Radio Biological Factors

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    The multifraction regimens commonly used in conventional radiation therapy are a consequence largely of radio biologic experiments performed in France in the 1920s and 1930s. It was found that a ram could not be sterilized by exposing its testes to a single dose of radiation without extensive skin damage to the scrotum, whereas if the radiation was spread out over a period of weeks in a series of daily fractions, sterilization was possible without producing unacceptable skin damage.


    Now, more than 80 years later, we can account for the efficacy of fractionation based on more relevant radio biologic experiments. We can appeal to the “four Rs” of Radiobiology,
Repair of sublethal damage
Reassortment of cells within the cell cycle

The basis of fractionation in radiotherapy can be understood in simple terms. Dividing a dose into a number of fractions spares normal tissues because of repair of sublethal damage between dose fractions and repopulation of cells if the overall time is sufficiently long. At the same time, dividing a dose into a number of fractions increases damage to the tumor because of reoxygenation and reassortment of cells into radiosensitive phases of the cycle between dose fractions.


    Radiation dose for the tumour is very to site to site and also depending on the critical structures associated with that. So many protocols has devised used widely, RTOGs protocol are considered as most standard protocol in all over world.


• Generally, the total dose that can be tolerated  depends on the volume of irradiated tissue
• It also depend on the structural organization of the  tissue in terms of the spatial arrangement of  functional sub-units, FSU‟s (the nephron in the  kidney, the lobule in the liver):
 If FSUs are arranged in a series, elimination of any unit is critical to the organ function (example: spinal cord)

If FSUs are arranged in parallel, elimination of a single unit is not critical to the organ function (kidney and lung)


1. Fractionation depends on 

a) Growth of tumour

b) size

c) Four R’s Radiobiology

d) All


1. c)Four R’s of Radiobiology


1. Radiobiology for the radiologist by E.J.Hall, Amato J. Giaccia


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