Beam Modelling & Inhomogeneity Corrections

Question 1

What is the goal of commissioning and clinical acceptance testing?

Answer 1

To determine a set of values for the adjustable parameters that generate calculated dose distributions.

Question 2

What are some examples of adjustable parameters?

Answer 2

1. a set of energies
2. off-axis beam softening
3. off-axis changes in the in-air photon fluence
4. photon fluence through the
   collimators.
5. Scatter from the flattening filter;
6. Scatter from the beam
   modifiers
7. The dimensions of the photon source
8. Electron contamination

Question 3

What is PDD?

Answer 3

Percentage of the absorbed dose at any depth ‘d’ to the absorbed dose at a fixed reference depth ‘do’ along the central axis of the beam.

Question 4

What is beam modelling?

Answer 4

Acquisition and comparison of the Linac and TPS profiles

What the treatment planning system provides compared to the actual recorded data from the Linac

Question 5

Profile Analysis?

Answer 5

• Field size
• Depth
• Wedges
• Off-axis factors
• As the depth increases the the difference between the TPS measured and recored increases

Question 6

What are algorithms?

Answer 6

A procedure for solving a mathematical problem in a finite number of steps that frequently involves repetition of an operation.

Question 7

Why do we need dose algorithms?

Answer 7

Measurements are performed under these conditions

• -> Fixed square field
• ->Fixed depth
• ->homogenous medium
• ->Flat surface

Don’t represent the real world!

Question 8

How is the effect of inhomogeneity calculated?

Answer 8

-Indirectly, through a correction factor.

OR

-Directly, inherent to the algorithm.

Question 9

Isodose distributions in patients are determined by?

Answer 9

1. Correction/measurement based algorithms

2. Model based algorithms

Question 10

What are types of corrections for contour irregularities?

Answer 10

• –> Effective SSD
• —>TAR (tissue air ratio)
• –> TMR (tissue maximum ratio)
• –>isodose shift method

Question 11

What are types of corrections for tissue inhomogeneities?

Answer 11

• ->TAR (tissue air ratio)
• –>Effective path lengths
• ->Batho power law method

Question 12

What is the TAR (tissue air ratio method)?

Answer 12

-Ratio of dose (Dd) at a given point in phantom to the dose in free space (Dfs) at the same point.

Question 13

What is the TAR equation?

Answer 13

CF=TAR (z-h, Aq)/TAR(z, Aq)

CF= Correction factor
z=depth
h=Thickness of missing tissue
A=Field size

Question 14

What is the effective path length?

Answer 14

•Models the primary dose variation.
•Unreliable for regions of e- disequilibrium (lung treated
with high energy photons).
•Best for dose calculation far away from inhomogeneity

Question 15

Why is optimising the linac performance important?

Answer 15

1. PDD- Beam Quality/Energy
2. Output
3. Flatness
4. Symmetry

Question 16

Which methods correct for inhomogeneities?

Answer 16

1. TAR method
2. Effective paths lengths
3. Batho power law method; 4. Equivalent TAR method

Question 17

What is the Batho power law method?

Answer 17

Accounts for both primary beam attenuation and scatter changes in water, below a single inhomogeneous slab.

Question 18

What is a limitation of the Batho power method?

Answer 18

Works well below a large inhomogeneous layer with e- density less than that of tissue.
􏰀 If the e- density is greater than that of tissue, the method over-estimates the dose.

Question 19

What is the equivalent TAR method (ETAR)?

Answer 19

correctly accounts for the primary component of dose

􏰀 It ignores changes in scattered dose

Question 20

What is a limitation of the equivalent TAR method?

Answer 20

It ignores changes in scattered dose