Doris C MacClelland, RN
Sterilization by ionizing radiation
Ionizing radiation for sterilization is an established procedure in industry for the processing of medical products. It has replaced other techniques because of its reliability, economy, and freedom in packaging. In most in-
Doris C MacClelland, RN, M S , is a Commander in the US Navy Nurse Corps. She is operating room supervisor at the Naval Regional Medical Center, S a n Diego, Calif A graduate of Emanuel Hospital School of Nursing, Portland, Ore, she received her BA degree from San Francisco State College and her M S from Indiana University, Indianapolis. The opinions and assertions contained herein are those of the author and are not to be construed as official or reflect the views of the Department of the Navy.
stances, irradiation processing requires less energy than conventional methods, which may become increasingly important as we face energy shortages. More sophisticated medical techniques and a continuing increase in the number of patients have put additional constraints on the availability of sterile supplies, thus making sterilization methods an economic as well as a technical problem. Specialty nurses, who are responsible for ensuring the availability of sterile supplies for use within the operating suite, must be knowledgeable about the changing concepts and trends of sterile supply management. This article provides sufficient information on radiation sterilization techniques so that decisions concerning use of supplies sterilized in this way can be made knowledgeably and confidently. History. Wilhelm Conrad Roentgen’s discovery of penetrating x-rays in 1895, closely followed by Antoine Henri Becquerel’s identification of rays emitted from uranium compounds and Pierre and Marie Curie’s discovery of radium-emitting alpha, beta, and gamma rays, became the groundwork for nuclear radiation physics. Research in this field has led to radiation techniques such as sterilization.
AORN Journal, October 1977, Vol26, No 4
676
World War I1 accelerated this research, and in the postwar years, numerous technical organizations and governmental, academic, and industrial agencies worldwide were involved in the investigation and development of radiation sterilization techniques and their practical applications for medical and industrial use. The first commercial use of radiation for sterilization of medical products, such as sutures, disposable needles, and syringes, was introduced by Ethicon in 1956.’About the same time, Denmark’s Atomic Energy Commission opened Europe’s first CD. balt 60 commercial radiation sterilization plant. In the early 1960s,the United Kingdom and the USSR began using cobalt 60 for commercial sterilization of medical products. One of the largest cobalt 60 plants designed for industrial sterilization was built in 1959 in Dandenong, Australia, for sterilization of imported goat hair bales used in the manufacture of carpets.* No longer needed for this purpose, it is now used on a contract basis for sterilization of medical goods and biological tissues. Radiation process. Radiation is the emitting of energy in the form of rays or particles. The rays consist of an electromagnetic spectrum that includes electric waves or household current, radio waves, infrared rays, visible rays that allow for sight and color, ultraviolet rays, x-rays, and gamma rays. The particles are minute portions of matter that move through space and are capable of transferring energy. These particles include electrons, alpha particles, fission fragments, and others. Cobalt 60 gamma-ray emitters and electron accelerators are two methods of eliciting the energy transfer processes. Radiation may be either ionizing or
676
nonionizing. An ion is an electrically charged particle. Ionizing radiation is therefore any electromagnetic or particulate radiation capable of producing ions, directly or indirectly, by interaction with matter. The direct action may be energy absorption or transfer within a molecule, causing the molecule to react with cellular constituents and create an observable biological effect. The indirect action is brought about by energy being absorbed in material surrounding the target molecule; the surrounding material then conveys damage to the target m ~ l e c u l e .The ~ resulting damage interferes with the enzyme and genetic systems of the cell, thereby producing cell death by the inability of the cell to propagate. Nonionizing radiation, since it does not involve electrical energy transfer, destroys organisms in several other ways-ultrasonic waves disrupt cells, infrared rays produce heat, and ultraviolet rays interfere with metabolism. Because nonionizing radiation is not as effective for sterilization as ionizing radiation, it is most commonly used for disinfection in hospitals and operating rooms as a means of reducing bacterial counts in the air. Measuring radiation. Precise measurement of radiation exposure is important for the efficient and safe operation of radiation plants as well as for consistently reliable radiation sterilization processes. The basic unit of measurement for electromagnetic radiation is the roentgen; however, because the roentgen does not measure particulate radiation, it was necessary to establish the radiation absorbed dose (rad) received by any subject. Technically, a rad is a unit of absorbed dose of ionizing radiation equal to an energy of 100 erg/gm of irradiated material. The megarad (Mrad) is a
AORN Journal, October 1977, Vol26, No 4
C
ombinations of heat and radiation act synergistically to kill cells.
