photons do not penetrate sufficiently to scatter at the surface)

1.01

22
Q

What is the Back Scatter Factor of 300kV?

A

Compton dominant and backscatter to surface

(The photons have enough energy tp penetrate the material and scatter back to the surface - at this energy they tend to be scattered backwards so relatively high backscatter factor)

BSF 1.5

23
Q

What is the Back Scatter Factor of >2MV?

A

Compton dominant and forward scattering

(At higher energies photons are predominantly forward scattered - much fewer are back scattered back to the surface)

1.02

24
Q

What causes PDD to increase?

A
  1. Increased energy
  2. Increased field size
  3. Increased Focus surface Distance
  4. Physica wedge
25
Q

Why does increasing energy increase the PDD?

A

Right shift of Dmax on graph

On avergae the photons in the beam are of a higher energy and are therefore more penetrating.

This means Dmax is deeper due to the build up effect of the higher energy beam. The beam is less attenuated.

26
Q

How does increasing field size increase PDD?

A

The larger the field size the more scatter and increased surface dose due to electron contamination

27
Q

How does increasing the FSD increase PDD?

A

Increasing the FSD decreases the effect of z on the inverse square law

28
Q

What causes surface dose/ build-up?

A
  • Electron backscatter
  • Contamination electrons from the machine
  • Bolus will increase dose to skin surface
29
Q

What increases surface dose?

A
  • Increased field size - increased electron contamination with larger area
  • Lead trays/blocks - increased electron contamination due to more material to interact with
  • Increasing beam angle obliquity
  • Lower energy electrons
30
Q

What decreases surface dose?

A
  • Increased FSD
31
Q

What does increased FSD result in?

A
  • Greater MU is needed
  • Increased PDD
  • More penetrating beam - inverse square law decreases
32
Q

What does a decrease in FSD require?

A

Increased MU

33
Q

How

A

Physics for FRCR CO1 (9 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions