RADIATION BIOLOGY AND PROTECTION
It is the study of effect of ionizing radiation on living system.
It may be:
- Direct effect.
- Indirect effect.
Effect of ionizing radiation also divided into:
- Deterministic effect.
- Stochastic effect.
- When energy of photon or secondary electron transferred directly to biologic macro molecule the effect is termed direct effect.
Indirect effects are those in which hydrogen and hydroxyl free radicals, produced by the action of radiation on water, interact with organic molecules.
Probability of having effect and dose
Probability of effect independent of dose. All individuals show effect when dose is above threshold.
Frequency of effect proportional to dose. The greater dose the greater chance of having effect.
Severity of clinical effects and dose
Clinical effects are proportional to dose. Greater the dose the greater effect.
Clinical effect is independent of dose. Individual either has effect or does not.(all-or-none response).
Killing of many cells
Sublethal damage to DNA
Yes, sufficient cell killing required for changes.
No, even 1 photon cause change in DNA.
Radiation induced mucositis
Radiation induced cancer
- Nucleic Acids
- Breakage of one or both DNA strands.
- Cross-linking of DNA strands within the helix, to other DNA strand or to protein.
- Change or loss of base.
- Disruption of hydrogen bonds between DNA strands.
- Primary structure of the protein is usually not significantly altered.
- Secondary and tertiary structures are affected by breakage of hydrogen or disulfide bonds.
- Inactivation of enzymes.
- It is more sensitive in dividing cells.
- Sensitive site is DNA within chromosomes.
- Chromosomal aberrations
- Depends upon stage of cell cycle.
- Irradiation of the cell after DNA synthesis results in single arm (chromatid) aberration that goes unrecognized as they get repaired.
- Irradiation before DNA synthesis results in double arm aberration that are complicated due to lack of intact template strand and thus miss repair are common.
- Increased permeability of plasma membrane to sodium and potassium ions.
- Swelling and disorganization of internal cristae.
- Focal cytoplasmic necrosis.
- Mitotic Delay
- Mild dose: Mild mitotic delay.
- Moderate dose: Longer mitotic delay.
- Severe dose: Profound delay with incomplete recover
- Cell Death
Reproductive death in a cell population is loss of the capacity for mitotic division.
The three mechanisms of reproductive death are:
- DNA damage.
- Bystander effect.
- DNA DAMAGE
Single strand break can repair.
Double strand break is responsible for:
- Cell death.
- Bystander effect
- It is the phenomenon in which un-irradiated (normal) cells exhibit irradiated effects as a result of signals received from nearby irradiated cells.
- This effect causes chromosome aberrations, cell killing, gene mutations, and carcinogenesis.
- Also known as programmed cell death occurs during embryogenesis
- Common in hemopoietic and lymphoid tissues.
- Cells round up, draw away from their neighbors and condense nuclear chromatin.
- Highly sensitive
Cells that are dividing regularly, long mitotic feature, undergo no or little differentiation between mitosis.
- Spermatogenic cells.
- Erythroblastic stem cells.
- Basal cells of oral mucosa.
- Intermediate sensitive
Divide occasionally in response to demand for more cells.
- Vascular endothelial cells.
- Acinar or ductal salivary gland cells.
- Parenchymal cells of liver, kidney and thyroid.
- Low sensitive
- Highly differentiated, when nature are incapable of division
- Striated muscle cells.
- Squamous epithelial cells.
- Short-term effects are determined by the sensitivity of its parenchymal cells.
- If continuously proliferating tissues (e.g., bone marrow, oral mucous membranes) are irradiated with a moderate dose, cells are lost primarily by mitosis linked death.
- Tissues composed of cells that rarely or never divide (e.g., muscle) demonstrate little or no radiation induced hypoplasia over the short term.
- Long-term deterministic effects depend primarily on the extent of damage to the fine vasculature.
- Irradiation of capillaries causes swelling, degeneration, and necrosis that lead to slow progressive fibrosis around the vessels.
- Deposition of fibrous scar tissue is increased around the obliteration of vascular lumens and impairs the transport of oxygen, nutrients, and waste products and results in death of all cell types.
