Standard Calibration Units
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
Application of Correction Factors
- 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
Examples of Correction Factors
- Output Factor
- Wedge Factor
- Transmission Factor
- FSD Factor
Output Factor
- 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
Equivalent Square
- 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
Formula for Equivalent Square calculation
Area = 2ab/(a+b)
Transmission Factor
• 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
FSD Factor
• 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
What causes Beam Intensity to Reduce?
• The beam intensity reduces as it passes through the body due to:
- Attenuation processes
- The inverse square law
Importance of knowing %DD
- 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
What is %DD
- 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)
Equation for %DD
%DD = (Absorbed dose at depth / Absorbed dose at reference depth) x 100
Reference Depths for Linear Accelerator Beams
- 4MV : 1 cm below surface
- 6MV : 1.5 cm below surface
- 8MV : 2 cm below surface
- 10MV: 2.5 cm below surface
When does %DD increase?
%DD increases with increasing beam energy as higher energy beams have greater penetrating power
Central Axis Dose Depth Charts
- 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
Need for Tissue Maximum Ratio
- 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.
Tissue Maximum Ratio Calculation
• 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
What do TMR’s account for?
TMR’s only account for changes in tissue thickness whereas PDDs account for changes in tissue thickness AND distance from the source
Inhomogeneity Factor
- 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
Weighting Factor
- 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
Shielding
- 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
