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

LINAC & RADIOTHERAPY
Medical sciences is touching new milestones everyday and it is quite evident from the new technologies that are available in the market today. The diagnosis and treatment of various diseases has attained a new level of accuracy and precision lke never before. With this article I would like to aquaint you about Clinical Linear Accelerator(LINAC) and Radiotherapy. Radiotherapy is one of procedures to be followed in treatment of cancer along wth surgery and chemotherapy.In most of the cases, taking care of some vital factors like age, sex, type of cancer, medicine etc. Radiotherapy can be used to treat patients fighting against it in all the three stages of cancer. In Radiotherapy patient is subjected to very high energy radiations of X-rays (also known as photons) and electrons. These radiatons have energies in the range of Megavolts (MV) for photons and Mega electron volts (Mev) in case of electrons. The machines used to produce such high energy radiatons are called as Clinical Linear Accelerator where the term clinical comes from its medical application.Other machine are also avaialble for the same type of treatment under the category of radiotherapy such as Cobalt machine which uses radioactive cobalt(C60) which emits Gamma radiation on its decay and the patient is irradiated, please keep in mind that only the affected area is irradiated not the whole body of the patient, Gammaknife- this again uses a radioactive source and find its application specifically in treatment of brain tumours and many more modalities like that but I will restrict myself to talk only about LINACs. As I have told earlier the treatment for cancer is a three pronged fork: 1) Surgery 2) Radiotherapy 3) Chemotherapy Usually a combination of one or more of these procedures are followed for the treatment of cancer patients. Lets say for a patient with breast cancer firstly surgery is done (Mastectomy-removal of breasts) in order to remove the affected area and after this patient has to undergo radiotherapy. In Chemotherapy treatment is done via medicines. The history of use of Linearaccelerators in Radiotherapy is not so old, say 15-20 years in India. The treatment done in those days is called as Conventional Radiotherapy (CRT) which were very basic and not much accurate in nature. But now a days we have got a lot of spacial techniques like IMRT, IGRT, VMAT to increase the accuracy of treatment and to reduce the amount of dosage to healthy tissue surrounding the cancerous growth. 2-Dimensional, 3-Dimansional images of patients are taken each time to reduce the scope of error and hence increasing the accuracy of the treatment. Here in India there is a long list of hospitals which provide these kind of treatments whcich includes All India Instute of Medical Sciences (AIIMS), Tata Memorial Hospital Mumbai, Ragiv Gandhi Cancer Hospital New Delhi, Narayana Hospital Bangalore and other private players like Apollo, Max etc. Stay tuned for more on LiNAC, different techniques of treatment and its procedures.....