First Year Exam: TPS and Treatment Planning Flashcards
What does AAA stand for?
Anisotropic Analytical Algorithm
What kind of kernels does AAA use?
Pencil beam kernel
What is a kernel? How are they made?
Kernels are a known distribution of dose deposition of secondary electrons set in motion by the primary interaction(s).
They are pre-calculated using monte carlo codes.
What is the difference between a pencil beam kernel and a point kernel? Which is more accurate? Which is faster?
A pencil beam kernel calculates the fluence of the primary beam on the patient skin and sums the pencil beam kernels from skin to medium.
A point kernel uses point-by-point dose results at each interaction site/voxel. It involves first figuring out where the interaction occurs, then superimposing the point kernel onto the geometry.
Point kernel is more accurate. Pencil beam is faster.
What is the difference between convolution and superposition?
Convolution is a mathematical operation that shows degree of overlap between two functions (Ex. TERMA and Kernel). It does not involve heterogeneity corrections.
Superposition uses density to scale the kernel prior to the convolution. Allowing it to account for heterogeneity.
What are the inherent limitations of kernels?
They have radial symmetry and do not account for side scatter very well. They do account for forward scatter well however. They are pretty simplistic.
What is the Linear Boltzmann Transport Equation?
A function that describes the macroscopic behavior of radiation particles traveling through matter.
What assumptions does the LBTE make?
Particles only interact with the medium and not with each other
No magnetic fields
In broad terms, what does LBTE mean?
It’s simply balancing the particles to say
Particles transported + those absorbed = total number of source particles
What is the goal of Acuros?
To discretize the LBTE and directly solve for it
How do you improve the accuracy of your monte carlo calculation?
Simulate a larger number of particles and histories
How do you improve accuracy of your acuros calculation?
Minimize discretizations
What is a “history”?
A single particle and all of its descendants
what is the equation for noise in a monte-carlo calc?
1/sqrt(N) where N is the number of simulated particle histories
What is a phase space file?
It’s a precalculated file describing radiation emanating from the machine head. It can be used for all calculations to follow, assuming that radiation from machine head does not change.
What does monte-carlo calculate, that most other calc algorithms do not?
Monte-carlo calculates dose to medium
All other algorithms calculate dose to water
What are the pros of using Dw over Dm?
All historical prescribing is based on Dw
Accelerator reference dosimetry is based on Dw
What are the pros of using Dm over Dw?
Dm is more clinically relevant
Accounts for heterogeneity in material by considering the density AND the effective Z, whereas Dw calculations only account by scaling density of water.
How do Dw and Dm account for hetereogeneity? How much of an error do you expect from these two methods?
Dw scales the density of water, ignoring the material type
Dm scales the density of the material and considers the effective Z
In tissue, error is 1-2%
In Bone, error can be as high as 15%
What is the general rule of thumb for determining calc grid size?
<= 3x3 cm2 field, you use 1-2 mm
> 3x3cm2 field, you use 2-3 mm
Give the general workflow of an acuros calculation
A) Transport fluence from source model to patient
B) Calculate scattered photon fluence in patient
C) Calculate scattered electron fluence in patient
D) Dose calculation
Step a is performed for all field orientations. Steps b-d are only performed once.
Up to what energy beams can Acuros be used to calculate?
25 MeV (well below our clinical use)
True or false, Acuros utilizes the same source model as AAA?
True
True or false, for Acuros dose can be calculated to either medium or water?
True
Recall for Acuros, you need to know the material anyway, meaning you already know the effective Z.