Wk02 Linac Flashcards
What is the source axis distance (SAD) of a Co-60 linac?
80 or 100 cm
What are the problems of Co-60 linac machines?
- Short half life (approx 5 years) —> need to change source every 5 year
- Difficult of dose calculation and computerisation
What are the different external beam radiotherapy (EBRT) modes and their machine respectively?
- Contact x ray = <50kvp x ray unit
- Superficial x ray = 50-150kvp x ray unit
- Orthovoltage x ray = 150-400 kvp x ray unit
- Teletherapy = Co-60 unit, Cs-137 unit
- MV x rays = linac
- MV electron beam = linac
- Hadron therapy = cyclotron / synchrotron / synchrocyclotron
When high speed electrons hit the high Z-material target, most of the electron’s kinetic energy is transformed into _____?
Heat
X ray photons which formed by X ray tubes are divided into which two categories?
- Characteristic x rays
- when incident electrons ejected a K-shell electron - Bremsstrahlung photons
- distant interaction with the nuclei —> low energy
- close interaction with the nuclei —> moderate energy
- direct impact with the nucleus —> max. Energy
The relative proportion of the number of characteristic photons to bremsstraahlung photons in an x ray beam spectrum varies with:
- Kinetic energy of the electron beam striking the x ray target
- Atomic number of the target
High energy x ray machines have __________ targets; low energy x ray machines have _________ type targets.
Transmission ; reflection
diagnostic range photons are produced at ____ degrees from the direction of electrons striking. While in megavoltage range, photons are produced at ____________ direction of electrons striking.
90; the same (180 degree)
Name 4 x ray beam qualities specifiers.
- Complete x ray spectrum
- Half-value layer (HVL)
- Tissue-phantom ratio (TPR)
- Percentage depth dose (PDD)
What is the term ‘beam quality’ is used to indicate.
The ability of a beam to penetrate a water phantom.
What is linear accelerator (linac)?
A device use high frequency electromagnetic waves to accelerate electrons to high energies through a linear tube.
What are the major components of a linac?
- Drive stand
- Klystron / magnetron
- waveguide
- water cooling system - Gantry
- electron gun
- accelerator structure - Treatment head
- bending magnet
- flattening filter
- scattering foil - Imaging systems
- MV/ KV imaging
What are the components of a treatment head under x ray therapy mode?
- x ray target
- primary collimates
- flattening filter
- ion chamber
- secondary collimated
- slots for wedges / blocks / compensators
What are the components of a treatment head under electron therapy mode?
- primary collimating
- scattering foil
- ion chamber
- secondary collimating
- accessory mount
- electron applicator
Modern linacs collimating are achieved with which 3 devices?
- Primary collimator
- provides fixed maximum aperture for the x ray beam - Secondary collimator (X and Y jaws)
- tungsten jaws
- 1cm x 1cm to 40cm x 40cm - Multileaf collimator
- each lead are driven by motor independently
- used in IMRT and VMAT
What is the function of a flattening filter?
The profile of x rays generated at the target has a cone shape distribution, metal flattening filter turns the cone shape profile into a flat profile, to facilitate manual and forward computer planning.
What are the benefits of flattening filter free (FFF) x ray beam?
- Increase output efficiency
- a large % of the x ray beam produced is absorbed by the flattening filter originally - Increase output dose rate
- increase from 400-600 to over 1200
What is the function of in chamber in linac?
- Determine treatment beam dose
- Redundant check of dose rate and integrated dose
- Allow feedback for flatness and symmetry
Why the edges of multi-leaf collimator are rounded?
To minimise transmission penumbra
What are the problems with wedged beam?
- Reduce dose rate
- Slow down treatment flow
- Limits the useable maximum field size
What are the components that achieved electron beam collimating?
- Primary collimator
- Secondary collimator
- Electron applicator (Cone)
- Electron inserts
What are the difference between electron and photon beam collimation?
- X ray target is removed
- The flattening filter is replaced by a scattering flattening filter foil to produce a broad electron beam
- the use of electron applicator
- Electron beam current is only 0.1% of that for x ray beam.
What do the optical light field simulates?
The geometry of the radiation beam
What are the functionalities of on-board imaging (OBI) system?
- Radiographic imaging
- Fluoroscopic imaging
- Cone beam CT (CBCT)
What are the characteristics of linac photon beams
- surface dose
- buildup region
- %PP
- Dmax
- dmax
What contributes to the surface dose?
Surface dose is caused by secondary electrons produced by interaction with air and objects like flattening filter, mirror and collimator
The __________ the energy beam, the __________ the buildup effect
Higher; larger
What it the definition of percentage depth dose (PDD) curve?
It is a measure of the variation of absorbed dose at different depth (d) along the central axis of the beam in water
Surface dose __________ with beam energy, ________ with field size
Decreases ; increases
For nominal beam energy (4-10MV), what is the equation to calculate the depth of maximum dose (dmax)?
Energy / 4
What is the manufacturer specification of photon beam energy?
The percentage depth dose (PDD) at 10cm depth in water with beam size 10cm x 10cm.
What is tissue phantom ratio (TPR)20,10
The ratio of doses on the beam central axis at depths 20cm and 10cm in water obtained with a constant SAD = 100 and FS = 10 x 10 at the position of the detector
What is the definition of field size
Width of the beam at 50% dose level
What is the definition of beam penumbra
Distance between 20% and 90% dose level in the beam profile
Penumbra width contributed and affected by________________.
- Physical field size of radiation source
2.geometry of collimator - Energy of radiation beam
- Medium of beam travelled
- Depth of measurement
- Scattered radiation
What is beam flatness?
F = 100 x [(Dmax-Dmin) / (Dmax+Dmin)]
With Dmax and Dmin values taken within 80% of the beam width and profile measured at 10cm depth
What is beam symmetry
S = 100 x [(left area - right area) / (left area + right area)]
Output factor ____________ with ___________ field size
Increases ; increases
Why electron beams have lower penetrating power than photon beams?
An electron will start interacting with the medium immediately, including orbital electrons and the nucleus. Due to strong Coulomb force interactions, electrons lose energy much faster than photons
Electron beams have _____________ surface dose than that of photon beams
Higher
The rate of electron energy loss per cm is _______________ and to the _________________.
Proportional to electron energy ; square of atomic number of medium (Z2)
What is R50
Electron beam quality is specified by R50.
R50 is the depth of water at which the absorbed dose falls to 50% of the maximum dose
What are the sources of x ray contamination of electron treatment?
- Scattering filter
- Ion chamber
- Jaws
- Metallic walls of treatment applicator
- Cone and inserts
- The patient
X ray contamination cause tails in PDD curves of electron beams