It assumed that 2Gy of dose administered daily bilaterally through 8X10cm fields over oropharynx, for weekly exposure of 10 Gy that continues for 6 -7 weeks until total of 64 – 70 Gy is administered. Following changes occurs:
- Radiation mucositis (Refer to chapter of oral cancer).
- Effect on salivary glands.
- Osteoradionecrosis (Refer to chapter of oral cancer).
- Radiation caries (Refer to chapter of oral cancer).
- Radiation induced fibrosis.
- Parenchymal component of glands is more radiosensitive (Parotid > sub mandibular > sublingual salivary glands).
- Marked loss of salivary secretion during 1st
- May reach to 0 at 60 Gy.
- Mouth become dry (xerostomia) & tender.
- Swallowing difficult & painful (Residual saliva loses its normal lubricating properties].
- Dry mouth more severe if parotids are irradiated bilaterally.
- Low pH of irradiated saliva 5.5 (Normal pH – 6.5).
- Decalcification of normal enamel (due to low pH).
- Buffering capacity of saliva falls by 44%.
- Radiation caries (oral microflora becomes more acidogenic, S. mutans, Lactobacillus & Candida count increased).
- If some portion of major salivary gland spared dryness of mouth usually subsides in 6 – 12 months because of compensatory hypertrophy of residual salivary gland tissue.
- The reduced salivary flow, if persist beyond a year is unlikely to show significant recovery.
- Destruction of tooth bud [Irradiation before calcification].
- Inhibition of cellular differentiation causing malformations & arresting general growth. (Irradiation after calcification).
- Retarded root development, dwarfed teeth or failure to form one or more teeth (permanent dentition) Irradiation of jaws in children.
- Pulpul tissue shows long – term fibroatrophy after irradiation.
- Eruptive mechanism of teeth is relatively radiation resistant.
- Radiation caries: refer to oral cancer.
It is collection of sign and symptoms in an individual after acute whole body exposure to radiation.
- Prodromal symptom (Dose=1 to 2 Gy)
- Hematopoietic systems (Dose= 2 to 7 Gy)
Rapid fall in circulating granulocytes, platelets, erythrocytes due to radio sensitivity of their precursors.
- Infection (due to lymphopenia and granulocytopenia).
- Hemorrhage (due to thrombocytopenia).
- Anemia (due to erythrocyte depletion).
If death occurs due to hematopoietic syndrome, it usually occurs 10 to 30 days after irradiation.
- Gastrointestinal syndrome (Dose= 7 to 15Gy)
Individuals exposed in this range may experience the prodromal stage within a few hours of exposure.
- The denuded mucosal surface.
- Loss of plasma and electrolytes.
- Loss of efficient intestinal absorption.
- Ulceration of mucosal lining.
- Hemorrhage into intestine.
- Loss of weight.
- Septicemia (due to invasion of endogenous intestinal bacteria).
- Death occurs within 2 weeks.
- Cardiovascular and Central Nervous System Syndrome (More than 50 Gy)
- Death occurs in 1 to 2 days.
- Collapse of circulatory system.
- Fall in BP in hours.
- Necrosis of cardiac muscles.
- Intermittent stupor.
- In coordination.
- Disorientation convulsions.
- It is period of apparent well being after radiations during which no signs or symptoms of radiation sickness occurs.
- Latent period is of few weeks at sublethal doses of less than 2Gy.
- Latent period extends from few hours to few days at supralethal doses of more than 5 Gy only.
- Antibiotic (during infections or leucopenias).
- Fluid & electrolyte replacement.
- Administration of platelets (to arrest hemorrhage).
- Whole blood transfusion (to treat anemia).
- Bone marrow graft indicated between identical twins [as, there is no risk for graft v/s host disease.
- They are highly radio sensitive.
- Most sensitive period between 18 – 45 days of gestation (organogenesis).
- The effects are deterministic in nature. Which include:
- Reduce growth.
- Reduce head circumference.
- Mental retardation.
- Small birth size.
- Genital & skeletal malformation.
- Chances of childhood cancers (Leukemias & solid tumors).
- Stochastic effects result from sub lethal changes in DNA of individual cells.
- Consequence of such damage is carcinogenesis.
