Intro to external beam radiotherapy equipment

Question 1

List the key differences between KV and MV therapy.

Answer 1

• KV:
• Relatively low capital & maintenance costs.
• Single modality (Photons).
• Simple design and operation.
• Simple collimation and field shaping.
• Shallow treatments with little or no ‘skin-sparing’.
• MV:
• High capital and maintenance costs.
• High maintenance workload.
• Multi-modality (Photons and Electrons).
• Complex design.
• Extremely complex treatments.
• Therapy of deep tissues.

Question 2

List some key components of the equipment used in KV therapy.

Answer 2

• Fixed target at shallow angle (30°-40°)
• Interchangeable applicators.
• Target at ground, cathode negative.
• Interchangeable filters.

Question 3

Why is a shallow target angle used in KV therapy?

Answer 3

• Minimises the heel effect thereby increasing field uniformity.
• Don’t require fine image detail so broad focal spot is acceptable.

Question 4

What are typical tube currents used in KV therapy?

Answer 4

-1-20mA (c.f. 1-1000mA in diagnostic)

Question 5

List two KV options and give KVp and mA ranges for both.

Answer 5

• Superficial (SXT):
• 50-150KVp.
• 6-18mA.
• Orthovoltage:
• 20-220KVp.
• 0-20mA.

Question 6

Draw a diagram of the basic tube components of a KV therapy tube.

Answer 6

• Metal housing and (ceramic, rubber, araldite).
• Filament.
• Focusing cup.
• Cathode.
• Fixed target.
• Cu earthed anode.
• Water cooled.
• Be window.

Question 7

Describe how electrons are produced from the cathode in KV therapy equipment and what does the tube current (mA) and peak kilovoltage (KVp) determine?

Answer 7

• Tungsten filament.
• e-s produced via thermionic emission.
• mA α no of e-s accelerated per unit time.
• Intensity of x-rays (no of photons per photon energy) I α mA.
• KVp gives max energy attainable by e-s during acceleration.
• Higher KVp produces more x-rays.
• I α KVp^2.

Question 8

Describe how the anode works in KV therapy equipment and give the interactions to produce x-rays and their relationship to Z.

Answer 8

• Non-rotating reflection target at ground potential.
• De-accelerates e-s.
• 99% heat, 1% x-rays.
• Tungsten (high melting point and high z).
• Anode embedded in conductive anode and water/oil cooled.
• Bremsstrahlung radiation α Z^2.
• Characteristic X-rays α Z^3.

Question 9

Why is filtration necessary for KV therapy and what inherent filtration do KV systems have?

Answer 9

• Preferentially absorb ‘softest’ x-rays.
• Not therapeutically useful, gives unwanted skin dose.
• Inherent filtration - lowest energies removed by target itself.
• 2.2mm Beryllium window.
• Additional metal filters used to ‘harden’ the beam.

Question 10

What are applicators used for (can you visualise what they look like)?

Answer 10

• Circular of rectangular collimation of the treatment beam.
• Fixed SSD (15-25cm).
• Range of diameters (1-15cm).
• Additional Pb cut-outs for better conformance (e.g. circles, ellipses or custom made).

Question 11

List and describe some MV equipment options, giving typical energy ranges.

Answer 11

• Cobalt-60 units:
• e.g. GammaKnife.
• Fixed emissions at 1.17Mev and 1.33MeV.
• Relatively shallow penetration.
• Linac:
• Conventional L-shaped gantry (Varian, Elekta, Siemens).
• Multi-modality.
• ~4-25MV range.
• Better penetration than Co-60.
• Specialised systems such as Tomotherapy and CyberKnife.

Question 12

What is the isocentre on an MV unit?

Answer 12

• The isocentre is a fixed point in space (e.g. 100cm from the focal point of the x-ray target).
• All rotations take place about this point (actually more of a sphere with d).

Question 13

List the basic components of an MV unit.

Answer 13

• Electron gun.
• Modulator.
• Microwave generator.
• Accelerating waveguide.
• Bending magnet.
• Head components.

Question 14

Describe the modulator component of an MV unit.

