External radiation therapy or Teletherapy:
When the radiation source is present outside the body it is called as external radiation therapy. It is mainly used in the treatment of cancer of the bladder, brain, breast, cervix, larynx, lung, prostate, and vagina. It is also used as a palliative therapy to treat symptoms of the advanced stage cancer patients. The beam of the ionizing radiation can be moved about and can be focused so that the tumor receives a high dose of radiation and the surrounding tissue gets a lower dose.
X-rays and Gamma rays are used in case of external radiotherapy. Low and medium energy X-rays are particularly suitable for treating cancers on the outside of the body. The main source of X-rays is linear accelerators, synchrotrons, and cyclotrons. To reach the internal tumors high-energy radiation (Gamma radiation) are produced from the radioactive source such as cobalt-60 or iridium-137. These radiations are more penetrating and are therefore more suitable for internal tumors.
Internal radiotherapy or Brachytherapy:
When the radiation source is present inside the body very close or inside the tumor then it is called as internal radiation. It is divided into two; the source is either placed in small sealed vials called the implants (sealed source radiotherapy) or can be directly injected in to the vein (unsealed source radiotherapy).
Implants may be in the form of thin wires, plastic tubes called catheters, ribbons, capsules, or seeds. The implant is put directly into the body. It is mainly used to treat tumors of the head and neck, prostate, cervix, ovary, breast, and perianal and pelvic regions.
In case of systemic radiotherapy, radioactive materials are taken by mouth or injected into the body. It is used to treat cancer of the thyroid and adult non-Hodgkin's lymphoma.
The patient carries the source inside them, so they normally need to stay in hospital while it is in place.
Sources: Iodine 131, Strontium 89, technetium-99
Stereotactic radiotherapy delivers multiple small fraction of the radiation dose to the target tissue. This technique would improve the outcome and minimize side effects. It is used to treat tumors in the brain as well as other parts of the body.
Total body irradiation: in this special radiotherapy technique the whole body is literally irradiated. This technique is used to abalate the bone marrow and reduce the response of the immune system prior to bone marrow transplantation or peripheral blood stem cell transplantation. It is also used as a part of high-dose treatment in case of leukemias and lymphomas. This is basically done to prevent the rejection of the transplanted cells.
Linear energy transfer (LET) is the rate at which a type of radiation deposits energy as it passes through tissue. Higher levels of deposited energy cause more cells to be killed by a given dose of radiation therapy. Different types of radiation have different levels of LET. X-rays, gamma rays are known as low-LET radiation. Neutrons, heavy ions, and pions are classified as high-LET radiation. But the high cost of the equipment and the specialized training needed to perform high-LET radiation therapy restricts its use in the clinical scenario.