The most important late somatic effects include –
- Reduced growth & development.
- Mental retardations.
- Radiation causes increased frequency of spontaneous mutations rather than inducing new mutations.
- Frequency of mutations increases in direct proportion to the dose, even at very low doses, with no evidence of a threshold.
- Majority of mutations are deleterious to the organism.
- Dose rate is important. At low dose rates the frequency of induced mutations is greatly reduced.
- Male are much more radiosensitive to than females.
- The rate of mutations is reduced as the time between exposure and conception increases.
According to National council on radiation protection & measurements
1- Natural or background radiation (83%)
- Cosmic (8%, 0.27msv).
- Terrestrial (8%, 0.28msv).
- Radon (56%, 2.00msv).
- Others (11%, 0.40msv).
2 – Artificial (17%)
- X-ray (0.39msv).
- Nuclear Medicine (0.14msv).
- Consumer and industrial products (0.10msv)
- The ICRP or the International Commission for radiation protection is the international regulatory body.
- Each country has its national counterpart of the ICRP.
- In America the counterpart is the NCRP or The National Commission for Radiological Protection.
- In India it is the AERB or the Atomic Energy.
- The International Commission of Radiation Protection (ICRP) was formed in 1928 on the recommendation of the first International Congress of Radiology in 1925.
- The commission consists of 12 members and a chairman and a secretary who is chosen from across the world based on their expertise.
- The first International Congress also initiated the birth of the ICRU or the International Commission on Radiation Units and measurements.
- The Indian regulatory board is the AERB, Atomic Energy Regulatory Board.
- The Atomic Energy Regulatory Board was constituted on November 15, 1983 by the President of India by exercising the powers conferred by Section 27 of the Atomic Energy Act, 1962 (33 of 1962) to carry out certain regulatory and safety functions under the Act.
- The regulatory authority of AERB is derived from the rules and notifications promulgated under the Atomic Energy Act, 1962 and the Environmental (Protection) Act, 1986.
- The mission of the Board is to ensure that the use of ionizing radiation and nuclear energy in India does not cause undue risk to health and environment. Currently, the Board consists of a full-time Chairman, an ex-officio Member, three part-time Members and a Secretary.
- It is an important radiation concept is the “As Low As Reasonably Achievable” (ALARA).
- ALARA means that every reasonable measure will be taken to assure that occupationally and nonoccupationally exposed person will receive the smallest amount of radiation possible.
There are three guiding principle:
- Principle of justification.
- Principle of optimization.
- Principle of dose limitation.
- Prescribing dental radiograph should be justified in order to minimize the unwanted exposure.
- Diagnostic radiography should be used only after clinical examination, consideration of patient’s history and consideration of both dental and general health needs of patients
Dentist should use every means to reduce unnecessary exposure to their patients and themselves.
Dose limits are used for occupational and public exposures to insure that no individuals are exposed to unacceptably high doses.
- Image receptor
- Film with faster speed should be used.
- Faster the speed more sensitive image receptor and less time required to some extent.
- Intraoral dental x-ray film is available in D, E, and F speed.
- Group E is almost twice as fast (sensitive) as film of group D and about 50 times as fast as regular dental x-ray film.
- F-speed film requires about 75% the exposure of E-speed film and only about 40% that of D-speed.
- Patient dose reductions of 75% compared with D speed film, 50% compared with E-speed film, and about 40% compared with F-speed film may be achieved using digital intraoral radiography.
- Intensifying screen
- Used for extra oral radiography.
- Contains rare earth elements Gadolinium, lanthanum that decreases the patient exposure by as 55% as compare to older tungstate screen.
- Further exposure achieved by using T grain film.
- T-grain film used with rare earth screens is twice as fast as calcium tungstate screen-film combinations and 1.33 times as fast as conventional rare earth screen-film combinations with no loss in image quality.
- Focal spot to film distance
- The combination of proper collimation and extended source-patient distance (focal spot-to-film distance) will reduce the amount of radiation to the patient.
- To focal spot film distance used in dental radiography.
- 20 cm (8 inch).
- 41cm (16 inch).
- Use of long FS to FD result in 32% reduction in expose tissue volume as greater distance the x ray beam is less divergent.
