Clinical Beam Characteristics Flashcards
(36 cards)
State 3 methods of describing MV photon beams
1) Depth dose
2) Beam profile
3) Isodose curve
Define percentage depth dose
The ratio of absorbed dose at a chosen depth to the absorbed dose at a reference depth (typically d_max).
Describe the percentage depth-dose curve of a 6 MV photon beam
The dose isn’t 100% at the surface due to the buildup region.
The curve is approximately exponential following d_max.
The change in the dose of an MV photon beam with depth beyond d_max is reduced by the ________ ______ ___ and by _________ and _________.
Inverse square law
Attenuation
Scattering
Give the equation for the variation of dose with depth of a clinical beam (beyond d_max)
D_d = dose
D_0 = dose at reference depth
SSD = source to surface distance
d_max = depth at maximum dose
Absorption = attenuation + scattering
Why is the absorption of an MV photon beam not perfectly exponential?
Because of scattering
Define tissue maximum ratio
The ratio of the dose at a given point to dose at the depth of maximum dose. This is a special case of the tissue phantom ratio.
Describe the difference between the change in relative depth dose with field size and the change in relative TMR with field size
The difference between the two curves is due to the inverse square law.
The change in relative depth dose/TMR with field size is equivalent to the change in actual depth dose/TMR with depth.
What is the penumbra width of a beam profile?
The region at the edge of a radiation field where the dose rapidly falls off from near-maximum to near-zero. It is the distance between the 80% and 20% isodose lines.
How can beam profiles be used to describe beam characteristics?
50% of a normalised beam profile equals the geometric width of the beam.
What is an isodose curve?
A line or segment where all points receive the same dose of radiation.
What is the isocentric definition of a monitor unit (MU)?
The monitor chamber reads 1 MU when 1 cGy is delivered to a point at a given depth in the phantom, with the phantom positioned so that the specified point is at the isocentre of the machine and the field size is 10cm x 10cm at the isocentre.
State 1 use of isodose curves
To visualise dose distribution in radiotherapy planning.
State 4 ways to characterise a kV X-ray beam
1) Spectrum (energy fluence)
2) Spectrum (kerma)
3) Change in penetration with filtration
4) Half value layer
Define energy fluence
The total energy of particles crossing a unit area within a given time.
What is kerma?
Kinetic
Energy
Released in
MAtter
(Kinetic energy released per unit mass)
This is the sum of the initial kinetic energies of all the charged particles liberated by uncharged ionizing radiation.
Which 2 materials are used to measure the HVL of kV X-ray beams
Aluminium for low energies
Copper for high energies
Define half value layer
The thickness of material required to reduce beam intensity to half its unattenuated value in the absence of scatter.
State 2 ways to characterise an electron beam
1) Depth dose curve
2) Beam shape
What is the practical range of an electron beam?
5E mm
State 4 measurements that can be carried out to specify the energy of an MV beam
- Depth of 80% isodose for a 10x10cm field at 100cm SSD (d80)
- Percentage depth dose at 10cm deep (PDD10)
- Tissue phantom ratio 20/10 (TPR20,10)
- Depth of dose maximum
Describe the difference between a flattened and unflattened (FFF) beam
A _________ beam is the standard way of treating patients when using MV photons.
Flattened
What material are flattening filters typically made of?
Steel or heavy metals like tungsten
