Nematodes

RADIATION HAZARDS AND SAFETY

            “It is the responsibility of veterinarian Incharge to enforce radiation safety measures in the radiological section and to educate other personals about the potential hazards of radiation”.

Biological Effects of Radiation

All types of radiation produce changes within the living tissues. X-ray beam while traveling the tissue forces the electrons to be ejected from the atomic lattice. The atom is thus left with surplus positive charge and cells within tissue come to high chemical reactivity, which can initiate biological effects. This, in turn, results into physiological and pathological changes resulting into ‘radiation sickness.’ The radiation effects may be ‘somatic’ or ‘genetic’. The somatic effects are harmful to a person but genetic effects affect generations. Leukemia and malignant tumors are common somatic effects. While genetics effects may not be manifested for several generations.

Direct and Indirect Effects of Radiation

¨    The direct effects appear due to the absorption of energy by the molecules.

¨    Indirect effects are caused by the products of radiation decomposition (radiolysis) of water and other solutes of the body. After the radiolysis of water in the cells, there is the formation of free radicals (H° and OH°) with unpaired electrons. These free radicals and the hydrogen peroxide (H2O2) formed by them are highly reactive and mutagenic. This is one of the chief mechanisms of radiation damage as 80% of the biological system is water.

·         These free radicals can readily break chemical bonds in vitally important macromolecules of the body such as proteins, nucleic acids, and lipids. The radiation effects secondary or tertiary structures of proteins or may cause a break in H⁺ or disulfide bonds which maintain the secondary or tertiary structures of proteins.

·         The various types of damages to the DNA molecules are a change of base (deamination), loss of the base, H⁺ bond breakage between chains, single-strand break, double-strand break, and cross-linking with other DNA molecules and proteins. Pyrimidines are more radiosensitive than purines. Thymine seems to be the most sensitive. These changes may lead to mutation, disturbing normal cell proliferation or other cellular activities.

·         The free radicals also attack the double bond between carbon atoms of the fatty acids.

·         In carbohydrates, depolymerization is the most common effect.     

Radiation Sensitivity of Different Body Cells

            This refers to the loss of reproductive capability of proliferating cells. Cells regularly proliferating seem to be most radiosensitive. Thus the cells, which do not proliferate regularly, such as nerves and muscles, are relatively radioresistant. Stem cells of hemopoietic systems and cells of gut, skin, and testes are highly radiosensitive. These cells in the order of decreasing sensitivity can be listed as lymphoid cells, epithelial cells of the small intestine, hemopoietic cells, germinal cells, epithelial cells of the skin, connective tissue cells, cartilage and growing bone cells, cells of the brain and spinal cord and cells of the skeletal muscles and mature bones.

Susceptibility of Different Species to Radiation

The LD 50/30 (dose that will kill 50% of the population in 30 days) is influenced by the source of radiation and the dose rate.  The LD 50/30 of X-rays for different species is as under Goldfish (2300 Rad*) followed by Rabbit (800 Rad), Man (250-450 Rad) and Dog (350 Rad).

*Rad = is a unit of absorbed dose for any ionizing radiation.

Early Radiation Effects

            The intensive irradiation of the entire body severely depletes radiosensitive cells in many organs simultaneously. The combined effect produces ‘radiation sickness’ (acute radiation syndrome). The resulting symptoms depend upon the radiation dose rate given. The clinicopathological changes include immediate and severe lymphopenia with a slow recovery in survivors. Major acute damage occurs in the stem cell pool. As the stem cell population is depleted it no longer provides cells to replace those being lost from the functional pool. The death caused by radiation damage to the haemopoietic system is the result of severe neutropenia, which permits the development of fulminating infections in the body.

Effect of Radiation on Different Tissues of the Body

1.      Lymphoid Tissue: lymphocytes of lymph nodes and spleen are highly radiosensitive. Within 15 minutes after a moderate exposure, there is a marked reduction in cell division and necrotic changes. Regeneration may take 2-5 hours. Large doses permanently inhibit regeneration and marked shrinkage of lymphoid tissue. Secondary changes include the formation of abscesses, ulcers, hemorrhages, or other lesions in the organs drained by a particular lymph node.

2.      Bone Marrow: precursors of RBCs, granulocytes and platelets are radiosensitive. Erythroblasts are more sensitive. Within a week or two of exposure, most of the radiosensitive cells disappear from the marrow leaving an aplastic marrow.

3.      Cardiovascular System: heart and large arteries are radioresistant but the endothelium of capillaries is radiosensitive. The occlusion of capillaries and reduced blood supply to tissue may result.

4.      Digestive System: most of the clinical symptoms arise as a result of changes in the digestive system.

a.       Ulcers and erosions in the buccal cavity due to large doses

b.      Large doses may result in the necrosis and sloughing of the epithelium of the oesophagus and pharynx

c.       Gastric ulcers and hemorrhages in the stomach

d.      Duodenum is the most sensitive part of the small intestine. The crypt cells show nuclear fragmentation and their mitotic activity stops. Large doses may shorten the villi.

e.       Ulceration and necrosis in the large intestine

(Net result = loss of fluid and decreased absorption, electrolyte imbalance, and secondary infections.)

