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TP2 > Beam modelling > Flashcards

Beam modelling Flashcards

1
Q

What is beam commissioning?

A

determine a set of values for the adjustable parameters and generate calculated dose distributions

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2
Q

Beam commissioning parameters that can be adjusted?

A
  • Energy
  • Off axis beam softening
  • cone angle and radius
  • transmission factor for photon fluence through collimators
  • dimensions of photon source etc
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3
Q

What are the two types of beam profiles?

A

Cross plane
In plane

measurements taken by the detector to characterise the LINAC

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4
Q

What does PDD mean?

A

Absorbed dose at any depth in reference to an absorbed dose at a fixed reference depth of R0 along the central axis of the beam

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5
Q

What is LINAC beam modelling based on?

A
  • Central axis dose distribution

- Beam profile

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6
Q

What does beam modelling take into account?

A

Changes in PDD

Influence of side scatter-beam flatness

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7
Q

What does profile analysis take into account?

A

Field sizes
PDD
Wedges
Off axis factors

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8
Q

What is Beam modelling?

A

Involves the acquisition and comparision of LINAC and TPS profiles

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9
Q

What is profile analysis?

A

Overlaying TPS and Linac profiles

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10
Q

What is an Algorithm?

A

A procedure for solving mathematical problems involving a finite number of steps and involves repetition

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11
Q

What are the two ways inhomogeneities are included in the calculation?

A

Indirectly-Through a correction factor

Directly-Inherent to the algorithm

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12
Q

What are the two types of correction algorithms?

A

Correction/measurement based (indirect)

Model based (direct)-

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13
Q

What are correction based algorithms?

A

Rely on measured data in water to account for patient contour, internal anatomy and beam modifiers

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14
Q

What are Model based algorithms?

A

Use measured data to derive model parameters- predict the dose based on laws of radiation transport

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15
Q

What are the corrections for contour irregularities?

A
  • Effective SSD methods
  • TAR or TMR method
  • Isodose shift method
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16
Q

What are corrections for tissue inhomogeneities?

A

TAR (tissue to air ratio)
Effective path lengths
ETAR (Equiv TAR)
Batho power law

17
Q

What do inhomogeneities result in?

A

changes in the absorption of the primary beam

Changes in electron fluence

18
Q

What does TAR take into account?

A

Takes the depth of calculation point and field size into account

Ratio of dose at given point in phantom to dose in free space at same point

19
Q

What does Effective path lengths model?

A

Primary dose variation

  • Unreliable for regions of e- disequilibrium
  • Best for dose calculation away from inhomogeneity
20
Q

What does Batho Power law method account for?

A

primary beam attenuation and scatter changes in water

21
Q

What does ETAR account for?

A

Uses water equivalent depth that correctly accounts for primary component of dose.
- Ignores changes in scattered dose.

22
Q

What does the lung correction do?

A
  • Build up inside lung higher dose beyond
  • Decrease side scatter because of increase in electrons outside of the geometrical extent of the beam
  • Increased loss of side scatter leads to Decreased dose on Central axis
  • Significant in small fields
23
Q

For lung and bone correction what is PD dependent on?

A

Energy

24
Q

What is the effect of PDD on entrance with bone?

A

Dose increase to adjacent tissue due to backscatter

25
Q

What is the effect of PDD on exit with bone?

A

Forward scatter of electrons from bone

Build up of electronic equilibrium in soft tissue

26
Q

What is ETAR best for?

A

Low energy beams (,6MeV)

27
Q

What is TAR known for?

A

Overestimates the dose for all energy

28
Q

What is Batho power law suited for?

A

Greater than 10MeV

29
Q

What do correction methods depend on?

A

Energy
Field size
Location and size of inhomogeneity
Location of calculation point

30
Q

What are the pinnacle treatment planning algorithms?

A

Fast convolve
Adaptive convolve
CC convolution

31
Q

What are the differences between Pencil beam, fast convolve and collapsed cone?

A

PB: fast but not good with airgaps
FC: fast but not used for treatment
CCC: Not fast but accurate

32
Q

What are monte carlo methods?

A

Made via statistical summaries of individual radiation events (energy, momentum, process angles)

33
Q

Why should the calculation Grid be large enough?

A

To cover structure for dose calculation but not so large that TPS takes too long

34
Q

How is a calculation grid calculated?

A

interpolates points throughout the patient.

35
Q

Effect of bone and air?

A

B-Increased attenuation and scatter
decreased %PDD after increased dose on boundary

-Air: decreased attenuation, scatter Increased DD after decreased dose on boundary

TP2 (5 decks)

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