Absorbed Dose Distributions Flashcards
What is the Absorbed Dose Formula
D=E/m (Dose = Energy/mass)
PDD
Percentage Depth Dose Distributions. Dependent on energy.
Off-axis Profiles
Beam profiles.
Dependent on energy, field size and normalization.
Contain a penumbra and horns.
CAX
Central Axis
Beam Attenuation process
Beam enters surface = surface dose = Ds
Dose rapidly rises under surface = maximum dose = Dmax at max depth dmax
Dose exits patient = exit dose = Dex
Skin Sparing Effect
In MV X-Ray beams the Ds is lower than the Dmax. Skin sparing effect improves with higher energy MV beams.
Typical Surface dose values (4)
- Superficial + orthovolatge = 100%
- Colbolt-60 gamma rays = 30%
- 6 MV X-ray beams = 15%
- 18 MV X-ray beams = 10%
3 Contributors to Surface Dose
- Photons scattered from collimators, flattening air filter and air.
- Photons and electrons back scattered from inside the patient.
- Electrons produced by photon interactions in air.
Build up Dose Region
Region between the surface (d=0) and depth of max dose (dmax) in MV photon beams.
2 Contributors to depth of Dmax
Photon beam energy and field size.
PDD Graph
Lines displaying build up dose going up then down.
Beam Profile Graph
Multiple square shaped lines showing relative dose in comparison to distance from central axis. Depth labels for lines. PDD line down center.
How to measure field size on a beam profile graph
Distance on x-axis between two 50% dose points.
What is a Penumbra
The region where dose changes rapidly.
How to measure a penumbra on a beam profile
The x-axis distance between the 20% and 80% dose points.
What are the 3 components to a penumbra?
- Geometric (finite source size)
- Scatter (in patient photon scatter)
- Transmission (beam transmission through collimator)
What are horns?
Slightly higher dose regions near the edges of a beam.
What is the purpose of horns?
Give a more uniform dose profile at greater depths by compensating for:
- lower photon energy in edges of beam
- increased scatter at edges of beam
Isodose Curves Graph
Lines connecting points of equal dose in an irradiated medium. Drawn at regular intervals of absorbed dose.
What 2 Normalization Conventions are in use with Isodose Curves
- Dose maximum normalized to 100%
- Isocenter normalized to 100%
