Treatment Planning Algorithms Flashcards
What are the 2 main types of algorithms?
Model based and correction based algorithms
Whats the main difference between photons and electrons (in relation to filters)?
With photons we have a flattening filter (bow-tie shaped) that removes low energy x-rays.
With electrons we have a scattering foil (circle shaped) that scatters electrons more
What are the 2 parts of model based algorithms?
(1) a part of the algorithm that MODELS THE BEAM and represents the fluence distribution BEFORE THE BEAM ENTERS THE PATIENT. (e.g. models electrons striking Linac, models propagation through Linac head)
(2) A part that MODELS THE PATIENT (e.g. models interactions within the patient).
What is the most common algorithm?
Convolution/Superposition algorithms
(All 3 algo’s on Pinnacle are convolution algorithms)
Have very good accuracy.
What are the main photon algorithms?
Pencil Beam
Convolution
Superposition
Monte Carlo
What are the components of Convolution Superposition algorithms?
Component 1: TERMA - total energy released per unit mass (represents the energy of primary photon interactions)
Component 2: Kernel - represents the energy deposited by a primary
photon interaction site.
REMEMBER: TERMA + KERNEL = DOSE
What are the 2 parts of a kernel?
Primary kernel (calculates primary dose) and the scatter kernel
What is the simple definition of a kernel?
The summation of all the interactions that gives a dose.
What are the limitations of pencil beam algorithms?
It uses measurement based approaches (i.e Batho and ETAR) to account for imhomogeneties (not accurate).
Doesn’t account for the increased electron scatter contribution in air.
The shape of the convolution kernel is wider than than the pencil beam kernel - what does this mean?
It is broader because the convolution algorithm accounts for the scatter contribution when it interacts with air. (This is why it more accurate)
What is a superposition algorithm?
It is a type of convolution algorithm. It has been modified to take into account radiobiological path length.
It is the same shape as the convolution kernel and is also called convolution-superposition.
What are advantages of convolution-superposition algorithms?
Models the interactions between the beam and the LINAC head.
Accounts for beam shaping and modifiers
It has efficient dose calculation
What are disadvantages of convolution-superposition algorithms?
Relatively time consuming (especially for CNS/ large volumes )
Need to conduct beam commissioning measurements.
What is the AAA algoritm?
anisotropic analytical
algorithm. It has a short computation time but it has accuracy close to Monte Carlo
What are the types of electron algorithms?
Clarkson
Pencil Beam
Electron Monte Carlo
Why are photons and electron beams different?
Electron beams have more pronounced side scatter (lateral equilibrium) - this influences dose distribution
So we need an algorithms that measures scatter
What is the Clarkson algorithm?
A measurement based algorithm. It is not accurate because it is based on a phantom than was measured in water - so it does not accurately account for side scatter.
What is a pencil beam (electron) algorithm?
Its good at predicting dose.
However has problems in regions of inhomogeneity
Pinnacle uses PB for electrons.
What is the Monte Carlo algorithm?
It picks events from a probability distribution using random generated numbers
Uses probability to estimate the amount of photoelectric, compton scatter and pair production interactions. - guesses all the possible interactions the x-rays could have.
More accurate than using a water phantom.
What are limitations of Monte Carlo?
It is subject to statistical uncertainty in dose.
This is proportional to the number of histories (the # of particles generated by the source model)
How do you increase the speed of a Monte Carlo algorithm?
You must decrease the uncertainty.
E.g. If
you decrease the uncertainty by a factor of 2, you need 4
times more particle histories. This increases the calculation
time by a factor of 4.
However this can reduce.
See lecture: 1:00:00
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