Dose fractionation effects are utilised in the treatment of

cancer Cancer is a group of diseases involving Cell growth#Disorders, abnormal cell growth with the potential to Invasion (cancer), invade or Metastasis, spread to other parts of the body. These contrast with benign tumors, which do not spread. Po ...

with

radiation therapy Radiation therapy or radiotherapy (RT, RTx, or XRT) is a therapy, treatment using ionizing radiation, generally provided as part of treatment of cancer, cancer therapy to either kill or control the growth of malignancy, malignant cell (biology), ...

. When the total dose of

radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or a material medium. This includes: * ''electromagnetic radiation'' consisting of photons, such as radio waves, microwaves, infr ...

is divided into several, smaller doses over a period of several days, there are fewer

toxic Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a subst ...

effects on healthy cells. This maximizes the effect of radiation on cancer and minimizes the negative

side effects In medicine, a side effect is an effect of the use of a medicinal drug or other treatment, usually adverse but sometimes beneficial, that is unintended. Herbal and traditional medicines also have side effects. A drug or procedure usually used ...

. A typical fractionation scheme divides the dose into 30 units delivered every weekday over six weeks.

Background

Experiment An experiment is a procedure carried out to support or refute a hypothesis, or determine the efficacy or likelihood of something previously untried. Experiments provide insight into cause-and-effect by demonstrating what outcome occurs whe ...

s in radiation biology have found that as the

absorbed dose Absorbed dose is a dose quantity which represents the specific energy (energy per unit mass) deposited by ionizing radiation in living matter. Absorbed dose is used in the calculation of dose uptake in living tissue in both radiation protecti ...

increases, the number of cells which survive decreases. They have also found that if the radiation is ''fractionated'' into smaller doses, with one or more rest periods in between, fewer cells die. This is because of self-repair mechanisms which repair the damage to

DNA Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...

and other biomolecules such as

proteins Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, re ...

. These mechanisms can be over expressed in

cells, so caution should be used in using results for a cancer cell line to make predictions for healthy cells if the cancer cell line is known to be resistant to cytotoxic drugs such as

cisplatin Cisplatin is a chemical compound with chemical formula, formula ''cis''-. It is a coordination complex of platinum that is used as a chemotherapy medication used to treat a number of cancers. These include testicular cancer, ovarian cancer, c ...

. The DNA self repair processes in some organisms is exceptionally good; for instance, the bacterium

Deinococcus radiodurans ''Deinococcus radiodurans'' is a bacterium, an extremophile and one of the most radiation-resistant organisms known. It can survive cold, dehydration, vacuum, and acid, and therefore is known as a polyextremophile. ''The Guinness Book Of World ...

can tolerate a 15 000 Gy (1.5 MRad) dose. Effectofselfrepair

In the graph to the right, called a cell survival curve, the dose vs. surviving fraction have been drawn for a

hypothetical A hypothesis (: hypotheses) is a proposed explanation for a phenomenon. A scientific hypothesis must be based on observations and make a testable and reproducible prediction about reality, in a process beginning with an educated guess or tho ...

group of cells with and without a rest time for the cells to recover. Other than the recovery time partway through the irradiation, the cells would have been treated identically. The

human body The human body is the entire structure of a Human, human being. It is composed of many different types of Cell (biology), cells that together create Tissue (biology), tissues and subsequently Organ (biology), organs and then Organ system, org ...

contains many types of cells, and the human can be killed by the loss of a single type of cell in a vital

organ Organ and organs may refer to: Biology * Organ (biology), a group of tissues organized to serve a common function * Organ system, a collection of organs that function together to carry out specific functions within the body. Musical instruments ...

. For many short-term radiation deaths due to what is commonly known as

radiation sickness Acute radiation syndrome (ARS), also known as radiation sickness or radiation poisoning, is a collection of health effects that are caused by being exposed to high amounts of ionizing radiation in a short period of time. Symptoms can start wit ...

(3 to 30 days after exposure), it is the loss of

bone marrow Bone marrow is a semi-solid biological tissue, tissue found within the Spongy bone, spongy (also known as cancellous) portions of bones. In birds and mammals, bone marrow is the primary site of new blood cell production (or haematopoiesis). It i ...

cells (which produce

blood cells A blood cell (also called a hematopoietic cell, hemocyte, or hematocyte) is a cell produced through hematopoiesis and found mainly in the blood. Major types of blood cells include red blood cells (erythrocytes), white blood cells (leukocytes), ...

), and the loss of other cells in the wall of the

intestines The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The tract is the largest of the body's systems, after the cardiovascular system. ...

, that is fatal.

Radiation fractionation as cancer treatment

Fractionation effects are utilised in the treatment of

with

. When the total dose of

is divided into several, smaller doses over a period of several days, there are fewer

effects on healthy cells. This maximizes the effect of radiation on cancer and minimizes the negative

. A typical fractionation scheme divides the dose into 30 units delivered every weekday over six weeks. Hypofractionation is a treatment regimen that delivers higher doses of radiation in fewer visits. The logic behind this treatment is that applying greater amounts of radiation works to lower the effects of accelerated tumor growth that typically occurs during the later stages of radiotherapy. Hyperfractionation is dividing the same total dose into more deliveries. Treatments are given more than once a day. Hyperfractionated radiation therapy is given over the same period of time (days or weeks) as standard radiation therapy. Accelerated fractionation (two deliveries per day and/or deliveries on weekends as well) has also been investigated.

References

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