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40 Cards in this Set
- Front
- Back
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Convert from Exposure to Absorbed Dose
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D med/max = f med (X) BSF(r)
r = field size fmed = conversion factor X = exposure |
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What effects backscatter?
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1. It increases with Field Size
2. Increase is higher with square fields than rectagular 3. It increases with beam quality until you reach HVL of 0.8-10.0 mm Cu and thuis decreases as energy increases then 4. It is indepdent of SSD |
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Peak scatter Factor equation
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PSF = (Dose in phantom at Dmax) / (Dose at same point in air)
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Back Scatter vs. Peak Scatter
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Peak scatter is useful for MV sources as maximum dose is at depth of Dmax and not the skin surface. Back scatter is only important at kV energies.
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Co 60 and Dmax
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0.5 cm
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4MV and Dmax
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1.0 cm
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6MV and Dmax
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1.5 cm
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10 MV and Dmax
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2.5 cm
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15MV and Dmax
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3.0 cm
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20 MV and Dmax
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3.5 cm
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25 MV and Dmax
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4.0 cm
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34 MV and Dmax
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5.0 cm
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What effects Dmax?
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1. Energy of primary photons
2. Field size |
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Percentage Depth Dose Definition
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The ratio of the dose along the central axis of the beam at a given depth divided by the dose at Dmax.
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What effects Percentage depth dose?
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1. Beam quality - as it increases along with energy, for a given depth PDD increases
2. Depth as depth increases, PDD decreases due to attenuation except in the build up region 3. As field size increases, so does PDD because of increased scatter 4. As SSD increases so does PDD |
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Mayneord Factor Definition
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The factor that helps calculate a PDD for a new SSD. It overestimates the PDD at large SSD.
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Maynard factor Equation
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F = ((SSD2 + Dmax)/(SSD1 + Dmax) x (SSD2 + d)/(SSD1 +d))^ 2
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Equation to calculate a new PDD based on a new SSD
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PDD newSSD = PDD oldSSD x F
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Tissue Maximum Ratio: Definition
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The ratio of the dose at d and the dose at Dmax.
TMR = (dose d) / (dose at Dmax) |
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TMR advantage over TAR
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TAR can not be calculated for energies above 3 MeV
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Isodose lines
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Lines representing radiation dose distribution where a line connects points of equal dose. The curves are usualy drawn at intervals and expressed as a percentage of the dose at a reference point.
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Isodose chart
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A set of isodose lines. It shows how the nergy of a radiation field is deposited within the patient.
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Light field
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A visible field projected onto the surface of a patient by LINACs whose edge is aligned with the 50% isodose line on machines
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Penumbra
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Region of rapid dose fall off on an isodose chart
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Physical penumbra
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The distance between the 90% and the 20% isodose line at Dmax
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What effects the shape of isodose lines?
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1. Beam quality - the depth of each line increases with energy. The penumbra width increases with decreasing increasing because of more side scatter.
2. Flattening filter produces flatness specified at 10 cm. Shallow to that horns are present. This effect is mollified by scatter produced in the treatment head. 3. Field size, large sizes have larger horns and smaller fields have isodose lines that are shaped like bells. |
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Definition of Flatness. Target value and equation.
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Dose of maximum and minimum points measured within central 80% of field width at 10 cm. A good field has a flatness of 5% or less.
F =((D max - D min) / (Dmax + Dmin) x 100 |
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Definition of Symmetry. Target value and equation.
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S = ((area Lt-area RT)/(area Lt + Area RT)) x 100
The left and right borders are measured between 80% of width. Target is 2% or less at Dmax. |
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Definition of wedge filter
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A block of wedge shaped material usually lead, copper or steel to change the standard isodose distribution. The thick end is called the heel and small end the toe.
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Why must wedges be atleast 15 cm from skin surface.
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It creates low energy contaminants which will reduce skin sparing.
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Definition and equation of wedge factor?
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As wedges decrease the output of a beam, this can be taken into account using a wedge factor.
WF = (dose with wedge beam for field size r ) / (dose without wedge in beam for field size r ) Usually measured at a reference depth 5 cm, 10 cm or Dmax. |
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How do you know if wedged field has had the wedge factor applied?
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The isodose line at Dmax is not 100%.
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What effects the wedge factor?
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Field Size and depth
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What Types of Wedges are there?
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1. Individualized - the thickness at the central axis changes with field size. Most common on Co 60. WF changes for field size. Toe of wedge set at edge of light field.
2. Universal wedge, thickness at central axis remains the same for all sizes. These wedges have set field limitations. Toe set beyond light field edge. |
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What important measures do wedges change?
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TAR/TMR/TPR/PDD
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What are some special wedge techniques?
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1. Asymmetric wedge
2. Flying wedge 3. Dynamic wedge |
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What is the definition of a dynamic wedge?
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Done by moving the collimator jaws across the field during treatment. Depending on the time spent at each location any angle wedge may be created. Segmented treatment table controls the LINAC to goveren jaw position and dose rate to create the wedge.
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Isocenter
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Point in space that is the same distance from the source for all gantry angles. Field size is set at the isocenter for all LINACs. The position of the isocenter is fixed.
LINACs 100 cm Co 60 - 80 cm |
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SSD technique: Definition
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The isocenter of the machine is placed on the patient's skin. Hence field size is defined on the patient's skin.
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SSD technique: Definition
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The isocenter of the machine is placed inside the patient at a proper depth. The field size is specified in the patient.
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