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.

1. Biologic Effects of
Ionizing Radiation

2. These initial interactions occur almost instantaneously, within 10−13 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-radiation–induced 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.

10. Stochastic and Deterministic Effects

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 Bergonié 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 Bergonié 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.