- It is required by federal law to collimate (limit) the field of radiation at patient’s skin surface to a circle having a diameter of no more than 7 cm (2.75 inch).
- Use of a rectangular PID having an exit opening of 3.5 x 4.4cm (1.38 x 1.34 inches) reduces the area of the patient’s skin surface exposed
- by 60% over that of a round (7 cm) PID.
- Film holders using round and rectangular collimation, rectangular collimation reduced the patient dose from intraoral examinations by about 60%.
- Filtration removes soft, low energy, long wave length x-rays from beam that absorb by the patient without contributing to the formation of radiographic image.
- X ray beam filtered with 3mm of Al reduces the patient exposure up to 20%
- 5mm Al equivalent filtration is required for 50 to 69kVp.
- 5mm Al equivalent filtration is required for 70kVp or more.
- In panoramic and cephalometric x-ray machines use of rare earth elements like samarium, erbium, yttrium, niobium, gadolinium, terbium activated gadolinium oxysulfide, and thulium-activated lanthanum oxy bromide in combination with aluminum filtration may reduce patient exposure by 20% to 80% compared with conventional aluminum filtration alone.
- Lead aprons and collars
- Leaded torso (body) aprons will reduce genetic exposure by 98% for panoramic radiography.
- Apron should cover both the patient’s chest as well as shoulders and back because tube head exposes the film from this position.
- Thyroid shield reduces the radiation exposure to thyroid gland approx. 50%.
- Lead apronsand/or skirt and vest garments need to be between 0.25 and 0.5 mm thick, properly stored, and inspected every 6 months to a year for cracks, creases, or rupture to ensure adequate protection.
- Protective 0.15-mm lead–equivalent glasses or goggles limit the eye lens dose and provide about 70% attenuation even in high energy (kVp) beams
- Kilovoltage (Kvp)
- X-ray machine should be operated at highest kilovoltage (kVp) consistent with good image, usually 70 to 90 kVp.
- X-ray machine incapable of being operated at least at 60kVp should not be used.
- High kVp produces more useful x-rays and fewer low energy rays that are absorbed by patient without contributing to quality of image.
- Milliampere-second (mAs)
- Image density is controlled by quantity of x ray produced which controlled by combination of milliamperage and exposure time, termed milliampere-second (mAs).
- Use of phototimer measures the quality of radiation reaching the film and automatically terminates the exposure when enough radiation reached the film to provide optimal density.
- This also reduces the unnecessary prolonged exposure.
- Choice of intraoral technique
- Paralleling long cone technique preferred.
- Film holders to be used instead of patient manual support.
- During processing the films
Film processing should be performed under the manufacturer-recommended conditions with proper processing equipment and a darkroom with safelights. Alternatively, an automatic processor with an appropriate safelight hood may be used.
Way to minimize unnecessary re-exposures:
- Proper exposure.
- Good quality processing solution.
- Time, temperature, processing.
- Well-equipped and maintained dark room.
- Correctly maintained automatic processors.
- Use of magnifying glass for studying of radiograph.
- Interpretation of the Image
- Radiographic images should be viewed under proper conditions with “an illuminated viewer to obtain maximum available information.
- By viewing the radiograph in semi darker room with light transmitted through the film, all extraneous light should be eliminated.
Shielding implies that certain materials (concrete, lead) will attenuate radiation (reduce its intensity) when they are placed between the source of radiation and the exposed individual.
There are four aspects of shielding in diagnostic radiology:
- X-ray tube shielding.
- Room shielding:
(a) X-ray equipment room shielding.
(b) Patient waiting room shielding.
- Personnel shielding.
- Patient shielding (of organs not under investigation).
- The x-ray tube housing is lined with thin sheets of lead because x-rays produced in the tube are scattered in all directions. This shielding is intended to protect both patients and personnel from leakage radiation.
- Manufacturers of x-ray devices are required to shield the tube housing so as to limit the leakage radiation exposure rate to 0.1 R hr-1 at 1 meter from the tube anode.
- AERB recommends a maximum allowable leakage radiation from tube housing not greater than 1mGy per hour per 100 cm2.
