Monitor Units

Question 1

Standard Calibration Units

Answer 1

The conditions in which LA’s are calibrated give a particular absorbed dose under

For an LA the dose rate on central axis is 1GY/100 MU (or 100cGy/100MU) at d-max, for a 10x10 cm field at 100 cm SAD

Question 2

Application of Correction Factors

Answer 2

• Once absorbed dose has been calculated, it is converted to a MU setting
• Machine is calibrated using standard calibration units
• Any deviations from the standard calibration require the application of correction factors

Question 3

Examples of Correction Factors

Answer 3

• Output Factor
• Wedge Factor
• Transmission Factor
• FSD Factor

Question 4

Output Factor

Answer 4

• Because the dose from the radiation depends on the contribution of scatter, this needs to be compensated for by using an output factor
• Increase in field size increases %DD, therefore if field size is >10cm square, factor needs to be applied which will reduce the MUs we need to set

Question 5

Equivalent Square

Answer 5

• We define for each rectangular field an equivalent square field that produces the
  same proportion of scatter, and so the same percentage depth dose
• Rather than produce tables of data for thousands of different rectangles, we can
  produce data for a few equivalent square fields

Question 6

Formula for Equivalent Square calculation

Answer 6

Area = 2ab/(a+b)

Question 7

Transmission Factor

Answer 7

• Anything placed in between the radiation beam and the patient will attenuate the
beam to some extent

• Each piece of equipment that could attenuate the beam (such as beam
modifiers), patient equipment needs a factor to increase the monitor units
accordingly

Question 8

FSD Factor

Answer 8

• The treatment machines are calibrated for a particular FSD, usually 100cm
• Treatments at different FSD’s need this taking into account when calculating the
applied dose
• This is done by using the inverse square law

• For a divisible factor…
• Factor = I(1) / I(2) = (FSD2)2 / (FSD1)2

Question 9

What causes Beam Intensity to Reduce?

Answer 9

• The beam intensity reduces as it passes through the body due to:

1. Attenuation processes
2. The inverse square law

Question 10

Importance of knowing %DD

Answer 10

• When planning radiation treatments, it is essential to know how much of the beam intensity is left by the time the beam has penetrated the tumor

Question 11

What is %DD

Answer 11

• Way of expressing the dose at a particular depth
• Ratio of absorbed dose at a depth (d) to the absorbed dose at a reference depth (dr) along the beam central axis
• Ratio is expressed as a percentage
• In other words, the absorbed dose at a depth (d) is expressed as a percentage of the dose at a reference depth (dr)

Question 12

Equation for %DD

Answer 12

%DD = (Absorbed dose at depth / Absorbed dose at reference depth) x 100

Question 13

Reference Depths for Linear Accelerator Beams

Answer 13

• 4MV : 1 cm below surface
• 6MV : 1.5 cm below surface
• 8MV : 2 cm below surface
• 10MV: 2.5 cm below surface

Question 14

When does %DD increase?

Answer 14

%DD increases with increasing beam energy as higher energy beams have greater penetrating power

Question 15

Central Axis Dose Depth Charts

Answer 15

• Informs us what percentage of the beam’s intensity (along the central axis) is left after passing through a certain depth of tissue
• This data can also be displayed as a central axis depth dose curve

Question 16

Need for Tissue Maximum Ratio

Answer 16

• The problem with percentage depth doses occurs when considering plans that have been isocentrically normalised.
• If the total percentage depth dose at the isocentre has been “scaled down” or normalised to 100%, it becomes difficult to calculate the applied doses needed for each beam.
• In effect, you would have to correct for the inverse square law. It is easier to use tissue-maximum ratios.

Question 17

Tissue Maximum Ratio Calculation

Answer 17

• The tissue-maximum ratio is defined as
(dose at depth / dose at dmax)

• Calculation point remains at a fixed distance from the source (i.e. Fixed FSD).
• As the depth increases, the surface moves towards the source
• Arranged in tables for field sizes and machines

Question 18

What do TMR’s account for?

Answer 18

TMR’s only account for changes in tissue thickness whereas PDDs account for changes in tissue thickness AND distance from the source

Question 19

Inhomogeneity Factor

Answer 19

• This takes into account the nature of the underlying tissue.
• If there is a lot of lung in the path of the beam, less radiation need to be given than if there is bone there.
• Planning computers apply these factors automatically using CT data

Question 20

Weighting Factor

Answer 20

• Not all fields will contribute the same amount of dose to the TD.
• A plan using 1 anterior and 2 post oblique fields will require more dose from the anterior to prevent a “hot-spot” at the posterior side.
• Planning computers take this into account automatically

Question 21

Shielding

Answer 21

• The use of shielding will affect the output factor
• There will be a reduced area for scatter to be contributed from.
• The equivalent square will be smaller. Because of this, more AD is needed
• Departments have protocols saying how much shielding requires a correction to be made, using MLCs this is calculated by the TPS