RT1 - Intro to equipment Flashcards
Difference between KV and MV systems?
KV
- Low cost
- Only Photons
- Simple Design & Operation
- Simple, shallow treatments
MV
- High Cost
- Photons & electrons
- Complex design
- Complex, deep treatments
List 4 basic KV tube components.
- Cathode
- Annode
- Filtration
- Applicators
Describe function of KV tube: Cathode.
- Electrically heated Tungsten (high z, high mpt)
- e- produced via thermionic emission
- Filament current (mA) determines:
- Tube current (mA ∝ Nphot/s)
- Intensity (I = Nphot/E ∝ mA)
- Focussing cup voltage (kV) determines:
- Max Ee- (kVp)
- I ∝ kVp^2
Describe function of KV Tube Annode
- Non rotating target @ ground potential
- De-accelerate electrons
- Energy lost via
- Collision (99%) - high mpt, water cooled
- Radiation (1%) - Characteristic x-rays ∝ Z^3 and brehmstrahlung ∝ Z^2
Describe function of KV Tube Filtration.
- Absorb soft x-rays
- Not useful for therapy becasue they only add skin dose
- Inherent filtering in target & Be window
- Additional Al or Cu filters to harden beam
Describe function of KV Tube Applicators.
- Circular/rectangular collimation
- Fixed SSD (15/25cm)
- Diameters 1-15cm
- Additional Pb cut-outs for conformity
Name and describe the two types of MV treatment systems.
Co60
- Fixed Energy (1.17MeV)
- Specialised e.g. Gamma-Knife
LINAC
- Multiple energies
- Better penetration
- Multi-modality
Name the 5 Basic Components (not the head components) of a LINAC.
- Modulator
- Electron Gun
- Microwave Generator
- Accelerating waveguide
- Bending magnet
- Head components
Describe the function of:
Modulator
- Two components
- Pulse forming network
- Thyratron (high-speed swithch)
- PRF 200-300Hz
- Supply approx. 1us pulse to cathodes to control:
- e- gun
- microwave generator
Describe the function of:
Electronn gun
- Heated element -> thermionic emission
- Cathode (20kV), concave shape to focus
- Grid: high speed switching of +- potential. Pulses are adjusted to phase match microwaves
- Annode @ 0V
- e-s injected into waveguide
Describe the function of:
Microwave Generation (2methods)
MAGNETRON
- Thermionic emission from cathode
- electrons follow spiral path due to (1) pulsed electric field and (2) permanent magnet
- E- field across cavities which determine resonant frequency
- e-s bunch at resonant frequency,
- circulating feedback to amplify power
- microwaves injected into waveguide
KYLSTRON
- Low power microwaves and electrons in 1st cavity
- Microwaves bunch electrons at input frequency
- Increased electric field density
- Microwaves injected into waveguide
Describe the function of:
Accelerating Waveguide
- Microwave power accelerates e- into MV
- Hollow tube where microwave phase velocity > c
- Phase velocity controled to keep electrons at particular phase
Buncher section:
- Initial cavities accelerate e’s up to 0.99c
- Electrons have varied phase at injection, varying acceleration leads to bunching
Drift section:
- Longer cavities to accomodate increasing velocity (~c)
- Velocity remains constant as mass increases
Describe the function of:
Bending Magnet
- MV Beams need long waveguides, which prohibits acceleration in vertical plane
- Horizontal beam deflected to pass through isocentre
- Different tytpes
- 270deg
- slalom
Order of head components:
photons (x7)
- Primary collimator
- photon target
- flattening filter
- Montior chambers
- Physical Wedge
- Secondary collimators
- Accessory holder
Order of head components:
electrons (x5)
- Primary collimator
- Scattering foils
- Montiro chambers
- Secondary Collimator
- Applicator
Head component function:
Primary Collimator
- Defines extent of treatment field
- High density material
- fixed geometry
- Surrounds photon target/ e- foil
Head component function:
Photon Target
- Transmission target (alloy with high z, high density, high mpt)
- Forward peaked beam
- Bremstrahlung
- High efficency
Head component function:
Flattening Filter
- Equalise intensity across profile
- High z, conical design
- Differential attenuation to offset forward peak
- Beam hardening
- Reduced dose rate
- energy specific height/thickness
- mounted on carousel
Head component function:
electron foils
- Broaden thin pencil beam
- Flatten beam
- Max scatter, Min attenuation, Min x-ray production
- Energy specific
Head component function:
Monitor chamber
- Transmission ion chamber
- Photons: sealed
- Electrons: unsealed
- Primary
- Dose rate (inner)
- Radial symmetry & flatness
- Secondary
- Dose rate (redundancy)
- Transverse symmetry & flatness
Head component function:
Secondary collimator
- Pairs of thick, high density jaws define 2D field
- MLCs
Head component function:
MLC
- 30-80 pairs leaf pairs
- width 0.5cm
- better shaping = better confinement
- Shaped/angled to minimise leakage
Head component function:
Wedges
- Wedged intensity distribution
- Physical
- removable (multiple angles)
- single angle (in treatment head)
- High z -> spectral changes & reduced dose rate
- Replaced by dynamic collimator jaws
Head component function:
Electron Applicators
- Attach to head
- Open or closed
- 95cm SSD
