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Biologic effects of Ionizing Radiation

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Dr Jinki Singha

1) Ionizing radiation interacts with biological molecules like DNA and water to cause damage through direct and indirect actions within seconds to hours. 2) This damage can manifest as cell death, mutations, and cancer over time from hours to generations depending on the extent of damage. 3) Radiation damages cells most during the G2 and mitosis phases of the cell cycle and causes cell death primarily through mitotic catastrophe and chromosomal abnormalities, except in lymphocytes and salivary glands where apoptosis is more common.

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Biologic Effects of Ionizing Radiation
These initial interactions occur almost instantaneously, within 1013 seconds after exposure. Subsequent modification of biologic molecules follows within seconds to hours, and the damage from these modifications may manifest in hours, days, years, and even generations, depending on the extent and type of damage.
Biologic effects of Ionizing Radiation
Chemical and Biochemical Consequences of Radiation Direct Actions In direct actions, biologic molecules (denoted RH, where R is the molecule and H is a hydrogen atom) absorb energy from ionizing radiation and within seconds form unstable free radicals. These free radicals quickly re-form into stable configurations by dissociation (breaking apart) or cross-linking (joining of two molecules).
Indirect Actions The initial interaction of a photon occurs with a water molecule. Indirect actions are the predominant mode of x-radiationinduced biologic damage. Ionizing radiation initiates a complex series of chemical changes in water, collectively referred to as radiolysis of water.
Deoxyribonucleic Acid and Chromosomal Damage and Damage Response Damage to a cell's deoxyribonucleic acid (DNA) is the primary cause of radiation-induced cell death, heritable mutations, and carcinogenesis. Ionizing radiations, via production of free radicals, produce many different types of alterations in DNA, including: Base damage Single-strand breaks Double-strand breaks DNA-DNA and DNA-protein cross-links
A proliferating cell moves in the cycle from mitosis phase when chromosomes are condensed and visible to gap 1 (G1) to the period of deoxyribonucleic acid (DNA) synthesis (S) to gap 2 (G2) to the next mitosis. Cells are most radiosensitive in the G2 and mitosis phase, less sensitive in the G1 phase, and least sensitive during the latter part of the S phase.
Biologic effects of Ionizing Radiation
Biologic effects of Ionizing Radiation
Stochastic and Deterministic Effects
Biologic effects of Ionizing Radiation
Biologic effects of Ionizing Radiation
Cell Killing Mitotic death. The predominant mode of radiation-induced cell killing is mitotic death (or mitotic catastrophe), resulting from lethal chromosomal and chromatid aberrations. A cell's sensitivity to this mode of death is determined by its mitotic rate and degree of differentiation. This relationship is referred to as the law of Bergoni辿 and Tribondeau, in honor of the radiobiologists that first described this principle. Cell Radiosensitivity= Mitotic Rate/ Degree of differentiation
Biologic effects of Ionizing Radiation
Apoptosis. Lymphocytes are the most radiosensitive mammalian cell and are an exception to the law of Bergoni辿 and Tribondeau. Likewise, serous acini of the salivary glands are highly radiosensitive, although they do not divide rapidly. In these cell types, apoptosis is the predominant mode of radiation-induced death.
Radiotherapy Involving the Oral Cavity Typically, 2 Gy is delivered daily for a weekly exposure of 10 Gy. The radiotherapy course continues for 6 to 7 weeks until a total of 60 to 70 Gy is administered.
Biologic effects of Ionizing Radiation
Biologic effects of Ionizing Radiation

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Biologic effects of Ionizing Radiation

  • 2. These initial interactions occur almost instantaneously, within 1013 seconds after exposure. Subsequent modification of biologic molecules follows within seconds to hours, and the damage from these modifications may manifest in hours, days, years, and even generations, depending on the extent and type of damage.
  • 4. Chemical and Biochemical Consequences of Radiation Direct Actions In direct actions, biologic molecules (denoted RH, where R is the molecule and H is a hydrogen atom) absorb energy from ionizing radiation and within seconds form unstable free radicals. These free radicals quickly re-form into stable configurations by dissociation (breaking apart) or cross-linking (joining of two molecules).
  • 5. Indirect Actions The initial interaction of a photon occurs with a water molecule. Indirect actions are the predominant mode of x-radiationinduced biologic damage. Ionizing radiation initiates a complex series of chemical changes in water, collectively referred to as radiolysis of water.
  • 6. Deoxyribonucleic Acid and Chromosomal Damage and Damage Response Damage to a cell's deoxyribonucleic acid (DNA) is the primary cause of radiation-induced cell death, heritable mutations, and carcinogenesis. Ionizing radiations, via production of free radicals, produce many different types of alterations in DNA, including: Base damage Single-strand breaks Double-strand breaks DNA-DNA and DNA-protein cross-links
  • 7. A proliferating cell moves in the cycle from mitosis phase when chromosomes are condensed and visible to gap 1 (G1) to the period of deoxyribonucleic acid (DNA) synthesis (S) to gap 2 (G2) to the next mitosis. Cells are most radiosensitive in the G2 and mitosis phase, less sensitive in the G1 phase, and least sensitive during the latter part of the S phase.
  • 13. Cell Killing Mitotic death. The predominant mode of radiation-induced cell killing is mitotic death (or mitotic catastrophe), resulting from lethal chromosomal and chromatid aberrations. A cell's sensitivity to this mode of death is determined by its mitotic rate and degree of differentiation. This relationship is referred to as the law of Bergoni辿 and Tribondeau, in honor of the radiobiologists that first described this principle. Cell Radiosensitivity= Mitotic Rate/ Degree of differentiation
  • 15. Apoptosis. Lymphocytes are the most radiosensitive mammalian cell and are an exception to the law of Bergoni辿 and Tribondeau. Likewise, serous acini of the salivary glands are highly radiosensitive, although they do not divide rapidly. In these cell types, apoptosis is the predominant mode of radiation-induced death.
  • 16. Radiotherapy Involving the Oral Cavity Typically, 2 Gy is delivered daily for a weekly exposure of 10 Gy. The radiotherapy course continues for 6 to 7 weeks until a total of 60 to 70 Gy is administered.