Answer 14

• Primarily consists of a pulse forming network (PFN).
• PFN stores electrical energy (charging cycle) to provide to thyratron (discharging cycle).
• Thyratron (high speed switch) initiates discharge cycle resulting in the formation of a pulse.
• Thyratron uses energy stored by PFN to deliver pulses to the magnetron/klystron and electron gun.
• Systems typically display a pulse repetition frequency (PRF) of ~200-300Hz (pulse intervals of ~3-5ms).
• Acts to supply a ~1μs pulse to cathodes.
• For triode gun, modulator timing controls the gate voltage.

Question 15

Describe the triode gun component of an MV unit and draw a diagram of it.

Answer 15

• Heater (~1000°C):
• Thermionic emission from cathode.
• Cathode (concave):
• Surrounded by constant cloud of e-s.
• Held at potential of ~20KV.
• Concave shape focuses emissions.
• Grid (concave):
• Held at higher -ve potential to prevent e- attraction to anode (but pulsed to +ve potential to phase match).
• High speed switching of +/- potential.
• Pulses adjusted to phase match e- injection with microwaves.
• Anode at 0V. e-s injected into accelerating waveguide.

Question 16

Describe the magnetron component of an MV unit (could alternatively be a klystron) and draw a diagram of it.

Answer 16

• Thermionic emission from heated cathode.
• Electrons follow a spiral path under the action of:
• Pulsed electric field.
• Permanent magnet.
• Creates electric field across cavities.
• Cavities determine resonant frequency.
• Electrons ‘bunch’ at this frequency and in circulating feed-back to amplify power.
• Microwaves injected into main waveguide.
• Several hundred pulses per second, each having μs duration and frequency in range 3GHz.

Question 17

Describe the klystron component of an MV unit (could alternatively be a magnetron) and draw a diagram of it.

Answer 17

• e-s produced from cathode via thermionic emission.
• e-s injected into drift tube via a p.d. of ~100KV.
• Low power microwaves are produced via a solid state ‘pilot’ oscillator.
• e- ‘bunch’ at input frequency, resulting in increased E field density.
• E-field amplification in drift tube.
• e- beam produces an E field in the resonant catcher cavity (tuned to resonate at bunch frequency) as it crosses it’s mouth.
• Enables acceleration > 10MV.
• peak power ~ 6MW.

Question 18

Describe the accelerating waveguide component of an MV unit and draw a diagram of it.

Answer 18

• Waveguide needed to accelerate e-s to MV range.
• Microwave power transferred to injected e-s.
• In hollow tube, Vp > c.
• Control Vp to maintain e-s at particular phase.
• Resonant cavities (Initially shorter but get longer as bunched e-s speed up so that phase velocity of microwaves matches than of e-s).
• ‘Buncher’ section:
• Initial cavities.
• Varying phase at electron injection produces differential acceleration, resulting in bunching.
• Majority of acceleration here (e-s exit section at ~0.99c).
• Beyond this, cavities lengthen to accommodate increasing electron velocity (until ~ c) then remain constant as mass increases (drift section).
• Resonance frequency depends on cavity diameter (not length).

Question 19

Describe the travelling wave and standing wave accelerating waveguides.

Answer 19

• Travelling wave:
• Simple design, relatively long with microwave injection gun at the end.
• Alternating cavities containing nodes and anti-nodes.
• No overall acceleration in cavities containing nodes.
• Standing wave:
• Complex design, reduced length with microwave injection at any point in waveguide.
• Nodes are moved off to side coupled cavities so acceleration occurs along the full length hence they can be made shorter.
• Microwave power reflected at both ends so that waves interfere to produce a standing wave pattern along the axis which does not advance like a travelling wave.

Question 20

Describe the bending magnet component of an MV unit and draw a diagram of it.

Answer 20

• Permanent or electromagnet to deflect beam towards isocentre.
• Acts as energy discriminator (e-s whose energies do not fall within a narrow range are either bent too much or not enough by B field).
• 270° systems described as doubly achromatic as they act to ensure electrons exit the system at the same point and in the same direction/at the same angle ensuring a small focal spot.
• Can also have 112.5° slalom bending magnet.

Question 21

List the head components of an MV unit when using photons and draw a diagram.

Answer 21

• Primary collimator.
• Photon target.
• Flattening filters.
• Monitor chambers.
• Physical wedge.
• Secondary/Tertiary collimators.
• Accessory holder.

Question 22

List the head components of an MV unit when using electrons and draw a diagram.