dose equivalent to 1 x lo6 rad and is the unit of measure used for measuring the dose range required for sterilization to take place. The monitoring devices in most cobalt 60 plants are capable of measuring absorbed doses in the range from about 0.5 to 5.0 Mrad. Since sterilization is dependent on total dose received, reliable and consistent predictions of kill can be made. Research studies have demonstrated that an exposure of 2.5 Mrad ensures a high degree of sterility, and this level has been adopted by industry as the safe dose for routine sterilization of hospital ~ u p p l i e s . ~As with other methods of sterilization, the end result is dependent on the degree of contamination present prior to the sterilization process; therefore, it is imperative that sanitation standards be established and maintained at the manufacturer’s level t o ensure that items will not be grossly contaminated when packaged for sterilization. To illustrate the destruction level of selected material with the standard 2.5-Mrad dose, rubber gloves and tubing will experience no loss of tensile strength after dosages of less than 8 Mrad, and many polymers will suffer no serious change below about 10 Mrad. Levels of exposure. There are three levels of exposure that may be at-
tained during ionizing radiation. The minimum exposure is the period necessary to reduce the specific bacterial count to the predetermined level; the target exposure is the minimum dose with added safety criteria; and the maximum exposure is that dose at which some undesirable effect occurs.5 During the sterilization process, all material does not receive the same amount of radiation even though all material receives the target dose. Two factors that account for the differences in exposure are the bulk density of the packages and the distance the packages are from the source of radiation. To counteract this effect, material may pass the source at different angles or at different distances on a programmed basis, or some packages may be rotated on a turntable. Effects of radiation on specific microorganisms. Environmental and natural conditions of microorganisms before, during, and after irradiation are important to radiosensitivity. The lethal dose of radiation for cells is influenced by the presence of oxygen, the degree of hydration of the cell, and the stage of growth cycle at the time of radiation. The presence of chemicals such as glycerol, aliphatic alcohols, hydrogen sulfide, and others can also reduce the lethal effects of radiation.6 Generally, viruses are more resistant than bacterial spores, and spores
AORN Journal, October 1977,Vol26, No 4
677
are more resistant than vegetative organisms, yeasts, and molds. Gramnegative organisms are more susceptible than Gram-positive organisms; therefore, Pseudomoms aeruginosa is more readily killed than Streptococcus or Staphylococcus. Because of its resistance to radiation, Bacillus pumilus is recommended for biologic control of radiation sterilization as B subtilis is for ethylene oxide and B stearothermophilus is for steam sterilization.' Since many organisms can be killed by either heat or radiation, recent data c o n k n that properly chosen combinations of heat and radiation act synergistically to kill cells.* Studies using dry B subtilis spores show that increases in temperature can lead to an appreciable acceleration of radiation sterilization cycles. Similar synergism has been observed in a variety of biological systems, including active proteins, viruses, spores, bacteria, yeasts, mammalian cells, and human cancers.9 Two potential benefits of this synergism are (1) because many radioresistant organisms are relatively heat labile, sufficient heat alone will tend to destroy these organisms and (2) organisms that are both radioresistant and heat resistant can be eliminated by designing the proper heat-radiation cycle to inactivate the organisms synergistically.10 Preliminary data also suggest that combinations of heat and radiation may produce fewer mutations, one of the major disadvantages of radiation sterilization. It has been demonstrated that mutations induced by radiation may be as high as 15%. However, the mutation rate among survivors of heat-radiation cycles is near the natural level expected for the organisms investigated .l1 Types of commercial radiation plants
670
Types of ionizing radiation Table 1
Cobalt 60 gamma rays Advantaoes Highly penetrating; products can be sterilized in shipping cartons Large volumes can be irradiated simuttaneously Simplicity and reliability of radiation source Dose easily controlled Penetrates liquids and solids, including bulk powders Plants operate continuously Disadvantages a Exposure times are long (8 to 24 hours) a Plants are elaborate and expensive to build Stringent personnel safety standards because of increased risk of radiation exposure More expensive than electron accelerators to operate
Electron accelerators Advantages Sterilizationachieved in seconds Plants require less space and are cheaper to build and easier to operate Required safety precautions not as stringent as with gamma emitters Disadvantages Less penetration capability: penetration dependent on acceleration of electrons Only small packages can be processed Maintenance of constant dose is more difficult
in use. For irradiation of medical products, gamma-ray emitters and high-energy electron accelerators are the only methods in common use. There are approximately 65 gammaray facilities in use throughout the world as compared to approximately
AORN Journal, October I977,V o l 2 6 , No 4
