Brainscape's Knowledge GenomeTM

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


See full index

Radiotherapy: custom cards > Characteristics of clinical beams > Flashcards

Characteristics of clinical beams Flashcards

1
Q

What does the treatment planning system contain and what information does it require?

A
  • The treatment planning system (TPS) contains a description of the physical and dosimetric characteristics of each linac.
  • Measurements are required for the dosimetric model but need to be made correctly. Is TPS model right?
  • We need to understand the dosimetric characteristics of clinical beams.
  • TPS needs the right information about our treatment machines, the beams they produce and how they interact with matter.
  • Need to measure lots of aspects of our clinical beams in order to create an accurate model in the software.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What is the dominant interaction with matter at the energies typically used in radiotherapy and how does the interaction vary with e- density and E?

A
  • Compton scatter.
  • Attenuation α electron density.
  • Attenuation α 1/E.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Draw typical isodose plots for 200KVp, 60Co, 4MV and 10MV beams.

A
  • Isodose lines describe boundaries of different dose levels.
  • Lower energy - less forward scattered radiation and greater penumbra.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

Draw a diagram of a single beam profile and label it.

A
  • Patient surface.
  • Build-up region.
  • Point of max dose.
  • Penumbra.
  • Profile.
  • Normalisation point 10cm depth.
  • Scatter and leakage from linac and scatter from patient.
  • Dose change with depth - PDD.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

Describe the central axis depth dose as a beam travels through a patient?

A
  • When photon beam is incident on a patient:
  • Dose builds up from surface dose Ds to a maximum Dmax at depth zmax.
  • It then falls almost exponentially until dose Dex at exit of patient at depth zex.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

Describe and explain the relationship between beam energy and Dmax.

A
  • Higher photon energies produce lower surface dose and deeper Dmax.
  • This is because Compton scatter is inversely proportional to photon energy therefore the photons have less chance of interacting with the matter i.e. are more penetrating.
  • Results in low skin dose - good for deeper targets but a problem for superficial targets.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

Describe how depth dose is measured and draw a diagram.

A
  • Detector Q, shown at depth z, moves up the beam axis.
  • Point P is at the depth of maximum dose Zmax.
  • Field size A is usually defined at surface f = 100cm SSD.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What is percentage depth dose (PDD)?

A

-Dose normalised to 100% at Zmax or Xcm.

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

What is relative depth dose (RDD)?

A

-Dose normalised to unity at Zmax or Xcm.

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

What are the components of a depth dose curve?

A
  • Primary beam.
  • Head scatter.
  • Phantom scatter.
  • Electron contamination.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

Explain the build-up effect and draw a diagram showing this.

A
  • Photons interact at different depths in the tissue and generate secondary electrons.
  • At each interaction, the recoil electrons travel, mostly forward, and deposit dose.
  • As more tracks overlap, the dose is built up until charged particle equilibrium (CPE) is reached.
  • A steady state would be reached if there were no photon attenuation/scattering.
  • Dose > 0 at surface due to some backscattered electrons from patient & contamination from linac.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What is the relationship between beam energy and surface dose, depth of dose max and dose at depth? Sketch graphs showing this.

A
  • As photon beam energy increases:
  • surface dose generally decreases.
  • depth of dose maximum increases.
  • Dose at depth increases.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

How dose the PDD curve for a given beam energy vary with beam size and why?

A
  • As the treatment beam gets bigger, the dose at a given depth generally increases due to:
  • More photons reaching the patient from the source (extended source of flattening filter).
  • More scattered electrons to measurement point from the irradiated volume.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

How dose the PDD curve for a given beam energy vary with SSD and why? Sketch a graph showing this.

A
  • Actual dose decreases with distance from source (inverse square law).
  • Relative dose, with respect to reference point, increases with SSD.
  • Compare two pairs of fixed-separation points (10cm apart) at different places on same inverse square law graph.
  • EXAMPLE: D(b)/D(a) = 0.83, D(d)/D(c) = 0.86.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What is the tissue phantom ratio (TPR) and how is it determined? Draw diagrams to help explain.

A
  • TPR(Z,C) = D(Z,C)/D(Zref,C)
  • Detector at fixed distance = SAD and overlying material thickness varied.
  • Field size AQ is defined at SAD.
  • Detector Q remains at SAD.
  • Measurement (a) at depth z is normalised to the measurement (b) at the reference depth zref.
  • Tissue maximum ration (TMR) is the special name when Zref=Zmax.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

Draw the relative depth dose (RDD) and tissue phantom ratio (TPR) curves on the same graph.

A
  • Different shape.
  • Not comparable.
  • Inverse square law effect.
  • Different scatter conditions.
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

How dose the TPR vs depth curve vary with beam size? Draw a graph showing this.

A
  • Varies similarly to PDD but usually normalised at greater depth.
  • Normally separate field size factor for flexibility of data use.
18
Q

List ways in which an electron beam interacts with tissue.

A
  • Excitation.
  • Ionisation.
  • Bremsstrahlung.
  • Characteristic radiation.
19
Q

Describe (and sketch) the energy spectrum of an electron beam at the accelerator window, the tissue surface and at depth d in tissue.

A
  • At accelerator window, the e- beam is almost monoenergetic exiting the accelerating waveguide, but the waveguide window, scattering foils, ionisation chamber, air, photon collimators, electron applicators, etc. generate interactions.
  • At tissue surface, energy spectra broadens and mean energy decreases.
  • Broader spectrum due to collisional & radiation energy loss.
  • At depth d in tissue, broader spectrum with more lower energy e-s.
20
Q

Describe build-up and skin sparing when using an electron beam.

