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4 R’s of Radiobiology

By Dr. Deepa Gautam1st yr resident, Radiotherapy

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• The four R’s of radiobiology are the concepts to explain the rationale behind the fractionation of radiotherapy.

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• Repair (few hrs)

• Reassortment/Redistribution (few hrs)

• Repopulation (5-7 wks)

• Reoxygenation (hrs to few days)

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Repair

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Direct and Indirect Actions of Radiation

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• Types of DNA damage:– Single strand break– Double strand break

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Types of radiation induced damage

• Lethal damage• Potentially lethal damage• Sublethal damage

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Lethal damage:

• irreversible and irreparable damage that leads to cell death.

Eg.• Dicentric chromosome• Ring chromosome• Anaphase bridge

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Potentially lethal damage:

• Causes cell death under ordinary circumstances but can be modified by postirradiation environmental conditions.

• If cells are prevented from dividing by creating suboptimal growth conditions for 6 hrs after irradiation, the damage can repair.

• Invitro: by keeping cells in saline or plateau phase

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Sublethal damage:

• Repairable in hours under ordinary circumstances unless additional sublethal damage is added

• Repair of sublethal damage reflects the repair of DNA breaks before they can interact to form lethal chromosomal abberations

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Repair

• Base Excision Repair• Nucleotide Excision Repair• DNA DSB Repair:– Homologous Recombination Repair– Nonhomologous End Joining

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Base Excision Repair

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Nucleotide excision repair

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Homologous Recombination Repair

• Occurs in late S/G2 phase• Undamaged sister

chromatid acts as template• slow process

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Nonhomologous End Joining

• Occurs in G1 phase of cell cycle

• Fast but error prone and thus potentially mutagenic

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• By splitting radiation into small parts, cells are allowed to repair the sublethal damage

• Damage repair depends upon the ability of cells to recognise the damage and activate the repair pathways and cell cycle arrest

• Malignant cells often have suppressed these pathways• Normal tissues are able to repair by the time next

fraction is given

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Reassortment/Redistribution

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• Cells may be in different phases of cell cycle during irradiation( S-phase being radioresistant and M-phase being most radiosensitive)

• Resistance and sensitivity depends upon the level of sulfhydryl compounds(radioprotector) in the cell.

• A small dose of radiation given over a short period will kill a lot of sensitive cells and less of resistant cells

• Surviving cells continue the cycle and may reach sensitive phase when second dose of radiation is given

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Repopulation

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• Repopulation is the process of increase in cell division seen in normal and malignant cells after irradiation

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Repopulation in normal tissues• The time to onset of repopulation after irradiation and the

rate at which it proceeds vary with the tissue• Acute-responding tissues(stem cells, progenitor cells, GI

epithelium, oropharyngeal mucosa,skin) begin repopulation early.

• Late-responding tissues(Renal tubular epithelium, oligodendrocytes, schwann cells, endothelium, fibroblasts) begin repopulation after completion of conventional course of radiation.

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Repopulation of malignant tissues

• The mechanism applies to malignant tissues as well.• Some tumours exhibit accelerated repopulation, a marked

increase in their growth fraction and doubling time and decrease in cell cycle time, at 4 - 5 wks. Eg. SCC of head and neck, cervix.

• It is a dangerous phenomenon that is countered if treatment time extends over 5 wks.

• It is mediated through radiation-induced receptor activation and cellular growth stimulation that occur after a single radiation exposure of 2 Gy.

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• The current standard treatment times confer a benefit by allowing regeneration of acute-responding tissues, which reduces toxicity.

• Attempts made to deliver the therapy more quickly has caused the acute responses to become more severe and dose-limiting.

• Growth factors like hematopoietic growth factors( G-CSF, GM-CSF, erythropoietin, IL-11), keratinocyte growth factor protect the tissues from radiation injury

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Reoxygenation

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• Tumours under 1mm size are fully oxic but beyond this size they develop the region of hypoxia.

• Hypoxia in tumours can result from two different mechanisms.

1. Acute Hypoxia2. Chronic Hypoxia

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Acute Hypoxia

• Develop in tumour as a result of the temporary closing or blockage of a particular blood vessel owing to the malformed structure which lacks smooth muscle and often has incomplete endothelial lining and basement membrane

• At the moment when a dose of radiation is delivered, a proportion of the tumor cells may be hypoxic, but if the radiation is delayed until a later time, a different group of cells may be hypoxic.

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Chronic Hypoxia

• It results from the limited diffusion distance of oxygen in respiring tissue that is actively metabolizing oxygen

• The distance oxygen can diffuse in respiring tissue is about 70µm

• Cells that are hypoxic for long periods become necrotic and die

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Reoxygenation

• The phenomenon by which hypoxic cells become oxygenated after a dose of radiation.

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Process of Reoxygenation

•Tumors contain a mixture of aerated and hypoxic cells.•A dose of x-rays kills a greater proportion of aerated than hypoxic cells.• The pre-irradiation pattern tends to return because of reoxygenation of hypoxic cells.• If the radiation is given in a series of fractions separated in time sufficient for reoxygenation to occur, the presence of hypoxic cells does not greatly influence the response of the tumor.

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Time Sequence of Rexygenation

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Mechanism of Reoxygenation• Reoxygenation in tumours have:– Fast component :• seen in acute hypoxia• occurs within hours • reoxygenation occurs when temporarily closed vessels reopen

– Slow component:• seen in chronic hypoxia• occurs within days • reoxygenation occurs when the tumor shrinks in size and the

surviving cells that were previously beyond the range of oxygen diffusion, come closer to a blood supply

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• The concept of reoxygenation applies mostly to animal tumours that are experimentally studied

• The human tumours are assumed to reoxygenate from the evidence that many tumours respond to the doses on the order of 60Gy in 30#s

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Radiosensitivity, the newer member of the R’s

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•Apart from previous 4 R’s, there is an intrinsic radiosensitivity or radioresistance in different cell types.•The radiosensitivity of the tumor cells is now thought to be the primary determinant of tumor response to radiation.

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Thank You