Materials suitable for ionizing radiation Table 2
Thermoplastics (most common) polystyrene polyethylene polyamide (nylon) polyester (Dacron, Mylar) polyurethane polymethyl methacrylate (Perspex, Plexiglas) ionorner (Surlyn-A) polypropylene cellulose esters polyvinyl chloride (discolors) Thermosetting resins epoxy phenolics melamine urea Rubbers natural latex segmented polyurethane butadiene-styrene neoprene (discolors) butyl (discolors) Other materials wool silk
cellulose metals glass (discolors) ten accelerator systems.12 Even though gamma-ray emitters are more expensive to operate than accelerator systems, 20 years ago when irradiation plants were first being built the majority of radiation sources being used were the cobalt 60 plants. The accelerators now available have undergone considerable technical improvement, resulting in a highly reliable process at a substantially lower cost than cobalt 60 emitters. For this
reason, electron accelerators will probably gain in popularity in years to come. Sterilization of medical products is only one use of commercial radiation plants, which are also used for processing plastics, vulcanization of rubber, curing paint, preserving food, and other industrial uses. Irradiation processing requires less energy than other processing methods, a major consideration in dealing with energy shortages. Also, irradiation processing is less expensive. For example, steam sterilization of medical disposable products is estimated to cost 2$ per pound, whereas radiation sterilization is 0.05c per pound. Cobalt 60 plants consist of four basic parts: source of radiation, protective shield, product conveyor or transfer mechanism, and dosimeter. The plants may be either automatic (continuous product flow) or batch. Since gamma rays are highly penetrating, they are more versatile in that they allow for complete freedom of packaging. The rate of energy absorption or absorbed dose is much slower with gamma-ray sources than with accelerators, requiring as much as 8 to 24 hours to deliver the standard 2.5-Mrad dose. Because the rate is slower, it is easier to monitor, thereby ensuring constant levels of exposure. Particle accelerators may be either an AC generator, such as the Van de Graaff accelerator, or a linear accelerator. In both models there is an electron gun, an acceleration tube, and a scanning device to spread the electrons over the desired target area. In contrast to the gamma-ray emitters, the penetration capability depends on the speed of the electrons, which can vary, but the absorbed dose is obtained within seconds. The rapidly absorbed dose makes monitoring of the process
AORN Journal,October 1977,Vol26, No 4
679
onizing radiation is virtually devoid of thermal reaction.
I
more difficult, and because the rays are less penetrating, the versatility of packaging is somewhat restricted. Table 1 lists advantages and disadvantages of gamma-ray and accelerator plants. In 1967, the International Atomic Energy Agency, in collaboration with the World Health Organization, prepared a ‘?Recommended code of practice for the radiosterilization of medical products,” which was reviewed and updated in 1974.13 Major categories discussed in the code include conditions for the manufacture of items to be radiosterilized, microbiological control of radiation facilities, and operation of radiation sterilization facilities. These recommendations will be subject to updating as new technical information and experiences are accumulated. Current applications. Originally used only for such items as sutures, needles, syringes, and other disposable hospital products, radiosterilization is now used for pharmaceuticals, biological tissues, prostheses, highly specialized surgical instruments, hospital-administered fluids, and numerous industrial uses. Although many of these uses are still in the research and development stage, current studies are establishing their safety and feasibility. Table 2 lists some of the materials used in medical products that are suit-
680
able for ionizing radiation sterilization.14 Pharmaceuticals: Dry pharmaceuticals and ointments are mostly stable when subjected to radiation, but pharmaceuticals in solution are mostly un~tab1e.l~ For example, dry antibiotics may discolor but generally suffer no appreciable change, whereas insulin shows severe degradation. Internally administered drugs that require excessive testing for safety and efficacy are being cautiously investigated as to their suitability for radiation sterilization. Serum albumin: Lyophilized human serum is stable to irradiation even in doses greater than 2.5 Mrad; in aqueous solution, however, human serum undergoes simultaneous polymerization, degradation, and denaturation. Biological tissues: Tissues, including fascia, cancellocortical bone grafts, cartilage grafls, dermoepidermal grafts, chorioamnionic grafts, and tendons, are being studied for feasibility of radiosterilization. Definitive studies concerning the use of radiosterilization for tissues and other biological substances will enhance the handling and management of these materials in the future. Advantages and disadvantages of ionizing radiation. With ionizing radiation gaining more prominence in commercial and industrial steriliza-
AORN Journal,October 1977, Vol26, No 4
