Radiation Therapy for Cancer

Daniel Le
May 25, 2017

Submitted as coursework for PH241, Stanford University, Winter 2017


Fig. 1: A linear accelerator used to create the beams of high-energy X-rays for radiation therapy. (Source: Wikimedia Commons)

Radiation therapy is a common type of treatment for cancer patients. Radiation therapy uses high energy photons with relatively smaller wavelengths on the electromagnetic spectrum such as X-rays, and gamma rays. These smaller wavelengths are higher in energy and have the capacity to do damage to biological systems. The goal of radiation therapy is to expose the cells to high energies and damage the DNA found in the cells, thus preventing the cells from making basic proteins for functioning which kills the cell. Cancer cells are cells in the body that have mutations that prevent the cell from regulating cell growth and division, which results in a tumor. The goal of radiation therapy is to hit these cells with enough energy to damage the cell's DNA so badly to the point where it is impossible for the cells to keep dividing or just cause cell death. The risks with radiation therapy however, is that normal and healthy cells can also be damaged by the treatment along with the targeted cancer cells. Doctors must take into account the potential damage to normal functioning cells when doing radiation therapy on a patient in order to prevent too much damage to healthy tissues. Two methods of giving radiation therapy to a patient are external-beam radiation therapy and internal therapy. [1]

External-Beam Radiation Therapy

This type of radiation uses photon beams with energies ranging from x-rays to gamma rays. Since large doses are needed to kill cancer cells, the surrounding healthy cells are at high risk of damage also. The way that doctors minimize the destruction of normal cells from the radiation beams is that the source of the beam is constantly rotating around an axis that is located right at the spot of the cancer cells. This essentially minimizes as much as possible the prolonged exposure to the surrounding cells in order to do as little damage to them as possible. While at the same time, having the axis of rotation be right at the tumor site allows the beam to always pass through the desired tumor area. Fig 1 shows what the machine that makes these high energy beams look like. Note how it is able to rotate around an axis where the patient would be. This allows the cancerous cells to get a high and prolonged dose and exposure to the radiation, which as explained earlier, destroys the DNA and kills the cancerous cells to prevent further growth. [2]

Internal Radiation Therapy

This type of radiation is also known as brachytherpy. The way brachytherapy works is that radioactive isotopes are injected directly into the tumor site. As the isotopes naturally decay , they give off radiation that damages the surrounding cancer cells that the isotopes were injected into. After a few weeks to months the isotopes completely decay off and no longer give off radiation. Again the goal is to damage the DNA of the cell to the point that the cancer cells can no longer do cell division and/or die. [3]

Another type of internal radiation therapy more specifically called systemic radiation therapy is when a patient swallows or receives an blood injection of a radioactive substance. This radioactive substance however, is bound to an antibody. The body has an innate antibody-antigen recognition system that the body uses in order to distinguish different cells, this type of therapy takes advantage of that by selecting an antibody that will recognize the cancer cell and every time the radioactive substance finds a cancer cell, it binds to that cell and kills the cancercells. [4]

© Daniel Le. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.


[1] T. S. Lawrence, R. K. Ten Haken, and A. Giaccia, "Principles of Radiation Oncology," in Cancer: Principles and Practice of Oncology, 8th Ed., ed. by T. DeVita Jr., T. S. Lawrence and S. A. Rosenberg (Lippincott, Williams and Wilkins, 2008).

[2] D. C. Giancoli, Physics: Principles With Applications, 7th Ed. (Pearson, 2013.

[3] A. Taylor and M.E. Powell, "Intensity-Modulated Radiotherapy - What Is It?" Cancer Imaging 4, 68 (2004)

[4] L. E. Gaspar and M. Ding, "A Review of Intensity-Modulated Radiation Therapy," Curr. Oncol. Rep. 10, 294 (2008).