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Introduction of Radiotherapy


Radiation therapy is based on the interaction of ionizing particles—typically x-rays, gamma rays, and electrons—with tissues at the molecular level. This interaction depends on the release of localized energy sufficient to break chemical bonds through the production of secondary charged particles, usually electrons. These secondary electrons produced by the primary beam are ultimately responsible for inflicting biologic injury.

X-rays and gamma rays are the forms of radiation most commonly used to treat cancer. They are both electromagnetic, nonparticulate waves that cause the ejection of an orbital electron when absorbed. This orbital electron ejection is called ionization. X-rays are generated by linear accelerators; gamma rays are generated from decay of atomic nuclei in radioisotopes such as cobalt and radium. These waves behave biologically as packets of energy, called photons.

Particulate forms
of radiation are also used in certain circumstances. Electron beams have a very low tissue penetrance and are used to treat skin conditions such as mycosis fungoides. Neutron beams may be somewhat more effective than x-rays in treating salivary gland tumors. However, aside from these specialized uses, particulate forms of radiation such as neutrons, protons, and negative mesons, which should do more tissue damage because of their higher linear energy transfer and be less dependent on oxygen, have not yet found wide applicability to cancer treatment.
  1. QMolecular mechanism of action of ionizing radiations:
    1. Action on enzymes                                       
    2. Direct DNA damage
    3. Indirect injury by free radical injury (80%)                   
    4. Linear energy transfer (LET)Q
  2. Physical Characteristics of Commonly Used Isotopes



Energy (MeV)

Half-Value Layer
(mm lead)

Exposure Rate Constant


1600 years

0.047-2.45 (0.83 avg)




5.26 years





30 years





74.2 days

1.36-1.06 (0.38 avg)




60.2 days

0.028 avg



  1. Radiosensitizers
    1. Many chemical agents are now available which can interact with radiation to improve cell kill induced by radiation therapy.
    2. Compounds that incorporate into DNA and alter its stereochemistry
      1. Halogenated pyrimidines, cisplatin.
      2. Hydroxyurea (another DNA synthesis inhibitor, also potentiates radiation effects.
    3. Compounds that deplete thiols (e.g., buthionine sulfoximine) can also augment radiation effects.
    4. Nitrosoimidazole group of compounds can mimic oxygen molecule and enhanced radiation damage like oxygen fixation, thereby enhance radiation damage at the DNA level.
    5. The useful nitrosoimidazole compounds are metronidazole, misonidazole, etanidazole, and pimonidazole.Q
    6. Other agents like pyrimidine analogues (Idoxyuridine IUdR, Bromo-deoxyuridine BUdR) can interlink with DNA and make the DNA chain unstable to be damaged by the radiation.
    7. Hypoxia is a major factor that interferes with radiation effects.
  2. Radioprotectors
    1. The agent amifostineQ is used commonly in clinical practice to protect from toxicities of chemoradiotherapy.
  3. Modes Of Radiotherapy:
    Ra-diation therapy is delivered with an external beam of radiation (teleradiation therapy) or with a radioactive implant or mold (Brachtherapy).
External beam radiation therapy entails generation of energy particles at some distance from the patient. Brachytherapy entails placement of radioactive sources near or within a tumor.
  1. Teletherapy (most commonly used)Q
  2. Brachytherapy:
    1. Interstitial           
    2. Intracavitary
  3. Systemic radiotherapy
  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. Cobalt - 60
    1. Another commonly used source for external beam irradiation in treating cancer patients is Cobalt – 60.
    2. Co-60 emits 2 Gamma rays per disintegration.
    3. 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.

Extra Edge
  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. Megavoltage Q
    2. Gamma ray beams
      1. Cobalt –60 beam      
      2. ​Cesium –137 beam
    3. Particle beams
      1. Electrons                  
      2. Protons          
      3. Neutrons      
      4. Deuterans
Extra Edge

Cobalt-60 is artificial radioactive cobalt having a half-life of 5.3 years, used as a teletherapy source and in tubes & needles for interstitial & intracavitatory 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 teletherapy source.

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