tion, it is important to determine benefits derived as well as limitations encountered. Advantages 0 There is freedom of choice for packaging materials, and packages may be hermetically sealed prior to sterilization, allowing for indefinite shelf-life under proper storage conditions. 0 The power of penetration into liquids and most solids eliminates the diffusion problems encountered with steam and ethylene oxide processing. 0 Since ionizing radiation is virtually devoid of thermal reaction (heat may increase zero to four degrees), heat-sensitive articles may be safely processed. 0 It is easy to predict constant and reliable penetration doses by using reasonably uncomplicated monitoring controls. 0 Since radiation is usually a continuous process, the monitoring of the absorbed dose is relatively simple compared to other methods of sterilization. 0 Once plants are established, irradiation is more economical than other means of sterilization. Disadvantages 0 Radiosterilization requires sophisticated equipment, expensive to build and operate and difficult to house, making hospital use impractical at this time. 0 Some organisms are radioresistant, and doses applicable to most products will only reduce the number of organisms rather than eliminate them, as would steam or gas sterilization.
Radiation is mutagenic, and damaged but surviving organisms may become resistant or acquire other unpleasant characteristics. 0 Ionizing radiation is damaging to certain types of materials. Radiation may not be satisfactory for use with metals under certain circumstances, since particles become attenuated as they strike the outside of the metal and lose penetration capability. The problem of a toxic residue forming on certain items initially sterilized by radiation and then subjected to ethylene oxide has prompted a great deal of discussion and research within the past several years. Original warnings against the resterilization of gamma-irradiated items have recently been modified, and complete elimination of this practice is no longer recommended. In light of present-day information and “provided recommended aeration times for PVC following EO sterilization are rigidly adhered to, previous gamma-irradiation of the item is not a contraindication to resterilization with ethylene oxide.”16 There is some damage to materials exposed to radiation. With most products, no visible changes are seen, even though minor chemical changes probably do occur. Unless these changes alter the function or characteristics of the materials, they usually do not cause problems. Some thermoplastic materials will discolor when exposed to radiation; Teflon is one of the few materials that is damaged excessively when exposed to high doses of radiation. The chemical changes that take place in organic substances and biological tissues present a somewhat different picture; hence, it is necessary to test these products individually for 0
AORN Journal, October 1977, Vol26, No 4
683
biological characteristics, pharmacopeial quality, and toxicity to determine their suitability for radiosterilization. Conclusion. Since the first commercially irradiated sutures were marketed in 1956, the use of radiation for the sterilization of hospital products has grown into a business estimated soon to reach $3 billion. The availability of ionizing radiation through cobalt 60 and electron accelerator plants has made a tremendous impact on sterilization methods, both technologically and economically. Although not the panacea for all sterilization problems and not yet practical for on-site, inhospital processing of supplies, it is nevertheless safe to conclude that it will become an even more versatile adjunct to the armamentarium of sterilization techniques. Notea 1. K ti Morganstern, "Appraisal of the advantages and disadvantages of gamma, electron, and x-ray radiation sterilization," Radiosterilization of Medical Products, 1974, Proceedings series (Vienna: International Atomic Energy Agency, 1975) 271. 2. Pamela A Wills, et al, "Industrial and dosimetric aspects of radiation sterilization in Australia," Radiosterilization of Medical Products, 1974, Proceedings series (Vienna: International Atomic Energy Agency, 1975) 101. 3. 6 L Gupta, "Chemical and biological effects of radiation sterilization of medical products." Radiosterilizationd Medical Products, 1974, Proceedings series (Vienna: International Atomic Energy Agency, 1975) 181. 4. Charles Artandi, "Sterilization by ionizing radiation," International Anesthesiology Clinics 10 (Summer 1972) 125. 5. Eugene T O'Sullivan, Martin ti Stein, "Gamma radiation as a microbiological control process," Bulletin of the Parenteral Drug Association 30 (January-February 1976) 50. 6. Gupta, "Chemical and biological effects of radiation sterilization," 186. 7. OSullivan, Stein, "Gamma radiation," 50. 8. C A Trauth, ti D Sivinski, "Synergistic effects of heat and irradiation treatment (thermoradiation) in the sterilization of medical products." Radiosterilizationof Medical Products, 1974, Pro-
684
ceedings series (Vienna: International Atomic Energy Agency, 1975) 26. 9. Ibid, 26. 10. lbid, 40. 11. Ibid, 41. 12. R N Mukherjee. H C Yuan, "Factors involved in planning radiation-sterilization practices and technology in the developing countries and the agency's promotional rde," Radiosterilization of Medical Products, 1974, Proceedings series (Vienna: International Atomic Energy Agency, 1975) 417. 13. "Recommendations for the radiation sterilization of medical products ," Radiosterilization of Medical Products, 1974, Proceedings series (Vienna: International Atomic Energy Agency, 1975) 513. 14. Altandi, "Sterilization by ionizing radiation." 127- 128. 15. Gupta, "Chemical and biological effects of radiation sterilization," 188. 16. Robert 6 Roberts, "X-rays + PVC + EO = OK," Respiratory Care 21 (March 1976) 223-224.
