TPS algoritms Flashcards
What are the two components of Dose?
Primary radiation Dprim
Scattered radiation Dscat
What are the properties of electron transport?
- Range of MeV electrons can be a few cm
- Particles in a forward motion
- Scattered considerably at the end of their range
Where are contamination electrons generated?
accelerator head and also between Linac and patient
Contribute to the surface and build up dose
-More significant for large fields
What is Beam Penumbra?
Distance between 90 and 20 percent isodose curves
What is field width?
distance between 50% isodose
What are the electron effects in Lung?
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
What is one inhomogeneity problem?
take accound of inhomogeneities in the lateral direction of the radiation field.
Types of kernel algorithms?
Not kernel based
Point Kernel
Pencil Kernel
What are the two types of algorithms?
- Factor based (Data driven based on measured data)
- Model based ( adjustable parameters approximation of reality)
What are model based algorithms?
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
What is the function of a dose algorithm?
To predict with as much accuracy as possible the dose delivered to any point in the patient
What is the most accurate out of Pencil beam, convolution and monte carlo?
Monte Carlo
What are common photon algorithms?
- Pencil beam
- Convolution
- Superposition
- Monte carlo
What are the benefits of convolution/ superposition algorithms?
Accuracy close to Monte carlo but quicker
What are the two components of all convolution algorithms?
- represents energy imparted to the medium (TERMA)
2. energy depositied about a primary interaction site, the kernel
What are the two kernel types?
Primary (primary dose) and secondary ( first and multiple scatter doses)
What is the definition of kernels?
Dose at a point is calculated by summing the effects from scattering elements
summation=Kernel
What does TERMA+Kernel equal?
Dose
What are the Kernel shapes?
slab, pencil beam, or a point
What is the pencil beam kernel?
energy deposition in a medium from a narrow geometry beam ‘pencil beam’
What are the limitations of Pencil Beams?
Use a density scaling approach (Batho, ETAR) for inhomogeneities
-Doesn’t account for increased electron lateral scatter in air.
What is the convolution dosimetry?
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
What is the superposition method?
Variant of the convolution
What are the effects on inhomogeneity for superposition?
Increased dose in the lateral direction
- Reduction of dose on the central beam
- TPS generally overestimates the dose adjacent to and in air cavities.
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)
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
What are the disadvantages of convolution superposition?
Time consuming
Beam commissioning measurements (empirical parameters need to be determined)
What are some convolution algorithms?
CC convolution
Pencil beam
AAA (uses spatially variant convolution scatter kernels and separate modelling for photons and electrons)
What are common algorithms for electrons?
Clarkson
Pencil beam
Electron Monte Carlo
What is different for electron modelling
More difficult
Side scatter of electrons is more pronounced
Why shouldn’t measured beam algorithms be used?
dangerously wrong in anything but a water phantom
Should Pencil beam algorithms be used?
OK but not trusted
What is the only reliable method for electrons?
Monte Carlo
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
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
How do Monte carlo simulations work?
Different MC codes make different assumptions to increase speed and accuracy
What is Monte Carlo simulation subject to?
statistical uncertainty in dose, proportional to 1/n^1/2
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).
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
How can Monte carlo affect planning?
Noise can affect DVH
Post-processing in form of DVH deconvolution
What characteristics for PB?
Fast
Accounts for scatter
Poor representation of inhomogeneity effects
Assumes monoenergetic beams
-Struggle to deal with irregular contours
What are the characteristics for Conv/Sup
Accounts for inhomogeneities
Accounts for spectrum of energies
Accounts for beam modifiers
Relatively Slow
Difficult to commission
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