A
  • Electrons deposit energy immediately, hence give a larger surface dose than with megavoltage photon beams.
  • Electron path becomes more oblique due to scattering as the beam passes through tissue - dose build-up to depth of max dose.
  • Beyond this point, numbers of electrons decline.
  • Steep dose fall-off beyond dmax as electrons are not energetic enough to penetrate.
  • Range straggling occurs, which increases at higher energies.
21
Q

Draw and label a graph of absorbed dose vs depth for an electron beam.

A
  • Peak/max dose.
  • Surface dose.
  • Therapeutic interval (distance between near and far useful dose level).
  • Depth of peak dose.
  • Depth of 50% absorbed dose.
  • Depth of practical range.
  • bremsstrahlung tail Dx.
22
Q

How do PDD curves vary with increased energy for electron beams. Sketch graphs.

A
  • With increased energy:
  • Surface dose increases.
  • Depth of dose max increases.
  • d50, d80 and Rp increase in depth.
  • Gradient of fall-off decreases.
  • Bremsstrahlung x-ray contamination level increases.
23
Q

What dose a beam profile describe?

A

-a beam profile describes the variation across the beam in tissue/water.

24
Q

How is the beam size defined?

A

-Beam size is defined as FWHM of beam at 10cm deep.

25
Q

How is the penumbra defined and why is it useful to know?

A
  • Penumbra usually defined as distance between 80% and 20% dose levels.
  • Sets limit on how much of a beam is useful and how much exposes normal tissue.
26
Q

How does the beam profile change with depth?

A
  • Beam widens with depth.
  • Penumbra widens with depth.
  • Dose changes with beam size at a given depth (think PDD).
  • Flattening filter gives flattest beam at 10cm deep- rounded beyond and ‘horny’ at shallow depths.
27
Q

How is the wedge angle defined?

A

-The wedge angle is defined as the angle between isodose line and the normal to central axis at 10cm deep (IEC 1217).

28
Q

What are the three causes of a penumbra? Draw sketches to explain these causes.

A
  • Geometric (extended radiation source).
  • Transmission through collimators.
  • Scatter in patient.
29
Q

What is the equation for the penumbra width, P, at depth d?

A

-Pd = s(SSD+d-SDD)/SDD

30
Q

What does the width of the transmission penumbra depend on?

A

-Depends on energy of beam - higher energy more penetrating.

31
Q

What does the width of the dosimetric/scatter penumbra depend on?

A
  • Scatter angle is energy dependent.
  • Lower energies have more lateral scatter.
  • Lower energies have wider dosimetric penumbrae.
  • Some contribution from in-air and collimator scatter.
  • At higher energies geometric and transmission penumbrae effects dominate.
32
Q

What are field size factors? Sketch graph of relative dose vs square field.

A
  • As beam size increases, dose to patient increases.
  • Compare dose at 10cm deep with reference beam for various beam sizes.
  • Can apply field size factors to reference condition.
  • ST = D(C)/D(Cref).
33
Q

What are the two components of dose to the patient in RT?

A
  • Incident beam.

- Scatter within patient.

34
Q

Explain what head scatter is and state how much it contributes to patient dose.

A
  • Head scatter’ mostly from flattening filter but some contribution from collimators.
  • Constitutes about 3-4% of dose to patient.
  • Variation from 10x10cm reference field is ~±5%.
35
Q

What is the head scatter factor and what does it depend on? Draw a diagram to help.

A

-Sc=Dmp(C)/Dmp(Cref) [Dmp is dose measured in mini-phantom].
Linac-specific, depends on head design.
-Quantifying absolute head scatter value is difficult.
-For dose calculations, need change relative to reference beam size.
-Exclude variation in patient-scatter by constant irradiated volume.

36
Q

What is the patient (phantom) scatter factor and what does it depend on?

A
  • Sp = ST/Sc
  • Not linac specific.
  • Beam quality dependent.
  • Difficult to measure:
  • Infer from ST and Sc.
  • Can use published tables (NCS12).
  • Variation from 10x10cm reference field is ~±10%.
37
Q

What are monitor units?

A
  • Ionisation chamber ‘monitors’ beam constantly.
  • Amplifier adjusted until 1 monitor chamber unit = 1 TPS dose unit (usually 1cGy) under ‘calibration conditions’.
  • Monitor chamber linked to beam control - when requested MUs are reached the beam is switched off.
  • TPS calculates MUs required to deliver prescribed dose.
38
Q

Why do calibration conditions need to be established on linacs and what does calibration define?

A
  • Need a relationship between Dose at Px and MUs required to deliver that dose.
  • Linac calibration defines the reference dose/MU for dosimetry system.
  • The calibration conditions and the MU calculation (dosimetry system) reference conditions need not be the same but there needs to be a known relationship between them.
39
Q

what factors result in deviations from the reference condition?

A
  • Depth.
  • Treatment distance.
  • Collimator setting.
  • Shape/size of irradiated area.
  • Attenuators etc.
  • Patient heterogeneities.
  • etc.
40
Q

Suggest a typical linac calibration condition.

A

-e.g. 1cGy/MU at dmax depth for 10*10cm field.

41
Q

Give an example of a relationship between linac calibration conditions and the dosimetry system reference conditions.

A

-e.g. 1cGy/MU at dmax = 0.0078Gy/MU at 10cm depth for isocentric 6MV 10*10cm field.

42
Q

What in an equivalent square?

A
  • A square field which has the same central axis depth dose characteristics as a given non-standard field.
  • Se = 2xy/(x+y)
  • This relationship only applies to photons.

Radiotherapy: custom cards (4 decks)

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