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  1. Linear accelerators capable of generating low-energy megavoltage x-rays (4 to 6 MeV) and high-energy megavoltage x-rays (15 to 25 MeV) along with electrons. This range of x-ray and electron energies allows one to tailor distribution of the radiation dose to the location of the cancer. Most patients are treated with megavoltage x-rays or gamma rays (photons), which are penetrating beams useful for managing a wide variety of cancers. The characteristics of an x-ray or gamma ray beam (as opposed to electrons) important in radiation therapy are its skin-sparing properties, its depth dose properties (penetration), and its isodose distribution (beam uniformity).
  2. Another commonly used source for external beam irradiation in treating cancer patients is Cobalt-60.
  3. Co-60 emits 2 Gamma rays per disintegration.
  4. The dose rate at any practical distance from a Gamma ray emitting point source in air can be obtained by means of inverse square law.Q

Radiation from any source decreases in intensity as a function of the square of the distance from the source (inverse square law). Thus, if the radiation source is 5 cm above the skin surface and the tumor is 5 cm below the skin surface, the intensity of radiation in the tumor will be 52/102, or 25% of the intensity at the skin. By contrast, if the radiation source is moved to 100 cm from the patient, the intensity of radiation in the tumor will be 1002/1052, or 91% of the intensity at the skin. Teletherapy maintains intensity over a larger volume of target tissue by increasing the source-to-surface distance. In brachytherapy, the source-to-surface distance is small; thus, the effective treatment volume is small.

  1. Beams used for external radiotherapy are:-
    1. X-ray beams
      1. Superficial: 40 to 120 kv                     
      2. Orthovoltage: 250 to 400 kv
      3. Supervoltage: 2,4,6,12 & 35 Mev        
      4. MegavoltageQ
    2. Gamma ray beams
      1. Cobalt –60 beam  
      2. Cesium –137 beam
    3. Particle beams
      1. Electrons              
      2. Protons        
      3. Neutrons
Cobalt-60 is artificiall radioactive cobalt having a half-life of 5.3 years, used as a teletherapy source and in tubes & needles for intertistial & intracavitary therapy. It decays (into nickel), and emits beta and gamma rays (1.33 mv) & has to be replaced at regular intervals of about 4-5 years.Q
  1. Selection of beam energy is based on the location of the tumor.
    1. Cancers 12 to 15 cm deep, such as cancer of the prostate or uterine cervix, usually are best managed with 15- to 25-MeV x-rays because these beams are more penetrating and have better skin-sparing properties than do lower-energy beams.
    2. Cancer of the head and neck, however, can be managed with 4- to 6-MeV x-rays or cobalt 60 gamma rays, at least initially. These tumors are located no more than 7 to 8 cm deep, and there is usually a need to treat the regional lymph nodes, which are superficial. However, 15- to 25-MeV x-rays occasionally are used to deliver additional treatment (a boost) to certain head and neck cancers, such as cancer of the base of the tongue or nasopharynx.
    3. Electron beams are useful for managing superficial lesions.
    4. Unlike x-rays, electrons have a finite range, so tissues deep to the tumor can be spared.
    5. A guideline for the useful range in centimeters of an electron beam is its energy in megaelectron volts (MeV) divided by three.
    6. For cancer treatment, 6-MeV electrons are commonly used for cancers of the skin or lip, 6- to 9-MeV electrons for cervical lymph nodes over the spinal cord, 9- to 12-MeV electrons for cancers of the buccal mucosa, and 15- to 18-MeV electrons for cancers of the tonsillar area or parotid gland.
    7. Electron beams have poorer skin-sparing properties than do photon beams. This is an advantage for superficial skin lesions, but the beams must frequently be combined with high-energy x-rays if high doses are planned to deep tumors to avoid high doses to the surface.

Currently Linear accelerator is the best tele-source.

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