LINEAR ACCELERATOR

With continuation of my last article I am back with some more updates on Radiotherapy and Linacs. And with this I am going to tell you about the basics of a Linac . Well the name may not give you a chunk of ideas about the physics and geography of this machine but it has been a topic of research in physics from a long time. The principle of linac is quite simple: here an electron beam is accelerated till it reaches a potential of the order of Megavolts, that is around the 0.9 times the speed of light
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In simple words, a linear accelerator is a machine in which very high energy radiations are produced by accelerating the electrons emitted by cathode which are then directed on to the affected area of the patient in form of electrons or photons (X-rays).

Working of Linac: in Linac source of electrons is a gun. In this gun there is a cathode filament made up of tungsten which is subjected to a high negative potential which usually is of the order of Kilovolts (around 50 Kv). The electrons thus generated are then made to enter the accelerator waveguide where the actual acceleration takes place. The energy required to accelerate the electrons comes from RF or microwaves. Inside a waveguide, as we all know RF and microwaves have got a very high energy, the RF wave is made to carry the electrons. In other words, electrons are superimposed on RF waves. The RF waves are cut on & off through a switchwhich works in coordination with a programmable pulse generator (PPG) and pulse forming network (PFN) whose job is to cut off the RF wave when it is at crest (at the peak energy level)and hence when the next wave comes it accelerates the electron further up to its crest level. In this manner, the electrons keep on gaining speed until they come out of the waveguide and hits the target.

The most commonly used source of producing RF waves in Linacs are Magnetron and Clystron. It is of prime importance to maintain the parameters of electron beam thus generated and for this the waveguide has go specials coils called steering, centring and focussing coils. When current is applied to these coils, they produce magnetic fields which perform the function of focussing centring etc. The orientation and the amount of current flowing through the coils regulate the magnetic fields and hence have an effect on beam geometry. The effect of gravity on the waveguide is also taken into account because G-force varies with the angle of the gantry wrt ground level, this clearly shows the importance of these coil. The pressure inside the waveguide is maintained below 10-5 mbar. The electron beam coming out of the waveguide is then made to fall on an target made up of tungsten to produce X-rays and is passed as it is through window n case we need an electron beam. The beam is then subjected to collimation. Here the shaping of the beam is done. This is done via Beam limiting devices (BLDs) and Multileaf Collimator (MLC). Invention of MLCs has been a revolution in the development of Linacs. Depending on the machine MLC may have 80, 120 0r 160 leaves. Each leaf in the MLC is a flat thin leaf like structure made of tungsten which together forms the shape of the tumour and allows the radiation to pass through it only. MLC has decreased the dose level to the normal tissue surrounding the tumour.

Today the Linacs are available with wide range of energies. Energies available are 4 MV, 6 MV, 15 MV, 18 MV in case of photons and 4 Mev, 6 Mev, 8 Mev, 10 Mev, 18 Mev etc in case of electrons