The Working of a LINAC

TELETHERAPY : 

Teletherapy refers to radiation therapy given by an external radiation source at a distance from the body

The isocentre is the point in space about which the gantry of the linear accelerator, the treatment head of the linear accelerator, and the couch rotate.

Beam AXIS – axis around which the collimator rotates 

Radiation Field – plane perpendicular to Beam axis 

A LINAC or linear accelerator

Electron gun   _____ Wave guide        _____       Bending magnet ____ TARGET

 I                                        I           

I                                   Magnetron 

I                                           I 

I                                           I 

Pulsed modulator ———- 

PARTS of a LINEAR ACCELERATOR

  1. Electron gun

2. Radio frequency power generation system: Magnetron/Klystron 

3. Wave guide 

4. Focussing coils (magnetic coils on side that focus e beam and prevent deviation) 

5. Steering coils (magnetic coils at start and end that keep e beam in centre) 

6. Bending magnet

7. Target 

8. Radiation beam

9. Components of Linac head

Pulses of voltage are used to generate power in a pulsed modulator. High bursts of energy is given in a short period of time. ( Power = Energy /time) to generate high power. This power feeds into electron gun and magnetron/klystron.

The power generated by the pulsed modulator heats the cathode of the electron gun. The electron gun is usually composed of a metal that on heating releases an electron cloud by thermionic emission. These electrons are then accelerated in the waveguide using energy derived from the RF/Microwave Power generation system. The accelerated electrons are bent in the bending magnet and focussed onto the target. 

The collision of high speed electrons and the matter in the target material releases high energy radiation by the process of brehmstrauhlung. This radiative energy is essentially composed of photons that act as the radiation beam.

These photons can be of variable energy. The energy of the beam (e.g. 8MeV) is the MAXIMUM energy of photons in a particular radiation beam.

In Electron therapy, all parts of the Linac are the same, except there is no target. The high energy electrons are focused with the help of a bending magnet and then leave the linac head through a scattering coil to create a radiation beam.

ELECTRON GUN:

Releases electrons by thermionic emission.

RF/Microwave Power GENERATION SYSTEM:

Radio frequency power generation system (RF/microwave range of EM radiation) can either be a magnetron or a klystron. These are used to generate RF waves which are then sent on to the wave guide.

Magnetron (Elekta) -, circular geometry, often used for low energy beams <5MV ish, small so can be gantry mounted, microwave production of 3000MHz, send microwaves to waveguide 

Structure: Involves circular spiral acceleration of electrons in a magnetic field. These electrons decelerate in resonant cavities and the energy is released microwaves

Klystron (Varian) – linear geometry, often used for higher energy beams >5MV, larger and more expensive but longer life, works on the principle of microwave amplification, sends high frequency microwaves to waveguide 

Structure: Cathode produces electrons, Low frequency microwaves are fed into a system that creates an alternating electric field. Electrons exposed to this field are bunched together and accelerate and decelerate in bunches. The energy lost from electrons fed into the low frequency microwaves to make them high frequency.

WAVEGUIDE: 

RF pulses are fed into the waveguide. The waveguide’s function is to accelerate electrons released by the electron gun in a linear fashion.

Travelling type waveguide: 

Electrons are captured by microwaves (like a surfer captures a wave) to be carried forward by it.

Standing wave guide:

More complicated to explain. Reflected microwaves are used to accelerate electrons forwards.

STANDING WAVE GUIDE: More expensive, more stable, shorter, used with klystron.

BEAM BENDING MAGNETS

1. 90 degree bending magnet

Not really used as the exit position depends on magnet strength (constant) but also on beam energy (variable) 

2. 270 degree BENDING MAGENT

Used practically  

Less variation with beam energy in the final focus exit point 

Bulky system in head of linac.

3. Slalom bending system ( 3 bending coils) 45 bend 45 bend and 112 bend

Offers advantages of 270 bend (more focus) but involves a less bulky head.

Bending magnets can filter out and select high energy electron and bend them to form final electron beam.

Thus 270 degree BM has a better focus than 90degree bending magnet, as longer time to select a narrow range of energy electrons which will result in a more collimated beam.  

