Monte Carlo Modelling

Question 1

What are the 2 types of basis for the TPS? Describe them.

Answer 1

Correction Based - Take measurements for a variety of fields. The TPS then selects the one most like the patient plan and applies corrctions for it.
Model Based - Take a few measurements for basic field, then rely on the modelling of the fundamental physics to make a plan

Question 2

How does a Monte Carlo algorithm differ from a Las Vegas algorithm?

Answer 2

It defines a maximum number of selection, k. Therefore the algorithm stops when the solution or k is reached

Question 3

Why are Monte Carlo algorithms considered the ‘Gold Standard’?

Answer 3

They give the most accurate modelling of photon interactions with the fewest assumptions

Question 4

What is the history of a particle?

Answer 4

The transport of the primary particle and all of the secondary particles that it creates - the number of histories is in reference to the number of calculation repetitions made

Question 5

What is the track of a particle?

Answer 5

The path taken in matter by a particle until in is absorbed - taking a number of steps

Question 6

What 4 components does a Monte Carlo simulator need?

Answer 6

Random number generator
Physical interaction model
Geometry
Scoring and Stats

Question 7

What is the random number generator used for?

Answer 7

Picks a number between 0 and 1, which are assigned to random variables. These variables have a probability of being chosen which is defined by the probability density function

Question 8

What does the physical interaction model contain?

Answer 8

The physics of individual particle interactions
Mathematical method of describing interactions
This is the most important aspect for characterising the radiation transport

Question 9

What is contained in the geometry?

Answer 9

Mathematical description of the bodies through which particles are transported - ie the linac head and the patient

Question 10

What are the definitions of scoring and statistics?

Answer 10

Scoring = accumulation of physically realistic data during simulation, with the results based entirely on probability of occurrence of a particular event
Stats = the associated uncertainty, which derives meaning from the simulation

Question 11

How is MC modelling used in Radiotherapy?

Answer 11

Used to determine kernels in TPS algorithms
Used to calculate the beam leaving the linac head and dose limiting devices - the phase space of the beam
Determine dose to the patient as each history gives dose to the voxel

Question 12

What are the stages for characterising the particle history?

Answer 12

Free path between events
Interaction
Energy loss and angular deflection
New particles

Question 13

What is phase space defined as?

Answer 13

Each particle’s position, energy, and direction as it leaves the head - defined above modifiers such as the MLCs

Question 14

What is electron energy transport and why is it needed?

Answer 14

There are too many particles to model them explicitly, take a condensed history where at each step electrons lose energy by a set amount to many collisions and radiative losses, and are scattered by many coulomb scattering events

Question 15

What are the uncertainties associated with the modelling of photon and electron transport in the patient?

Answer 15

Goal = 2%/2mm, can be fine tuned to 1% for a machine
In reality: uncertainties of 2-3%
So get accuracy of 5%/5mm

Question 16

How can the time taken in a simulation be reduced?

Answer 16

Decrease the calcuation time to achieve a defined statistical variation
Decrease the statistical variance to achieve a lower simulation time

Question 17

What are the methods of variance reduction techniques?

Answer 17

Tansport cut off - stop tracking particles when their energy is < threshold, assume all energy deposited at the last point
Bremsstrahlung splitting: Divide single photon event into several photon events
Russian Roulette: Combine several electron events into a single event
Geometry interrogation: How close is it to a density boundary?
Zonal discard: Does the electron have the energy to escape the voxel?

Question 18

In transport cut-off, what are the thresholds for photons and electrons?

Answer 18

Photon: 0.005-0.1MeV
Electrons: 0.1-1MeV

Question 19

How does the model change for an electron applicator?

Answer 19

Have source phase space of air fluence
Energy and direction component propagate through applicator to generate exit phase space - made up of direct and indirect electrons and Bremsstrahlung photons generated in head
Can include custom inserts

Question 20

How can simulation times be decreased?

Answer 20

Change the number of histories/cm^2
Change the calculation density matrix
No right answer just what gives acceptable plans