RT1 - Intro to equipment

Question 1

Difference between KV and MV systems?

Answer 1

KV

• Low cost
• Only Photons
• Simple Design & Operation
• Simple, shallow treatments

MV

• High Cost
• Photons & electrons
• Complex design
• Complex, deep treatments

Question 2

List 4 basic KV tube components.

Answer 2

1. Cathode
2. Annode
3. Filtration
4. Applicators

Question 3

Describe function of KV tube: Cathode.

Answer 3

• 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

Question 4

Describe function of KV Tube Annode

Answer 4

• 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

Question 5

Describe function of KV Tube Filtration.

Answer 5

• 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

Question 6

Describe function of KV Tube Applicators.

Answer 6

• Circular/rectangular collimation
• Fixed SSD (15/25cm)
• Diameters 1-15cm
• Additional Pb cut-outs for conformity

Question 7

Name and describe the two types of MV treatment systems.

Answer 7

Co60

• Fixed Energy (1.17MeV)
• Specialised e.g. Gamma-Knife

LINAC

• Multiple energies
• Better penetration
• Multi-modality

Question 8

Name the 5 Basic Components (not the head components) of a LINAC.

Answer 8

1. Modulator
2. Electron Gun
3. Microwave Generator
4. Accelerating waveguide
5. Bending magnet
6. Head components

Question 9

Describe the function of:

Modulator

Answer 9

• Two components
  
  • Pulse forming network
  • Thyratron (high-speed swithch)
• PRF 200-300Hz
• Supply approx. 1us pulse to cathodes to control:
  
  • e- gun
  • microwave generator

Question 10

Describe the function of:

Electronn gun

Answer 10

• 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

Question 11

Describe the function of:

Microwave Generation (2methods)

Answer 11

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

Question 12

Describe the function of:

Accelerating Waveguide

Answer 12

• 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

Question 13

Describe the function of:

Bending Magnet

Answer 13

• MV Beams need long waveguides, which prohibits acceleration in vertical plane
• Horizontal beam deflected to pass through isocentre
• Different tytpes
  
  • 270deg
  • slalom

Question 14

Order of head components:

photons (x7)

Answer 14

1. Primary collimator
2. photon target
3. flattening filter
4. Montior chambers
5. Physical Wedge
6. Secondary collimators
7. Accessory holder

Question 15

Order of head components:

electrons (x5)

Answer 15

1. Primary collimator
2. Scattering foils
3. Montiro chambers
4. Secondary Collimator
5. Applicator

Question 16

Head component function:

Primary Collimator

Answer 16

• Defines extent of treatment field
• High density material
• fixed geometry
• Surrounds photon target/ e- foil

Question 17

Head component function:

Photon Target

Answer 17

• Transmission target (alloy with high z, high density, high mpt)
• Forward peaked beam
• Bremstrahlung
• High efficency

Question 18

Head component function:

Flattening Filter

Answer 18

• 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

Question 19

Head component function:

electron foils

Answer 19

• Broaden thin pencil beam
• Flatten beam
• Max scatter, Min attenuation, Min x-ray production
• Energy specific

Question 20

Head component function:

Monitor chamber

Answer 20

• Transmission ion chamber
  
  • Photons: sealed
  • Electrons: unsealed
• Primary
  
  • Dose rate (inner)
  • Radial symmetry & flatness
• Secondary
  
  • Dose rate (redundancy)
  • Transverse symmetry & flatness

Question 21

Head component function:

Secondary collimator

Answer 21

• Pairs of thick, high density jaws define 2D field
• MLCs

Question 22

Head component function:

MLC

Answer 22

• 30-80 pairs leaf pairs
• width 0.5cm
• better shaping = better confinement
• Shaped/angled to minimise leakage

Question 23

Head component function:

Wedges

Answer 23

• Wedged intensity distribution
• Physical
  
  • removable (multiple angles)
  • single angle (in treatment head)
• High z -> spectral changes & reduced dose rate
• Replaced by dynamic collimator jaws

Question 24

Head component function:

Electron Applicators

Answer 24

• Attach to head
• Open or closed
• 95cm SSD