VACUUM: Electron gun, Wave guide, bending magnets are all in vacuum 

COOLING SYSTEMS are needed in the linac as all processes release large amounts of energy that is lost as heat. Cooling system are needed to keep the systems functional. 

Path of the Photon beam in the linac head:

Target —> photon beam —-> primary collimator —> Flattening Filter (FF) —> ionisation chamber —> Secondary collimator —> wedges 

Path of the Electron beam in the linac head:

Electron beam—> window —> primary collimator —> scattering foil —> ionisation chamber —> secondary collimator ——> applicator 

A carousel mounts the FF and scattering foil and can rotate to get either one in the beam path according to radiation beam desired (photons vs electron respectively)

FF – important to shape beam profile from forward peaked beam to flat beam. Also hardens beam more in the centre (as well as attenuates to make the flat shape) 

Beam profile: Plot normalised dose at 10cm depth vs position (width across central axis) in the Radiation field.

Beam flatness: Max and min dose over beam profile measured at 10cm depth, over the central 80% of beam width (if beam width is between 50% isodose lines). The reported maximum and minimum doses and the difference between the two values gives an indication of beam flatness.

Beam symmetry: Consider equidistant points on Left and right of the central axis at 10cm depth. The percentage ratio of the doses at above points gives an indication of beam symmetry. 

COLLIMATORS 

Primary collimators reduced beam scatter outside treatment head 

Secondary collimators shape beam lower down. Should not allow more than 1% transmission of primary beam through the collimator itself. 

PENUMBRA: 

The penumbra is the surface area outside the direct/primary radiation field that receives partial dose. Radiation penumbra as measured is the distance between 80% and 20% isodose lines at 10cm depth.

Composed of transmission penumbra + geometric penumbra + scattered radiation.

Transmission (though collimators) 

Geometric (penumbra caused by finite source size) 

Radiation penumbra: transmission + geometric penumbra + scattered radiation 

IONISATION CHAMBER 

3 layers of monitoring chambers 

First chamber – first check. Should terminate beam once prescribed MU have been delivered 

Second chamber – fail check of first 

(If both fail there will be timer which will switch beam off!) 

3rd chamber is divided into sectors. Comparing signal between them gives information on flatness, symmetry of beam profile 

Feedback from ionisation monitoring chambers sent to beam steering/bending magnet system. Any deviations from expected values, outside a set tolerance of 5% – beam terminates.  

MLC (multileaf collimator)

40 leaves 

Each leaf projects a 0.5 to 1cm shadow at isocentre 

Transmission through a leaf should be < 2% 

Transmission between leaves should be <4% 

Rounded edges MLC have a greater penumbra as cannot be aligned with radiation beam divergence as older collimators can 

However,  with an MLC the penumbra is the same size irrespective of position of MLC in beam profile

TYPES OF TELETHERAPY DELIVERY:

Arc or Rotation therapy:

Beam moves around patient 

Fixed field size as beam rotates – (NOT VMAT) 

Can have MLC shielding, but MLCs do not move during arc 

Can be varying degrees of arc – 100, 180, 360 degree 

Good for fairly central PTV, and equidistant from surface of patient in all directions. Ideally a cylinder patient shape would be perfect 

IMRT

Intensity modulated radiotherapy 

This can produce concave dose distributions and spare OARs 

Extension of using a wedge to improve conformity 

Intensity varies across single beam 

Changes in MLC position required to create complexity within beam 

Step and shoot – MLC move while beam is off 

Sliding window – MLC move while beam is on 

Larger number of beams are required to improve conformity 

Inverse planning or computer planning needed (too complex for forward plans) 

Based on optimisation criteria provided by planner (these are entered in form of constraints to organs and weighted as to how important it is to achieve this – and this information is fed into a computer which produces an optimal plan. Can change constraints and weight to change plan)

VMAT

Volumetric Modulated Arc Therapy 

Gantry and MLC move during treatment 

gantry will rotate usually 200- 360 degrees during treatment 

Improves efficiency and speed 

Increases low dose bath 

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s