Radio sensitivity

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Radio sensitivity is the relative vulnerability of cells, tissues, organs or organisms to the harmful effect of ionizing radiation. As ionizing radiation passes through living cells it destroys the capacity of reproduction in some cells. A very weak exposure over several years can be as potentially injurious as a large single exposure. The time between the exposure and the first signs of radiation damage is called the Latent period.

From the fundamental law of radiation biology, the law of Bergonie and Tribondeau in 1906; the X-rays are more effective on cells which have greater reproductive activity. Cells are least sensitive when in the S phase, then in the G1 phase, then in the G2 phase and most sensitive in the M phase of the cell cycle.

Examples of most sensitive cells are the epidermal stem cells, gastrointestinal stem cells, spermatogonia and erythroblasts. The least sensitive cells are the muscle fibers and the nerve cells. The very sensitive cells are oocytes and lymphocytes.

Physical Factors Affecting Radio Sensitivity:

a) LET (Linear Energy Transfer)

It is the rate at which energy is transferred from ionizing radiation to soft tissue. High LET attenuates the radiation more quickly, making shielding more effective and preventing deep penetration.

b) Relative Biological Effectiveness (RBE)

It is the ratio of the dose required to produce a given biological effect using a reference radiation of 250kVp X-rays and the dose required to produce the same effect using the radiation in question.

c) Fractionation and Protraction:

Fractionated dose refers to delivering same dose rate but in several fractions. It causes less effect because tissue repair and recovery occur between doses.

Protracted dose refers to delivering dose continually/continuously but at a lower dose rate. It causes less effect because of lower dose rate and length of time.

Biological Factors Affecting Radio Sensitivity:

a.  Oxygen effect

Tissue is more sensitive to radiation in the aerobic (oxygenated) state than in the anoxic (without oxygen) or hypoxic (low energy) state.

b. Oxygen Enhancement Ratio (OER)

It is the ratio of the dose necessary under anoxic condition to produce given effect and the dose necessary under aerobic conditions to produce the same effect. It depends on LET, whereby OER is greater for low LET radiation. Irradiation of tissue is usually conducted under conditions of full oxygenated.

c. Age

Cells are more sensitive to radiation before birth and at old age.

d. Gender

Females are 5-10% less sensitive radiation than males.

Cell Damage Classification:

1. Lethal damage. It is irreversible, irreparable and leads to cell death.

2.Sub-lethal damage. It can be repaired in hours unless additional sub-lethal damage is added that eventually leads to lethal damage.

3. Potentially lethal damage. It can be manipulated by repair when cells are allowed to remain in a non-dividing state.

Cells Recovery

Cells can recover from radiation injury if dose was not high enough to kill the cells before next cell division. They can also recover due to biochemical repair mechanism or by use of chemical agents which can modify response of cells .In whole body irradiation; recovery takes place through repopulation of cells. Radio protectors can be used to prevent cell damage.

REFERENCES

1. Hutchinson F (September 1996). “The molecular basis for radiation effects on cells”. Cancer Research.26 (9): 2045-52. PMID 5924966.

2. Kikwai RM. Fundamental Principles of Radiobiology. Nairobi. 2016.

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