5.      Skin: A germinal layer of epithelium adversely affected. The skin may become loose. Radiation erythema is the most prominent sign in individuals with light colour. At higher doses, dermatitis and ulceration develop.

6.      Respiratory System: radiation pneumonia and fibrosis of lungs may be observed.

7.      Urinary System: parenchymal cells are radioresistant. Damage may occur due to injury to blood vessels. Ureters and urinary bladder are radioresistant. 

8.      Organs of Vision: inflammatory reactions in conjunctiva and sclera.

9.      Male Reproductive System: epididymis and vas deferens are radioresistant. Spermatogonia are highly sensitive while mature spermatozoa at higher doses may not be able to fertilize ova. If fertilization occurs implantation fails. The sterility dose in man and animals is above 200-400 Rads.

10. Female Reproductive System: Ova and granulosa cells are highly sensitive.

Delayed Effects of Radiation

          Late effects of radiation are least understood. It is relatively easy to measure the effect of high levels of acute exposure by observing the immediate death of the animal or rather obvious clinical signs. But the delayed effects of an acute exposure or the consequence of low exposure over an extended period of time aren’t easily identified. But the results are believed to be as under:

1.      A premature aging effect

2.      An increased susceptibility to disease

3.      An increase in the incidence of neoplasia, specifically in squamous cell carcinoma and leukemia.

General Principles of Radiation Safety

1. Increasing the distance between the radiation source and personal: doubling the distance from the source will reduce radiation exposure by a factor of four. So the person should stay as far away from the tube as possible. The following points control this factor:

(a)   No individual other than the operator and essentially needed persons should be present in the room when exposure is being made.

(b)   Whenever possible the animal should not be restrained manually. Chemical and physical restraint methods should be used.

(c)   To avoid exposure to the primary beam a cassette holding device should be used in large animal radiography.

(d)   Persons being used for restraining and holding the cassettes with cassette holders should be rotated.

(e)   The operator should be in the shielding booth or behind a screen or at least 6 feet away from the X-ray source when the exposure is made.

(f)    No part of the body should be exposed to the primary beam.

2. Use of Protective Barriers: These barriers provide protection against the scatter radiation and not against the primary beam. The lead shielding material used in these barriers reduces the dose of scatter radiation well below the 1/20^th of the scatter radiation dose.

(a)   Aprons: these should have 0.25 mm lead equivalent for voltage up to 100 kV. The material used mostly is lead rubber covered with clothing or plastic impregnated with metallic lead. Protective aprons should never be folded but kept flat since lead shielding material tends to separate after repeated bending.

(b)   Gloves and Goggles: the lead gloves should preferably have a 0.5 mm lead equivalent. Gloves should be checked periodically by radiography for cracks. Lead goggles are used during fluoroscopic examinations.

(c)   X-ray Room and Equipment: following points should be kept in mind while constructing an X-ray facility:

a.       It should be away from public places

b.      X-ray equipment periodically checked for leakage

c.       Warning signs must be placed near the X-ray room regarding the potential hazards

d.      The wall of the X-ray room should be at least 22 cm thick and should be of concrete into which the iron may be introduced. In the case of certain rooms where the X-ray beam is mostly directed horizontally, the wall should have a lead lining sandwiched between plywood.

4.      Reduction of Exposure Factor and Unnecessary Radiography: correct exposure factors should be used in the first attempt. Repeated exposures enhance the radiation dose of a person. Unnecessary exposures should be avoided.

5.      Use of Radiation Monitoring Devices: radiation monitoring devices like film badges should be worn all the time by the personals involved in the radiographic work. Ideally, one badge should be at the belt level to monitor the whole-body exposure and one above the apron, at the neckline to estimate the exposure to the skin of the head and neck and eyes. 

·         Film badges. The film badge is the most commonly used radiation detection device today. It provides a reasonably accurate means of determining doses from beta, gamma, and x-radiation. Most film badges consist of a plastic holder containing radiation-sensitive film, usually of dental film size or 35-mm photographic film. The film badge also contains a variety of filters used to absorb radiations of varying energies. The variety of filters placed at different points on the film badge allows identification of a specified type and energy of the radiation-producing exposure. Films are developed and then evaluated by measuring the density of the blackening on the film. These measurements are compared to standard films that have been exposed to known radiation doses. The same film may be worn for a week, or more commonly a month. The length of time depends on the sensitivity of the film and the amount of radiation to which the radiation worker is exposed. Radiation monitoring badges are available in several forms: l) rings, 2) clips, and 3) wrist badges.

6.      X-ray beam filters, collimators, grids: Already discussed.

Age and Sex of the Involved Personal: persons under 18 years of age should not be involved in radiographic exposures because of the sensitivity of the growing tissues. Because of the extreme sensitivity of the human embryos at certain stages of development, pregnant women should not be involved in radiography.