Dose fractionation
Encyclopedia
Experiments in radiation biology have found that as the absorbed dose
of radiation
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
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/D/DNArepair.htmlhttp://www.benbest.com/lifeext/aging.html#dna and other biomolecules such as proteins. These mechanisms can be over expressed in cancer
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
. The DNA self repair
processes in some organisms is exceptionally good; for instance, the bacterium Deinococcus radiodurans
can tolerate a 15 000 Gy (1.5 MRad) dose.
In the graph below, called a Cell survival curve
, the dose vs. surviving fraction have been drawn for a hypothetical 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
contains many types of cells, and the human can be killed by the loss of a single type of cells in a vital organ
. For many short-term radiation deaths due to what is commonly known as radiation sickness
(3 days to 30 days after exposure), it is the loss of bone marrow
cells (which produce blood cells), and the loss of other cells in the wall of the intestines, that is fatal.
Absorbed dose
Absorbed dose is a measure of the energy deposited in a medium by ionizing radiation per unit mass...
of radiation
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
increases, the number of cells
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
which survive decreases. They have also found that if the radiation is fractionated into smaller doses
Gray (unit)
The gray is the SI unit of absorbed radiation dose of ionizing radiation , and is defined as the absorption of one joule of ionizing radiation by one kilogram of matter ....
, with one or more rest periods in between, fewer cells die. This is because of self-repair mechanisms which repair the damage to DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/D/DNArepair.htmlhttp://www.benbest.com/lifeext/aging.html#dna and other biomolecules such as proteins. These mechanisms can be over expressed in cancer
Cancer
Cancer , known medically as a malignant neoplasm, is a large group of different diseases, all involving unregulated cell growth. In cancer, cells divide and grow uncontrollably, forming malignant tumors, and invade nearby parts of the body. The cancer may also spread to more distant parts of the...
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
Cytotoxicity
Cytotoxicity is the quality of being toxic to cells. Examples of toxic agents are a chemical substance, an immune cell or some types of venom .-Cell physiology:...
such as cisplatin
Cisplatin
Cisplatin, cisplatinum, or cis-diamminedichloroplatinum is a chemotherapy drug. It is used to treat various types of cancers, including sarcomas, some carcinomas , lymphomas, and germ cell tumors...
. The DNA self repair
DNA repair
DNA repair refers to a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as UV light and radiation can cause DNA damage, resulting in as many as 1...
processes in some organisms is exceptionally good; for instance, the bacterium Deinococcus radiodurans
Deinococcus radiodurans
Deinococcus radiodurans is an extremophilic bacterium, one of the most radioresistant organisms known. It can survive cold, dehydration, vacuum, and acid, and is therefore known as a polyextremophile and has been listed as the world's toughest bacterium in The Guinness Book Of World Records.-Name...
can tolerate a 15 000 Gy (1.5 MRad) dose.
In the graph below, called a Cell survival curve
Cell survival curve
A cell survival curve is a curve used in radiobiology.It depicts the relationship between the fraction of cells retaining their reproductive integrity and the absorbed dose....
, the dose vs. surviving fraction have been drawn for a hypothetical 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.
Human body
The human body is the entire structure of a human organism, and consists of a head, neck, torso, two arms and two legs.By the time the human reaches adulthood, the body consists of close to 100 trillion cells, the basic unit of life...
contains many types of cells, and the human can be killed by the loss of a single type of cells in a vital organ
Organ (anatomy)
In biology, an organ is a collection of tissues joined in structural unit to serve a common function. Usually there is a main tissue and sporadic tissues . The main tissue is the one that is unique for the specific organ. For example, main tissue in the heart is the myocardium, while sporadic are...
. For many short-term radiation deaths due to what is commonly known as radiation sickness
Radiation Sickness
Radiation Sickness is a VHS by the thrash metal band Nuclear Assault. The video is a recording of a concert at the Hammersmith Odeon, London in 1988. It was released in 1991...
(3 days to 30 days after exposure), it is the loss of bone marrow
Bone marrow
Bone marrow is the flexible tissue found in the interior of bones. In humans, bone marrow in large bones produces new blood cells. On average, bone marrow constitutes 4% of the total body mass of humans; in adults weighing 65 kg , bone marrow accounts for approximately 2.6 kg...
cells (which produce blood cells), and the loss of other cells in the wall of the intestines, that is fatal.