Beam modelling simplified Flashcards
What is beam commissioning?
Adjust a set of parameters to calculate dose distributions
What is beam modelling?
- Acquisition of LINAC and TPS profiles by taking into account PDD, Output, Flatness and symmetry
- Modelled based on CAX distance and beam profiles
What are beam profiles?
Measure the relative dose against the corresponding central axis distance
What are PDD?
Quotient expressed as a percentage of absorbed dose at any depth (Dd) relative to an absorbed dose at a fixed distance (d0)
What is profile analysis?
Comparision of LINAC and TPS profiles by taking into account field sizes, depth and off axis factors
What are the impacts of tissue inhomogeneities?
Change in absorption of the primary beam and electron fluence.
How can tissue inhomogeneities be corrected?
Two methods:
Indirectly-correction method
Directly- inherent to algorithm
What are correction methods for contour irregularities?
Extended SSDs
TAR or TMR
Isodose shift method
What are correction methods for tissue inhomogeneities?
TAR
ETAR
Batho power Law
Effective path lengths
What is the TAR method?
- Ratio of dose at a given point in a phantom to the same point in free space.
- Accounts for the depth of calculation point and field size
- Adjusts dose beyond inhomogeneity by assuming it causes the same attenuation of the beam equivalent to water (density and thickness)
- Doesn’t account for scatter
What is ETAR?
Equivalent TAR accounts for alteration in scatter by caluclating the sum of densitiy in each pixel
(similiar to TAR but longer, more accurate and better for low energies <6MeV)
What is Batho Power Law?
Applies a correction factor based on electron density of inhomogeneity
-Accounts for incoherent scatter (>10MeV)
What is the effective path length method?
- Models primary dose variation, best for large distances away from inhomogeneity
- Compares the water equivalent tissue ability to attenuate the radiation by the same amount of normal tissue.
- Doesnt take into account scatter, tissue thickness or density
What are some characteristics of lung correction?
- Lower density creates higher dose beyond lung (Build up)
- Increasing electrons outside the beam reduce side scatter
- loss of side scatter reduces dose on CAX
- Significant for small fields and high energy beams (PD dependent on energy)
Effects of bone on entrance?
Dose increased to adjacent tissue due to electron backscatter
Effects of bone on exit?
Forward scatter of electrons from bone
Build up of electronic equilibrium in soft tissue
Other factors of bone?
Increased dose on boundary, attenuation and scatter
decreased %DD
Effects of air?
Decreased Attenuation, dose on boundary and scatter
Increase %Percentage dose
What are the pinnacle algorithms?
Fast convolve-Fast not used for treatment
Adaptive convolve
CC convolution-Not fast but accurate
What are the Monaco algorithms?
Monte Carlo-Not fast very accurate
Collapsed Cone algorithms
-Pencil beam-Fast but not good with airgaps
What is the CCconvolution-superposition method?
- Calculate primary and scattered radiation separately
- For each voxel, attenuation factor is calculated and applied to the primary beam
- A kernel represents the average scatter from the same point is then applied to to each voxel, weighted accordingly to the corresponding amount of primary radiated attenuated at that point
- Sums primary and scattered radiation to be accurate.
What is the Monte carlo method?
- Models the fate of a single photon as they enter the patient or phantom
- Attenuation and scattering of photons and resulting electrons is taken into account by mapping their paths and calculating an average dose distribution.
What is a calculation grid?
- Dose calculated for points throughout the patient model
- Compromise between resolution and calculation
- Can alter grid (Coarse to fine)
- Isodose distributions by joining grid points of equal value and interpolating dose values between them.
What does Adaptive convolve do?
Speeds up dose calculation by adaptively varying the spatial resolution of the dose grid
What does fast convolve do?
Uses fewer ray directions or collapsed cones, should not be used for MU calculation
