Chapter 9 Kahn: The Physicso of Radiotherapy
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What is percentage depth dose?
- Is defined as the quotient of the absorbed dose at any depth d to the absorbed dose at a fixed depth d0 along the central axis of the beam, expressed as a percentage. P=[D(d)/D(d0)]x100
- For orthovoltage and lower voltage, reference depth is at the surface d0=0.
- For higher energies, reference depth is at the position of the peak absorbed dose (maximum dose) d0=dmax
What is the dose buildup region?
- The region between the surface and the point of maximum dose dmax.
- As higher energy photons enter the patient, high speed electrons are ejected from the surface and subsequent layers, depositing their energy at a significant distance away from their site of origin. The energy fluence and hence the absorbed dose increase with depth until they reach a maximum, but photon energy fluence and electron production continuously decrease with depth. Beyond a certain depth, dose begins to decrease.
What is the geometric field size?
The projection, on a plane perpendicular to the beam axis, of the distal end of the collimator as seen from the front center of the source. Usually defined at a predetermined distance such as the source to axis distance SAD.
What is the dosimetric field size?
The distance intercepted by a given isodose curve (usually 50%) on a plane perpendicular to the beam axis at a stated distance from the source.
The majority of the treatments encountered in clinical practice require rectangular and irregularly shaped fields, how do we solve this problem?
- We require a system of equating square fields to different field shapes. There is one method developed by Sterling et al for equating rectangular and square fields:
- A rectangular is equivalent to a square field if if they have the same area/perimeter (A/P)
- A/P=(axb)/[2(a+b)], where a is width and b is length.
- A/P=a/4 for square fields.
Increasing the beam energy from 6 MV to 18 MV does what to the percent depth dose curve?
The percent depth dose will decrease more gradually with increasing depth.
What happens to the percent depth dose near the source?
Although the dose rate at a point decreases with increasing the distance from the source, the percent depth dose -dose relative to a reference point- decreases more rapidly near the source than far away from the source.
What is the Mayneord F factor and how do we calculate it?
- The F factor gives us a quotient of the percent depth doses at a depth d if we have two different SSDs (f1, f2), and a square field size r (rxr).
- This is done without considering changes in scattering as the SSD changes.
We multiply the percent depth dose by the F factor to estimate the desired percent depth dose for the new SSD.
What is Tissue-Air Ratio?
- Is the ratio of the Dose (Dd) at a given point in the phantom to the dose in free space (Dfs) at the same point. For a given beam, TAR depends on depth d and field size rd at that depth:
- Because is the ratio of the two doses at the same point, it is independent of the distance from the source. Thus TAR represents modification of the dose at a point due only to attenuation and scattering of the beam compared with the dose at the same point in free space.
What is the Backscatter Factor?
- It is the tissue-air ratio at the depth of maximum dose on central axis of the beam, meaning the ratio of the dose on central axis at the depth of maximum dose to the dose at the same point in free space:
- BSF=Ddmax/Dfs=TAR(dmax,rdmax), where rdmax is the field size at dmax.
- It is too independent of the distance from the source and only depends on beam quality and field size.
What is the relationship between TAR and percent depth dose?
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