Radiation acts on living systems through direct and indirect effects. Free radical production: RH+ X-radiation →→R*+H++e- Free radical fates: Dissociation: R*→→X+Y* Or Cross-linking: R*+S*→→RS

Generation of free radicals occurs in less than 10−10 second after interaction with a photon. Approximately one third of the biologic effects of x-ray exposure result from direct effects. direct effects are the most common outcome for particulate radiation such as α particle.

INDIRECT EFFECTS water is the predominant molecule in biologic systems (about 70% by weight), Photon + H2O → H*+OH* hydroxyl free radicals H* +O2 → HO2* Hydroperoxyl free radicals HO2*+H* → H2O2 hydrogen peroxide HO2*+ HO2* → O2+ H2O2 Both peroxyl radicals and hydrogen peroxide are oxidizing agents and are the primary toxins produced in the tissues by ionizing radiation.

INDIRECT EFFECTS Indirect effects are those in which hydrogen and hydroxyl free radicals, Produced by the action of radiation on water, interact with organic molecules The interaction of hydrogen and hydroxyl free radicals with organic molecules results in the formation of organic free radicals. About two thirds of radiation-induced biologic damage results from indirect effects

Such reactions may involve the removal of hydrogen: RH+OH → R*+ H2O RH+ H* → R*+H2 Both direct and indirect effects are completed within 10−5 second. The resulting damage may take hours to decades to become evident.

CHANGES IN DEOXYRIBONUCLEIC ACID Damage to (DNA) is the primary cause of radiation-induced cell death, heritable (genetic) mutations, and cancer formation (carcinogenesis). Radiation-induced changes in protein, lipids, and carbohydrates after low or moderate doses (up to 10 Gy) of radiation are so slight that they do not contribute to radiation effects.

Effect of Radiation on DNA Breakage of one or both DNA strands Cross-linking of DNA strands within the helix to other DNA strands or to proteins Change or loss of a base Disruption of hydrogen bonds between DNA strands The most important of these types of damage are single- and double-strand breakage.

Most single-strand breakage is of little biologic consequence because the broken strand is readily repaired by using the intact second strand as a template. However, misrepair of a strand can result in a mutation and prevent cell division. Double-strand breakage is believed to be responsible for most cell killing, carcinogenesis, and heritable effects

DETERMINISTIC AND STOCHASTIC EFFECTS Radiation injury to organisms results from either the killing of large numbers of cells (deterministic effects) or sub lethal damage to individual cells that results in cancer formation or heritable mutation (stochastic effects).

RADIOSENSITIVITY AND CELL TYPE Different cells from various organs of the same individual may respond to irradiation quite differently. This variation was recognized as early as 1906 by the French radiobiologists Bergoni é and Tribondeau. They observed that the most radiosensitive cells have the following characteristics: A high mitotic rate Undergo many future mitoses Are most primitive in differentiation

RADIOTHERAPY IN THE ORAL CAVITY The oral cavity is irradiated during radiation therapy of radiosensitive oral malignant tumors, usually Squamous cell carcinomas. Radiation therapy for malignant lesions in the oral cavity is usually indicated when the lesion is radiosensitive, advanced, or deeply invasive and cannot be approached surgically. Combined surgical and radiotherapeutic treatment often provides optimal treatment. Increasingly, chemotherapy is being combined with radiotherapy and surgery.

RADIATION EFFECT ON ORAL TISSUES Typically 2Gy is delivered daily, bilaterally through 8 × 10cm fields over the oropharynx(occasion small implants containing radon or iodine 125 are placed directly in a tumor mass) for a weekly exposure of 10Gy this continues typically for 6 to 7 weeks until a total of 64 to 70Gy are administered.

Oral Mucous Membrane The oral mucous membrane contains a basal layer composed of radiosensitive stem cells. near the end of the second week of therapy, as some of these cells die ---------} redness and inflammation (mucositis) ------------} white to yellow pseudomembrane ---------} food intake is difficult. After irradiation is completed; the mucosa begins to heal rapidly. Healing is usually complete by about 2 months.

Taste Buds Patients often notice a loss of taste acuity during the second or third week of radiotherapy. Alterations in the saliva may partly account for this reduction, which may proceed to a state of virtual insensitivity. Taste loss is reversible and recovery takes 60 to 120 days.

Salivary Glands Parotid glands are more radiosensitive than submandibular or sublingual glands. A marked and progressive loss of salivary secretion (hypo salivation) is usually seen in the first few weeks after initiation of radiotherapy.

