Different Types of Algorithms
• Segmentation
• Advanced margin
• Image Registration
• Inverse Plan Optimisation (specifically for IMRT or VMAT)
• Dose Calculation
Types of TPS and their corresponding Linac
• Monaco = Elekta TPS
• Eclipse = Varian TPS
• Pinnacle3 = Phillips
Values required for TPS Commissioning
- Output Factors
- Depth Dose Data
- A large proportion of data is measured on the central axis
Two Components of Dose
- Primary radiation (Dprim)
- Scattered radiation (Dscat)
Does Penumbra alter the accuracy of the dose calculation algorithm?
Not all algorithms can calculate dose accurately in the penumbra region
Different Classes of Algorithms
Factor-Based Algorithms
Model-Based Algorithms
Factor Based Algorithms
- Based on measured data
- Example: Clarkson’s Technique
Model Based Algorithm (Parts and Examples)
Consists of two parts:
- a part of the algorithm that models the beam, and provides a representation of the fluence distribution before the beam enters the patient
- a part that models the patient, usually based on a tomographic representation of the patient tissues
- Examples: Monte-Carlo, CCC, AAA, PB
What is the intent of algorithms?
To predict with as much accuracy as possible the dose delivered to any point in the patient
Speed and accuracy of most common algorithms
- PB (Least accurate, Fastest)
- Convolution (Intermediate)
- Monte Carlo (Most accurate, Slowest)
Examples of Photon Algorithms
PB, Convolution, Superposition, Monte Carlo, AcurosXB
What is a Convolution?
- Product of two functions to create a third
- Dose at any point can be calculated from the convolution of TERMA and kernel
- Dose is a product of TERMA and Kernel
Main Components of a Convolution
- energy imparted to the medium by the interactions of primary
photons, called TERMA - The energy deposited about a primary photon interaction site, the kernel.
Main Parts of a Kernel
- The primary kernel calculates the primary dose
- The scatter kernel calculates the first and multiple scatter doses.
What is a Scatter Kernel?
The summation of effects from scattering elements to calculate dose at a desired point
Scatter Kernel Shape
Tear-Drop Shape
What are some Variants of the Convolution Method?
- Super position or Convolution Superposition Algorithms
- Used by the TPS ‘CMS XiO’
- Variant of the Superposition-Convolution Algorithm = CCC
PB Algorithm
- Still clinically used for IMRT/VMAT optimisation (not dose calculation)
- Used in the first stage of IMRT/VMAT optimisation (finds initial set of control points)
- Faster speed of PB is preferable to CCC or MC
Pencil Beam Convolution Algorithms
- anisotropic analytical algorithm (AAA) used by Eclipse TPS is based on PB-Convolution technique
- AAA uses spatially variant convolution scatter kernels, from Monte Carlo simulation, to separate modelling for primary photons,
scattered photons, and contaminant electrons. - AAA is attractive option for routine clinical use because of its relatively short computation time and accuracy comparted to Monte Carlo
Electron Algorithms
- Dose calculations are more complicated than photons
- Side scattering of the electrons is more pronounced and has more influence on the dose distribution
Algorithm Examples
- Clarkson
Clarkson Algorithm
- Used on the CMS-XiO TPS
- Measurement based method
- Not accurate dose calculation
Pencil Beam model for Electrons
- Pinnacle3 uses PB for electron
- Has problems in regions of inhomogeniety
-Tends to underestimate the effects of sharp discontinuities in density
Monte-Carlo Dose Calculation Simulation Process
⇒ 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).
Monte Carlo Simulation
- Is a stochastic integration method
- Monte Carlo calculated quantities are subject to statistical uncertainties
- One must simulate an infinite number of histories for a zero uncertainty