The lead lined walls of Radiology department are referred to as protective barriers because they are designed to protect individuals located outside the X-ray rooms from unwanted radiation.
There are two types of protective barriers:
- Primary Barrier: is one which is directly struck by the primary or the useful beam.
- Secondary Barrier: is one which is exposed to secondary radiation either by leakage from X-ray tube or by scattered radiation from the patient.
The shielding of X-ray room is influenced by the nature of occupancy of the adjoining area. In this respect there are two types of areas:
- Control Area: Is defined as the area routinely occupied by radiation workers who are exposed to an occupational dose. For control area, the shielding should be such that it reduces exposure in that area to <26 mC/kg/week.
- Uncontrolled areas: Are those areas which are not occupied by occupational workers. For these areas, the shielding should reduce the exposure rate to <2.6mC/kg/week.
- X-ray examination room shielding
- Rooms housing diagnostic X-ray units are located as far away as feasible from areas of high occupancy and general traffic that are not directly related to radiation and its use.
- The room housing an X-ray unit is not less than 18 m2 for general purpose radiography and conventional fluoroscopy equipment.
- In case the installation is located in a residential complex, it is ensured that:
- Wall of the X-ray rooms on which primary X-ray beam falls is not less than 35 cm thick brick or equivalent.
- Walls of the X-ray room on which scattered X-rays fall is not less than 23 cm thick brick or equivalent,
- There is a shielding equivalent to at least 23 cm thick brick or 1.7 mm lead in front of the doors and windows of the X-ray room to protect the adjacent areas, either used by general public or not under possession of the owner of the X-ray room.
- Unshielded openings in an X-ray room for ventilation or natural light are located above a height of 2 m from the finished level outside the X-ray room.
- Rooms housing fluoroscopy equipment are so designed that adequate darkness can be achieved conveniently, when desired, in the room.
- Patient waiting area
- Patient waiting areas are provided outside the X-ray room.
- A suitable warning signal such as red light and a warning placard is provided at a conspicuous place outside the X-ray room and kept ‘ON’ when the unit is in use to warn persons not connected with the particular examination from entering the room.
- AERB has laid down recommendations for personnel protection of radiation workers.
- The protective barrier between the operator and X-ray tube should have a minimum lead equivalence of 1.5mm.
- Protective aprons and gloves should have a minimum lead equivalence of 0.25mm, and gonadal shields should have a minimum lead equivalence of 0.5mm.
- Any additional radiation protection devices which would be necessary for specialized radiological investigations should have a minimum of 0.5mm lead equivalence
The walls must be of sufficient density and thickness walls must be coated with lead and even with gypsum wall board (mentioned above)
- The operator should stand at least 6 feet from patient at an angle of 90˚ to 135˚ to the central ray of x ray beam when the exposure is made.
- In this position most scatter radiation is absorbed by the patient’s head.
- Operator should never hold the films in place, if correct film placement and retention are not possible ask any patients attendant to hold the film (lead apron given to him)
- During making exposures radiographic tube should not be hold.
- The instruments used to detect radiation are referred to as radiation detection devices.
- Instruments used to measure radiation are called radiation dosimeters.
There are several methods of detecting radiation, and they are based on physical and chemical effects produced by radiation exposure. These methods are:-
- Photographic effect.
- The ability of radiation to produce ionization in air is the basis for radiation detection by the ionization chamber.
- It consists of an electrode positioned in the middle of a cylinder that contains gas.
- When x-rays enter the chamber, they ionize the gas to form negative ions (electrons) and positive ions (positrons).
- The electrons are collected by the positively charged rod, while the positive ions are attracted to the negatively charged wall of the cylinder.
- The resulting small current from the chamber is subsequently amplified and measured.
- The strength of the current is proportional to the radiation intensity.
The photographic effect, which refers to the ability of radiation to blacken photographic films, is the basis of detectors that use film.
- Luminescence describes the property by which certain materials emit light when stimulated by a physiological process, a chemical or electrical action, or by heat.
- When radiation strikes these materials, the electrons are raised to higher orbital levels. When they fall back to their original orbital level, light is emitted.
- The amount of light emitted is proportional to the radiation intensity.