Congress deadline dates Nov 7-Up-tedate local membership roster information to be in at Headquarters Nov 22-Membership computer printout and delegate forms mailed from Headquarters to chapter treasurers Jan 28-Postmark deadline for Congress advance registrations Jan 30-Postmark deadline for forms listing delegates and alternates to be sent to Headquarters Feb 13-Written change of delegate status to be received at Headquarters Feb 20-Letters outlining duties mailed from Headquarters to delegates and alternates March 31-Postmark deadline for written request for refund of registration fee
AORN Journal,October 1977, Vol26, No 4
Sterilization by ionizing radiation
Ionizing radiation for sterilization is an established procedure in industry for the processing of medical products. It has replaced other techniques because of its reliability, economy, and freedom in packaging. In most in-
Doris C MacClelland, RN, M S , is a Commander in the US Navy Nurse Corps. She is operating room supervisor at the Naval Regional Medical Center, S a n Diego, Calif A graduate of Emanuel Hospital School of Nursing, Portland, Ore, she received her BA degree from San Francisco State College and her M S from Indiana University, Indianapolis. The opinions and assertions contained herein are those of the author and are not to be construed as official or reflect the views of the Department of the Navy.
stances, irradiation processing requires less energy than conventional methods, which may become increasingly important as we face energy shortages. More sophisticated medical techniques and a continuing increase in the number of patients have put additional constraints on the availability of sterile supplies, thus making sterilization methods an economic as well as a technical problem. Specialty nurses, who are responsible for ensuring the availability of sterile supplies for use within the operating suite, must be knowledgeable about the changing concepts and trends of sterile supply management. This article provides sufficient information on radiation sterilization techniques so that decisions concerning use of supplies sterilized in this way can be made knowledgeably and confidently. History. Wilhelm Conrad Roentgen’s discovery of penetrating x-rays in 1895, closely followed by Antoine Henri Becquerel’s identification of rays emitted from uranium compounds and Pierre and Marie Curie’s discovery of radium-emitting alpha, beta, and gamma rays, became the groundwork for nuclear radiation physics. Research in this field has led to radiation techniques such as sterilization.
