Radiation Therapy Principles

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How does radiation work to treat cancer?

Radiation is energy that’s carried by waves or a stream of particles. Radiation works by damaging the genes (DNA) in cells. Genes control how cells grow and divide. When radiation damages the genes of cancer cells, they can’t grow and divide any more. Over time, the cells die. This means radiation can be used to kill cancer cells and shrink tumors.

The cell cycle

To understand how radiation works as a cancer treatment, it helps to know the normal life cycle of a cell. The cell cycle has 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. This 5-phase process is controlled by proteins known as cyclin-dependent kinases (CDKs). Because CDKs are so important to normal cell division, they too have a number of control mechanisms.

The cell cycle

      G0 = Cell rests (it’s not dividing) and does its normal work in the body

      G1 = RNA and proteins are made for dividing

      S = Synthesis (DNA is made for new cells)

      G2 = Apparatus for mitosis is built

      M = Mitosis (the cell divides into 2 cells)

Steps of the cell cycle

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

G1 phase: The cell gets information that determines if and when it will go into the next phase. It starts making more proteins to get ready to divide. The RNA needed to copy DNA is also made in this phase. This phase lasts about 18 to 30 hours.

S phase: In the S phase, the chromosomes (which contain the genetic code or 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: More information about if and when to proceed with cell division is gathered during this phase. The G2 phase happens 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.

Cells and radiation

The cell cycle phase is important because usually radiation first kills the cells that are actively dividing. It doesn’t work very quickly on cells that are in the resting stage (G0) or are dividing less often. The amount and type of radiation that reaches the cell and the speed of cell growth affect whether and how quickly the cell will die or be damaged. The term radiosensitivity describes how likely the cell is to be damaged by radiation.

Cancer cells tend to divide quickly and grow 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 causes unwanted side effects. Radiation therapy is always a balance between destroying the cancer cells and minimizing damage to the normal cells.

Radiation doesn’t always kill cancer cells or normal cells right away. It might take days or even weeks of treatment for cells to start dying, and they may keep dying off for months after treatment ends. Tissues that grow quickly, such as skin, bone marrow, and the lining of the intestines are often affected right away. In contrast, nerve, breast, brain, and bone tissue show later effects. For this reason, radiation treatment can cause side effects that might not be seen until long after treatment is over.


Last Medical Review: 10/27/2014
Last Revised: 10/27/2014