- Lithium fluoride, for example, will emit light when stimulated by heat.
- This is the fundamental basis of thermoluminescence dosimetry (TLD), a method used to measure exposure to patients and personnel.
- Scintillation refers to a flash of light.
- It is a property of certain crystals such as sodium iodide and cesium iodide to absorb radiation and convert it to light.
- This light is then directed to a photomultiplier tube, which then converts the light into an electrical pulse.
- The size of the pulse is proportional to the light intensity, which is in turn proportional to the energy of the radiation.
- Personnel dosimetry refers to the monitoring of individuals who are exposed to radiation during the course of their work.
- Monitoring is accomplished through the use of personnel dosimeters such as the pocket dosimeter, the film badge or the thermoluminescent dosimeter.
- The radiation measurement is a time-integrated dose, i.e., the dose summed over a period of time, usually about 3 months.
- The dose is subsequently stated as an estimate of the effective dose equivalent to the whole body in mSv for the reporting period.
- Dosimeters used for personnel monitoring have dose measurement limit of 0.1 – 0.2 mSv (10-20 mrem).
- The pocket dosimeter monitors dose to personnel.
- It consists of an ionization chamber with an eyepiece and a transparent scale, as well as a hollow charging rod and a fixed and a movable fiber.
- When x-rays enter the dosimeter, ionization causes the fibers to lose their charges and, as a result, the movable fiber moves closer to the fixed fiber.
- The movable fiber provides an estimate of gamma or x-ray dose rate.
- These badges use small x-ray films sandwiched between several filters to help detect radiation.
- Film badges are inexpensive, easy to use, and easy to process.
- They are useful for detecting radiation at or above 0.1 mSv (10 mrem), they are not sensitive enough to capture lower levels of radiation.
- Their susceptibility to fogging caused by high temperatures and light means that they cannot and should not be worn for longer than a 4-week period at a stretch.
- Other major drawback to film badge monitoring is that it is an enormous task to chemically process a large number of small films and subsequently compare each to some standard test film.
- In India, film badges have recently been replaced by TLD badges.
- The limitations of the film badge are overcome by the thermoluminescent dosimeter (TLD).
- Thermolumine-scence is the property of certain materials to emit light when they are stimulated by heat.
- Materials such as lithium fluoride (LiF), lithium borate (Li2B4O7), calcium fluoride (CaF2), and calcium sulfate (CaSO4) have been used to make TLDs.
When an LiF crystal is exposed to radiation, a few electrons become trapped in higher energy levels. For these electrons to return to their normal energy levels, the LiF crystal must be heated.
- As the electrons return to their stable state, light is emitted because of the energy difference between two orbital levels.
- The amount of light emitted is measured (by a photomultiplier tube) and it is proportional to the radiation dose.
- Whereas the TLD can measure exposures to individuals as low as 1.3µC/kg (5 mR), the pocket dosimeter can measure up to 50 µC/kg (200 mR). The film badge, however, cannot measure exposures < 2.6 µC/kg (10 mR).
- In step 1: the TLD is exposed to the radiation.
- In step 2: the LiF crystal is placed in a TLD analyzer, where it is exposed to heat. As the crystal is exposed to increasing temperatures, light is emitted. When the intensity of light is plotted as a function of the temperature, a glow curve results.
- The glow curve can be used to find out how much radiation energy is received by the crystal because the highest peak and the area under the curve are proportional to the energy of the radiation.
- These parameters can be measured and converted to dose.
The recommendations of various authorities are as follows:
- ICRP: In pregnant females, a supplementary equivalent dose limit of 2mSv applied to the surface of her lower abdomen for the remainder of her pregnancy.
- NCRP: Recommends dose limit 0.5 mSv per month for the embryo/fetus for occupational pregnant workers].
- AERB: Recommends that once pregnancy is established the equivalent to surface of pregnant woman’s abdomen should not exceed 2 mSv for the remainder of the pregnancy.
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- Langlang OE, Langlais RP Rdilogic health and protection. Book of principles of dental imaging 1st edition 1997;311-320.
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- AERB Safety Code, (Code No. AERB/SE/MED-2), Mumbai 2001:1-20.
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