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Flashcards in Beam modelling Deck (35)
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1

What is beam commissioning?

determine a set of values for the adjustable parameters and generate calculated dose distributions

2

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

3

What are the two types of beam profiles?

Cross plane
In plane

measurements taken by the detector to characterise the LINAC

4

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

5

What is LINAC beam modelling based on?

-Central axis dose distribution
-Beam profile

6

What does beam modelling take into account?

Changes in PDD
Influence of side scatter-beam flatness

7

What does profile analysis take into account?

Field sizes
PDD
Wedges
Off axis factors

8

What is Beam modelling?

Involves the acquisition and comparision of LINAC and TPS profiles

9

What is profile analysis?

Overlaying TPS and Linac profiles

10

What is an Algorithm?

A procedure for solving mathematical problems involving a finite number of steps and involves repetition

11

What are the two ways inhomogeneities are included in the calculation?

Indirectly-Through a correction factor
Directly-Inherent to the algorithm

12

What are the two types of correction algorithms?

Correction/measurement based (indirect)

Model based (direct)-

13

What are correction based algorithms?

Rely on measured data in water to account for patient contour, internal anatomy and beam modifiers

14

What are Model based algorithms?

Use measured data to derive model parameters- predict the dose based on laws of radiation transport

15

What are the corrections for contour irregularities?

-Effective SSD methods
-TAR or TMR method
-Isodose shift method

16

What are corrections for tissue inhomogeneities?

TAR (tissue to air ratio)
Effective path lengths
ETAR (Equiv TAR)
Batho power law

17

What do inhomogeneities result in?

changes in the absorption of the primary beam

Changes in electron fluence

18

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

19

What does Effective path lengths model?

Primary dose variation
-Unreliable for regions of e- disequilibrium
-Best for dose calculation away from inhomogeneity

20

What does Batho Power law method account for?

primary beam attenuation and scatter changes in water

21

What does ETAR account for?

Uses water equivalent depth that correctly accounts for primary component of dose.
- Ignores changes in scattered dose.

22

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

23

For lung and bone correction what is PD dependent on?

Energy

24

What is the effect of PDD on entrance with bone?

Dose increase to adjacent tissue due to backscatter

25

What is the effect of PDD on exit with bone?

Forward scatter of electrons from bone

Build up of electronic equilibrium in soft tissue

26

What is ETAR best for?

Low energy beams (,6MeV)

27

What is TAR known for?

Overestimates the dose for all energy

28

What is Batho power law suited for?

Greater than 10MeV

29

What do correction methods depend on?

Energy
Field size
Location and size of inhomogeneity
Location of calculation point

30

What are the pinnacle treatment planning algorithms?

Fast convolve
Adaptive convolve
CC convolution