Medical Linear Accelerators Flashcards
What is SAD?
source to axis distance
What is the SAD in most modern C-arm linacs?
100 cm
Starting from Bending magnet list the components of the linac head for photon mode
Bending magnet –> Target (W) –> Primary collimator –> Flattening filter –> Monitor chamber –> Jaws (Secondary collimator) –> Multileaf collimator (MLC, also called tertiary collimator)
The number of MU registered in the
MU chamber that can be calibrated to dose; typically is set so that 1cGy is equal to 1 MU under some standard calibration conditions.
Define MU (Monitor Unit)
An MU is essentially one unit or one “tick” in the chamber that represents a certain amount of dose delivered.
In electron mode for LINAC, what is placed instead of flattening filter?
scattering foil
what materials are made of scattering foil? and what does it do?
low Z metal which scatters the beam to provide an approximately flat fluence across the face of the beam.
Starting from Bending magnet list the components of the linac head for electron mode
Bending magnet –> No target (No tungsten) –> Primary collimator –> Scattering foil instead of Flattening filter –> Monitor chamber –> Jaws (Secondary collimator) –> Multileaf collimator (MLC, also called tertiary collimator) –> Electron cone (applicator) which is usually positioned at 95 cm from the source (5cm from the patient skin)
What material do you use for beam-shaping devices for photon?
Wood’s metal (Cerrobend), density of 9.7 g/cm3, melting point of 70C.
Define penumbra
the dose profile across a beam does not have an extremely sharp falloff at the edge, but rather there is a “smearing” of the dose gradient. This is penumbra.
What is the equation of width (w) of penumbra in terms of SCD (source to collimator distance) and SSD (source to surface distance)?
Penumbra is composed of three components
geometric penumbra, transmission penumbra and scatter.
Describe the transmission penumbra
What are the 2 parameters that will increase penumbra width?
SSD and size of the source
