Radiation is energy that is carried by waves or a stream of particles. It can change the genes (DNA) and some of the molecules of a cell. Genes control how cells grow and divide. Radiation can damage the genes of a cancer cell so that it cannot grow and divide any more. So radiation is used to kill cancer cells and shrink tumors.

To understand how radiation works as a treatment, it is helpful to know about the normal life cycle of a cell. The cell cycle goes through 5 phases, one of which is the actual splitting of the cell. When a cell splits, or divides, into 2 cells, it's called mitosis.

The cell life cycle

The Cell Cycle

G0 phase (resting stage): The cell has not yet started to divide. Cells spend much of their lives in this phase. Depending on the type of cell, this step can last for a few hours or many years. When the cell gets the signal to reproduce (divide), it moves into the G1 phase.

G1 phase: During this phase, the cell starts making more proteins to get ready to divide. Single strands of DNA, called RNA, are also made. This phase lasts about 18 to 30 hours.

S phase: In the S phase, the chromosomes that contain the genetic code (DNA) are copied so that both of the new cells to be made will have the same DNA. This phase lasts about 18 to 20 hours.

G2 phase: The G2 phase is just before the cell starts splitting into 2 cells. It lasts from 2 to10 hours.

M phase (mitosis): In this phase, which lasts only 30 to 60 minutes, the cell actually splits into 2 new cells that are exactly the same.

Cell cycle and radiation

The cell cycle is important in cancer treatment because radiation usually works best on cells that are actively or quickly dividing. It doesn't work as well on cells that are in the resting stage (G0) or are dividing slowly. Radiosensitivity is a term used to describe how at risk a cell is to radiation damage.

Cancer cells tend to divide quickly and out of control. Radiation therapy kills cancer cells that are dividing, but it also affects dividing cells of normal tissues. The damage to normal cells is what causes side effects. Each time radiation therapy is given it involves a balance between destroying the cancer cells and sparing the normal cells.

Radiation does not always kill cancer cells or normal cells right away. It may take days or even weeks of treatment for cells to begin dying, and they may continue to die off for months after treatment ends. For this reason, there are long-term side effects that may come with radiation treatment, and some of these may not be seen until long after treatment is over.

In the past, it was thought that once an area was treated with radiation it could not be treated again with radiation because of damage to the normal cells in the treatment area. But recent research suggests that in some cases, a second course of radiation therapy can be given.

Go back to Radiation Principles.

Last Medical Review: 07/17/2009
Last Revised: 07/17/2009