Characteristics of Clinical Beams: Photons Flashcards
What happens to the isodose plot in the patient as the energy of the beam increases?
Greater penetration
Less side scattered dose - loss of side lobes on the isodose plot.
How is a percentage depth dose (PDD) curve obtained from and isodose plot?
Linear profile of the dose down the central axis of the plot.
How does an increase in energy affect the PDD?
- Greater depth of d(max)
- Shallower drop off after d(max)
- Shallower build up region
- Lower entrance surface dose
What are the major characteristics of a clinical photon beam?
- Build up region
- Point of max dose
- Normalisation point (5, 7 or10cm, depends on centre)
- Dose reduction with depth
- Penumbra
- Beam profiles at depth
- Dose from scatter (linac head and patient) and leakage
What causes the build-up effect in a clinical beam?
- Photons interact at different depths in the tissue and generate secondary electrons.
- At each interaction, the recoils electrons travel (mostly) forward and deposit dose
- As more tracks overlap, the dose builds up until Charged Particle EquillibriumM (CPE) is reached.
- A steady state would be reached if there was not photon attenuation (absorption or scatter)
- Dose>0 at the surface due to back0-scattered electrons and electron contamination.
How dose increasing the beam size affect the PDD?
Increases the dose at depth due to:
- More photons reaching the patient from the source
- More scattered electrons to the measurement point from the irradiated volume.
How does increasing the SSD affect the PDD?
Increases dose at depth > d(max) (i.e. shallower drop-off)
Absolute dose would decrease with increase of SSD, but percentage dose relative to a fixed point increases
Define the Tissue-Phantom Ratio.
Ratio od the dose obtained at a fixed field size and SSD, but with different thicknesses of “tissue” between the surface and the detector.
How does the TPR vary with increasing field size?
- Steeper build-up region
- Lower depth of TPR(max)
- Shallower fall-off after TPR(max)
Why is there a steeper fall-off from the TPR compared to a PDD?
- TPR does not have an Inverse Square Law effect
- Scatter conditions of the TPR and PDD are different.
How is the beam size defined on a beam profile?
Defined at the Full-Width Half-Maximum
How are the penumbra defined on a beam profile?
Difference between the 80% and 20% dose points.
How does the beam profile change with increasing depth?
- Beam and penumbra widen
- Flattest beam at 10cm (with flattening filter)
- Before 10cm beam is “horny”
- After 10cm beam is peaked
- Horniness and Peakedness increase with distance from 10cm depth.
- Dose is reduced as with PDD.
How is the wedge angle defined?
Wedge angle is the angle between the isodose line and the normal to the central axis at 10cm deep.
What are the three causes of penumbrae in a clinical beam?
- Geometric
- Transmission trough collimators
- Scatter in Patient.