Teeth Children receiving radiation therapy to the jaws may show defects in the permanent dentition such as retarded root development, dwarfed teeth, or failure to form one or more teeth.

Radiation Caries Caries results from changes in the salivary glands and saliva, including reduced flow, decreased pH, reduced buffering capacity, increased viscosity, reduced remineralization, and altered flora.

Dose units and dosimetry The term dose is used to describe the amount of energy absorbed per unit of mass at a site of interest Exposure is a measure of radiation on the basis of its ability to produce ionization in air under standard conditions of temperature and pressure (STP). The basic unit used to measure ionizing radiation is the gray (Gy)

EXPOSURE MEASUREMENT The term exposure refers to the measurement of ionization in air produced by x-rays. The traditional unit of exposure for x-rays is the roentgen (R). The roentgen is a way of measuring radiation exposure by determining the amount of ionization that occurs in air. A definition follows: Roentgen: The quantity of x-radiation or gamma radiation that produces an electrical charge of 2.58 × 10–4 coulombs in a kilogram of air at standard temperature and pressure (STP) conditions. One roentgen is equal to the amount of radiation that produces approximately two billion, or 2.08 × 109, ion pairs in one cubic centimeter (cc) of air. The roentgen measures the amount of energy that reaches the surface of an organism but does not describe the amount of radiation absorbed.

DOSE MEASUREMENT Dose can be defined as the amount of energy absorbed by a tissue. The radiation absorbed dose, or rad, is the traditional unit of dose. Unlike the roentgen, the rad is not restricted to air and can be applied to all forms of radiation. Rad: A special unit of absorbed dose that is equal to the deposition of 100 ergs of energy per gram of tissue (100 erg/g). Using SI units, 1 rad is equivalent to 0.01 joule per kilo gram (0.01 J/kg). The SI unit equivalent to the rad is the gray (Gy), The gray, which replaced the rad (radiation absorbed dose)in 1978, is a large unit and the centigray (cGy), which equals 1 rad, is commonly used in clinical practice.

DOSE EQUIVALENT MEASUREMENT Different types of radiation have different effects on tissues. The dose equivalent measurement is used to compare the biologic effects of different types of radiation. The traditional unit of the dose equivalent is the roentgen equivalent (in) man, or rem. Rem: The product of absorbed dose (rads) and a quality factor specific for the type of radiation. Each type of radiation has a specific quality factor (QF) based on different types of radiation producing different types of biologic damage. For example, the QF for x-rays is equal to 1. The SI unit equivalent of the rem is the sievert (Sv). Conversions for the rem and sievert can be expressed as follows: 1 rem = 0.01 Sv

Gray: the dose of ionizing radiation which results in the absorption of one joule of energy per Kilogram of material irradiated. The gray replaced the rad (radiation absorbed dose) in 1978 It is a large unit and the centigray (cGy), which equals 1 rad, is commonly used in clinical practice.

The Sievert (Sv) is the unit used in radiological protection and is a measure of the absorbed dose (measured in grays) and a biological quality factor Q which depends upon the nature of the radiation and the tissue damage it produces, so: sv= gy X Q . The quality factor for diagnostic X-ray is unity so for practical purposes the Sievert equals the gray. The Sievert is a large unit and the millisievert (mSv) is generally used in clinical practice.

Q/what are the Clinical situations possibly requiring radiographs?

The unit of equivalent dose is the sievert (Sv) The unit of equivalent dose is the sievert (Sv). For diagnostic x-ray examinations 1 Sv equals 1 Gy. The traditional unit of equivalent dose is the rem (roentgen equivalent man). One sievert equals 100 rem.

Equivalent Dose The equivalent dose (H T ) is used to compare the biologic effects of different types of radiation on a tissue or organ. Particulate radiations have a high LET and are more damaging to tissue than is low-LET radiation such as x rays. This relative biologic effectiveness of different types of radiation is called the radiation-weighting factor (W R ). For instance, deposition of 1 Gy of high-energy protons causes five times as much damage as 1 Gy of x-ray photons. The W R of photons, the reference, is 1. The W R of 5 keV neutrons and high-energy protons is 5 and the W R of α particles is 20. To account for this difference, the H T is computed as the product of the absorbed dose (D T ) averaged over a tissue or organ and the W R : Ht = Wr x Dt

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