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TPS algoritms Flashcards

1
Q

What are the two components of Dose?

A

Primary radiation Dprim

Scattered radiation Dscat

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2
Q

What are the properties of electron transport?

A
  • Range of MeV electrons can be a few cm
  • Particles in a forward motion
  • Scattered considerably at the end of their range
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3
Q

Where are contamination electrons generated?

A

accelerator head and also between Linac and patient

Contribute to the surface and build up dose

-More significant for large fields

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4
Q

What is Beam Penumbra?

A

Distance between 90 and 20 percent isodose curves

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5
Q

What is field width?

A

distance between 50% isodose

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6
Q

What are the electron effects in Lung?

A

Range of electrons in lung 3-5 times that in water

  • Larger penumbra
  • Loss of lateral equilibrium- e- travel further laterally
  • Actual dose beyond lung is lower than calculated
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7
Q

What is one inhomogeneity problem?

A

take accound of inhomogeneities in the lateral direction of the radiation field.

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8
Q

Types of kernel algorithms?

A

Not kernel based

Point Kernel

Pencil Kernel

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9
Q

What are the two types of algorithms?

A
  • Factor based (Data driven based on measured data)

- Model based ( adjustable parameters approximation of reality)

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10
Q

What are model based algorithms?

A

one part models the beam and provides a representation of the fluence distribution before beam enters the patient

Another part models the patient based on tomographic representation of patient tissues

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11
Q

What is the function of a dose algorithm?

A

To predict with as much accuracy as possible the dose delivered to any point in the patient

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12
Q

What is the most accurate out of Pencil beam, convolution and monte carlo?

A

Monte Carlo

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13
Q

What are common photon algorithms?

A
  • Pencil beam
  • Convolution
  • Superposition
  • Monte carlo
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14
Q

What are the benefits of convolution/ superposition algorithms?

A

Accuracy close to Monte carlo but quicker

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15
Q

What are the two components of all convolution algorithms?

A
  1. represents energy imparted to the medium (TERMA)

2. energy depositied about a primary interaction site, the kernel

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16
Q

What are the two kernel types?

A

Primary (primary dose) and secondary ( first and multiple scatter doses)

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17
Q

What is the definition of kernels?

A

Dose at a point is calculated by summing the effects from scattering elements

summation=Kernel

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18
Q

What does TERMA+Kernel equal?

19
Q

What are the Kernel shapes?

A

slab, pencil beam, or a point

20
Q

What is the pencil beam kernel?

A

energy deposition in a medium from a narrow geometry beam ‘pencil beam’

21
Q

What are the limitations of Pencil Beams?

A

Use a density scaling approach (Batho, ETAR) for inhomogeneities

-Doesn’t account for increased electron lateral scatter in air.

22
Q

What is the convolution dosimetry?

A

Model the initial incident fluence array

Assumes mono-enrgetic parallel beams

Require beam spectrum

convolve the primary energy fluence at each point with an energy kernel

23
Q

What is the superposition method?

A

Variant of the convolution

24
Q

What are the effects on inhomogeneity for superposition?

A

Increased dose in the lateral direction

  • Reduction of dose on the central beam
  • TPS generally overestimates the dose adjacent to and in air cavities.
25
How does superposition differ from convolution?
convolution is modified to take account of radiological path length (accounts for perturbation in first scatter photons which is the dominant scatter)
26
What are the advantages of convolution-superposition?
correctly models the beam passing through the Linac head then through the patient - Beam shapin and modifiers are explicitly accounted for - Efficient for volumetric dose calculations
27
What are the disadvantages of convolution superposition?
Time consuming Beam commissioning measurements (empirical parameters need to be determined)
28
What are some convolution algorithms?
CC convolution Pencil beam AAA (uses spatially variant convolution scatter kernels and separate modelling for photons and electrons)
29
What are common algorithms for electrons?
Clarkson Pencil beam Electron Monte Carlo
30
What is different for electron modelling
More difficult | Side scatter of electrons is more pronounced
31
Why shouldn't measured beam algorithms be used?
dangerously wrong in anything but a water phantom
32
Should Pencil beam algorithms be used?
OK but not trusted
33
What is the only reliable method for electrons?
Monte Carlo
34
What is the Clarkson Algorithm?
Measurement based method which uses a modified sector integration method based on tissue phantom TPR data generated from measured percent depth doses Not accurate
35
What are Pencil beam algorithm?
Electron beam penetration in matter is extremely complicated crude approximation Can be extended to 3D Problems in regions of inhomogeneity tend to underestimate the effects of sharp discontinues in density
36
How do Monte carlo simulations work?
Different MC codes make different assumptions to increase speed and accuracy
37
What is Monte Carlo simulation subject to?
statistical uncertainty in dose, proportional to 1/n^1/2
38
What is the process of Monte-carlo simulation?
Start with an electron exiting from waveguide. Follow it and its descendants through targets, primary collimators, ion chambers etc. Track it through patient-dependant structures (jaws, MLC etc.). Track it through the patient (as modelled from CT data set).
39
How does the monte carlo method work?
MC calculated quantities are subject to statistical uncertainties must simulate an infinite number of histories for a zero uncertainty noisy representation of true dose distribution
40
How can Monte carlo affect planning?
Noise can affect DVH Post-processing in form of DVH deconvolution
41
What characteristics for PB?
Fast Accounts for scatter Poor representation of inhomogeneity effects Assumes monoenergetic beams -Struggle to deal with irregular contours
42
What are the characteristics for Conv/Sup
Accounts for inhomogeneities Accounts for spectrum of energies Accounts for beam modifiers Relatively Slow Difficult to commission
43
What is the SVD algorithm?
separates the lateral and depth direction of a pencil beam distribution in water thereby saves memory and increases calculation speed

TP-3 (4 decks)

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