Radiation therapy, often known as radiotherapy (abbreviated as RT, RTx or XRT), is the use of energy waves such as light or heat to treat cancer and other conditions. A high energy type known as ionizing radiation is used in cancer therapy. The radiation helps in breaking up the DNA of cancer cells which leads to cellular death (apoptosis). Thus, radiation can kill cancer cells, preventing or slowing the spread of the disease.
The use of ionizing radiation as a treatment of cancer dates back to the late 19th century. In 1895, Wilhelm Conrad Roentgen discovered a different type of ray which can penetrate through an opaque substance and produce fluorescence. Roentgen named them as X-rays and thus, the field of radiology and radiation oncology were born.  X-rays were then successfully applied to the other appearances of carcinoma and trials were done to treat leukaemia. 
After the discovery of X-rays, Henri Becquerel thought that phosphorescent materials such as uranium salts can emit X-ray like radiations. In 1898, Becquerelâ€™s doctoral students Marie and Pierre Curie isolated the first known radioactive elements, Polonium and Radium.  By 1902, Radium had been successfully used to treat a pharyngeal carcinoma in Vienna. By 1904, Radium tubes were directly implanted into tumours which became one of the first examples of interstitial brachytherapy treatments. 
The invention of nuclear reactors during World War II made the production of artificial radioisotopes possible. In 1949, Dr Harold E. Johns sent a request to National Research Council asking them to produce Cobalt-60 isotopes. In 1951, COBALT-60 radiations were used for the first time in medical history to treat a 43-year-old cervical cancer patient. Medical Linear Accelerators (LINACS) began replacing X-ray and cobalt units in the 1980s. These accelerators produced high energy beams, allowing the treatment of tumours located deep inside the body without causing excessive damage to the overlying skin.
External Beam Radiotherapy (EBRT): This is the traditional method of giving radiotherapy. The radiation source is placed outside the body.
Intensity Modulated Radiation Therapy (IMRT): This is an advanced method which enables doctors to precisely shape the radiation around the cancer cells while keeping the dose away from normal cells. 
Volumetric Modulated Arc Therapy (VMAT): Introduced in 2007,  Volumetric modulated arc therapy produces the same results as IMRT but has reduced radiation delivery time.
Stereotactic Body Radiation Therapy: By delivering the right amount of radiation in a shorter period of time, this treatment adds up to big overall dose as compared to normal radiotherapy techniques.
Particle Therapy: In this therapy, energetic ionizing particles are directed at the target tumour. Proton therapy is an example of particle therapy.
Brachytherapy: In this method, the radioactive source is placed either inside or near the tumour. This reduces the side effects as normal cells near the cancer cells receive less dose.
Intra-operative Radiotherapy (IORT): This method of treatment is usually for prevention of recurrence of cancer in those patients who have had their cancer removed via surgery. The radiation source is put directly where the cancer used to be and delivers radiotherapy to the area where there may be some left over cancer cells.
Unsealed Source Radiotherapy (systemic radioisotope therapy): This is a form of targeted therapy. The radioisotope can be delivered through ingestion or infusion.
Radiation therapy as a treatment is painless but there are several risks of side effects involved. The severity and duration of side effects depends on the organs receiving the treatment, type of radiation, dosage and overall health of the patient.
Some common side effects of radiotherapy are:
Long term side effects depend on the treatment site. They include:
Heart or lung problems (if radiation affects the chest)
Risk of another form of cancer due to over-exposure to the radiation
Currently, there are several medicines such as Cancertame which help to minimize such side effects when taken in recommended dosage.
In subsequent years, the field of radiation oncology has experienced multiple technological revolutions. With the newer technology available, the radiotherapy planning systems underwent a drastic makeover.
With the invention of computed tomography (CT) in 1971, the radiation oncology saw a shift from 2-D to 3-D radiation delivery. The CT based planning made it possible for physicians to accurately determine the dose distribution. In the 1990s, 3-D Conformal Radiotherapy was designed to target the radiation directly onto the tumour. This reduced the side effects of the surrounding cells received little to no radiation.
The arrival of new imaging technologies has helped radiation oncologists to target tumours in a better way which has resulted in better treatment outcomes, organ preservation and fewer side effects.  Magnetic resonance imaging (MRI), intensity modulated radiation therapy (IMRT) and image guided radiation therapy (IGRT) are a few examples of advanced imaging technologies.
While the access to radiotherapy is improving globally, more than half of the patients still do not have access to the therapy due to the high costs of the treatment.  Due to the cost effectiveness, reliability and ease of use, Cobalt-60 units remain the main radiotherapy machines used all over the world today. It is being used to treat approximately 70% of the cancer cases via radiation.
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7. History of Radiation Therapy: The Evolution of Therapeutic Radiology
8. "Closing in on cancer". The Economist. 16 September 2017
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