Answer 22

• Primary collimator.
• Scattering foils.
• Monitor chambers.
• Secondary/Tertiary collimators.
• Applicators/cut-outs.

Question 23

Describe the primary collimator component in the head of an MV unit.

Answer 23

• Defines maximum extent of treatment field.
• High density material.
• Fixed geometry.
• Surrounds photon target/electron foil(s).

Question 24

Describe the photon target component in the head of an MV unit.

Answer 24

• ‘Transmission’ Target:
• High Z (e.g. W-74 or Steel).
• High density.
• High melting point.
• May be alloy or composite.
• ‘Forward peaked’ beam.
• Bremsstrahlung radiation.
• High efficiency relative to reflection target.
• Water cooled only.
• Thick target: higher average photon energy.
• Thin target: Lower average energy but unwanted e-s may become part of beam.

Question 25

Describe the flattening filter component in the head of an MV unit and draw a diagram showing the affect of the flattening filter on the beam intensity profile.

Answer 25

• Equalises photon intensity across beam.
• High Z conical design results in:
• Differential attenuation across beam to off-set forward peak.
• Modified beam spectrum due to variation in beam hardening (beam softer away from central axis).
• Overall reduction of dose rate.
• Height / thickness is energy specific.
• Carousel mounted (micro switch enabled to ensure filter matches chosen energy).
• Latest machines flattening-filter-free (FFF).

Question 26

Describe the electron foils component in the head of an MV unit and draw a diagram showing the affect of the foils on the beam intensity profile.

Answer 26

• Primary foil broadens thin electron ‘pencil’ beam (1-3mm).
• Secondary foil flattens the broad beam.
• Ideal foil is designed to:
• Maximise scatter.
• Minimise attenuation.
• Minimise x-ray production.
• Energy specific filters mounted on carousel (micro switch enabled to ensure filter matches chosen energy).
• Some systems have a single foil to both broaden and flatten the beam.

Question 27

Describe the monitor chamber component in the head of an MV unit.

Answer 27

-Transmission ionisation chambers that fully intercept treatment beam.
-Dual chamber (primary and secondary monitors).
-Primary monitors:
-Dose/dose rate, radial symmetry and flatness.
Secondary monitors:
-Dose/dose rate (redundancy), transverse symmetry and flatness.
-Mode specific chambers on carousel.
-Sealed Chambers for photons using thick walls to prevent temp/pressure changes without significant beam attenuation (no need for output corrections).
-Unsealed chambers for electrons since chamber walls can cause attenuation.
-Air ionised by photons/electrons with charge collected by electrodes.
-Charge collected can be related to dose delivered.

Question 28

Describe the second/tertiary collimation components in the head of an MV unit.

Answer 28

• Pairs of thick, high density (W-74) jaws define field in 2-D.
• One pair may be replaced with Multi-Leaf Collimator (MLC).
• May use back-up jaws to minimise inter-leaf leakage.
• MLC:
• Composed of ~28-80 leaf pairs.
• Leaf widths ~0.25-1cm (at isocentre).
• Extra shaping for greater treatment conformance.
• Shaped and/or angled to minimise inter-leaf leakage.

Question 29

Describe the physical, dynamic and virtual wedge components in the head of an MV unit.

Answer 29

• Differential attenuation used to create a wedge shaped distribution.
• Physical wedges include:
• Removable wedges (multiple angles) for manual insertion into accessory holder.
• Single angle motorised wedge system within treatment head (Elekta).
• These have characteristics of:
• High Z materials.
• Spectral changes (beam hardening).
• Reduced overall dose rate.
• Now largely replaced by dynamic collimator jaws (e.g. VW, EDW).
• Dynamic/virtual jaws sweep across the field during beam-on to create a wedge like distribution.
• Virtual wedge is formed by a jaw moving with constant velocity and varying dose rate, whereas an enhanced dynamic wedge is formed by varying jaw speed and dose rate.

Question 30

Describe the electron applicator components in the head of an MV unit.

Answer 30

• Attach to head via accessory holder.
• Open or closed style.
• May be designed to:
• Attenuate lateral scatter from air.
• Produce further flattening.
• End typically 95cm from source.
• Additional cerrobend cut-outs may be inserted into bottom of applicators (cerrobend is high density, low melting point eutectic metal alloy composed of lead, tin, bismuth and antimony).