Beam modelling Flashcards
What is beam commissioning?
determine a set of values for the adjustable parameters and generate calculated dose distributions
Beam commissioning parameters that can be adjusted?
- Energy
- Off axis beam softening
- cone angle and radius
- transmission factor for photon fluence through collimators
- dimensions of photon source etc
What are the two types of beam profiles?
Cross plane
In plane
measurements taken by the detector to characterise the LINAC
What does PDD mean?
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
What is LINAC beam modelling based on?
- Central axis dose distribution
- Beam profile
What does beam modelling take into account?
Changes in PDD
Influence of side scatter-beam flatness
What does profile analysis take into account?
Field sizes
PDD
Wedges
Off axis factors
What is Beam modelling?
Involves the acquisition and comparision of LINAC and TPS profiles
What is profile analysis?
Overlaying TPS and Linac profiles
What is an Algorithm?
A procedure for solving mathematical problems involving a finite number of steps and involves repetition
What are the two ways inhomogeneities are included in the calculation?
Indirectly-Through a correction factor
Directly-Inherent to the algorithm
What are the two types of correction algorithms?
Correction/measurement based (indirect)
Model based (direct)-
What are correction based algorithms?
Rely on measured data in water to account for patient contour, internal anatomy and beam modifiers
What are Model based algorithms?
Use measured data to derive model parameters- predict the dose based on laws of radiation transport
What are the corrections for contour irregularities?
- Effective SSD methods
- TAR or TMR method
- Isodose shift method
What are corrections for tissue inhomogeneities?
TAR (tissue to air ratio)
Effective path lengths
ETAR (Equiv TAR)
Batho power law
What do inhomogeneities result in?
changes in the absorption of the primary beam
Changes in electron fluence
What does TAR take into account?
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
What does Effective path lengths model?
Primary dose variation
- Unreliable for regions of e- disequilibrium
- Best for dose calculation away from inhomogeneity
What does Batho Power law method account for?
primary beam attenuation and scatter changes in water
What does ETAR account for?
Uses water equivalent depth that correctly accounts for primary component of dose.
- Ignores changes in scattered dose.
What does the lung correction do?
- 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
For lung and bone correction what is PD dependent on?
Energy
What is the effect of PDD on entrance with bone?
Dose increase to adjacent tissue due to backscatter