AORN Journal, October 1977, Vol26, No 4
676
World War I1 accelerated this research, and in the postwar years, numerous technical organizations and governmental, academic, and industrial agencies worldwide were involved in the investigation and development of radiation sterilization techniques and their practical applications for medical and industrial use. The first commercial use of radiation for sterilization of medical products, such as sutures, disposable needles, and syringes, was introduced by Ethicon in 1956.’About the same time, Denmark’s Atomic Energy Commission opened Europe’s first CD. balt 60 commercial radiation sterilization plant. In the early 1960s,the United Kingdom and the USSR began using cobalt 60 for commercial sterilization of medical products. One of the largest cobalt 60 plants designed for industrial sterilization was built in 1959 in Dandenong, Australia, for sterilization of imported goat hair bales used in the manufacture of carpets.* No longer needed for this purpose, it is now used on a contract basis for sterilization of medical goods and biological tissues. Radiation process. Radiation is the emitting of energy in the form of rays or particles. The rays consist of an electromagnetic spectrum that includes electric waves or household current, radio waves, infrared rays, visible rays that allow for sight and color, ultraviolet rays, x-rays, and gamma rays. The particles are minute portions of matter that move through space and are capable of transferring energy. These particles include electrons, alpha particles, fission fragments, and others. Cobalt 60 gamma-ray emitters and electron accelerators are two methods of eliciting the energy transfer processes. Radiation may be either ionizing or
676
nonionizing. An ion is an electrically charged particle. Ionizing radiation is therefore any electromagnetic or particulate radiation capable of producing ions, directly or indirectly, by interaction with matter. The direct action may be energy absorption or transfer within a molecule, causing the molecule to react with cellular constituents and create an observable biological effect. The indirect action is brought about by energy being absorbed in material surrounding the target molecule; the surrounding material then conveys damage to the target m ~ l e c u l e .The ~ resulting damage interferes with the enzyme and genetic systems of the cell, thereby producing cell death by the inability of the cell to propagate. Nonionizing radiation, since it does not involve electrical energy transfer, destroys organisms in several other ways-ultrasonic waves disrupt cells, infrared rays produce heat, and ultraviolet rays interfere with metabolism. Because nonionizing radiation is not as effective for sterilization as ionizing radiation, it is most commonly used for disinfection in hospitals and operating rooms as a means of reducing bacterial counts in the air. Measuring radiation. Precise measurement of radiation exposure is important for the efficient and safe operation of radiation plants as well as for consistently reliable radiation sterilization processes. The basic unit of measurement for electromagnetic radiation is the roentgen; however, because the roentgen does not measure particulate radiation, it was necessary to establish the radiation absorbed dose (rad) received by any subject. Technically, a rad is a unit of absorbed dose of ionizing radiation equal to an energy of 100 erg/gm of irradiated material. The megarad (Mrad) is a
AORN Journal, October 1977, Vol26, No 4
C
ombinations of heat and radiation act synergistically to kill cells.
dose equivalent to 1 x lo6 rad and is the unit of measure used for measuring the dose range required for sterilization to take place. The monitoring devices in most cobalt 60 plants are capable of measuring absorbed doses in the range from about 0.5 to 5.0 Mrad. Since sterilization is dependent on total dose received, reliable and consistent predictions of kill can be made. Research studies have demonstrated that an exposure of 2.5 Mrad ensures a high degree of sterility, and this level has been adopted by industry as the safe dose for routine sterilization of hospital ~ u p p l i e s . ~As with other methods of sterilization, the end result is dependent on the degree of contamination present prior to the sterilization process; therefore, it is imperative that sanitation standards be established and maintained at the manufacturer’s level t o ensure that items will not be grossly contaminated when packaged for sterilization. To illustrate the destruction level of selected material with the standard 2.5-Mrad dose, rubber gloves and tubing will experience no loss of tensile strength after dosages of less than 8 Mrad, and many polymers will suffer no serious change below about 10 Mrad. Levels of exposure. There are three levels of exposure that may be at-
tained during ionizing radiation. The minimum exposure is the period necessary to reduce the specific bacterial count to the predetermined level; the target exposure is the minimum dose with added safety criteria; and the maximum exposure is that dose at which some undesirable effect occurs.5 During the sterilization process, all material does not receive the same amount of radiation even though all material receives the target dose. Two factors that account for the differences in exposure are the bulk density of the packages and the distance the packages are from the source of radiation. To counteract this effect, material may pass the source at different angles or at different distances on a programmed basis, or some packages may be rotated on a turntable. Effects of radiation on specific microorganisms. Environmental and natural conditions of microorganisms before, during, and after irradiation are important to radiosensitivity. The lethal dose of radiation for cells is influenced by the presence of oxygen, the degree of hydration of the cell, and the stage of growth cycle at the time of radiation. The presence of chemicals such as glycerol, aliphatic alcohols, hydrogen sulfide, and others can also reduce the lethal effects of radiation.6 Generally, viruses are more resistant than bacterial spores, and spores
AORN Journal, October 1977,Vol26, No 4
677
are more resistant than vegetative organisms, yeasts, and molds. Gramnegative organisms are more susceptible than Gram-positive organisms; therefore, Pseudomoms aeruginosa is more readily killed than Streptococcus or Staphylococcus. Because of its resistance to radiation, Bacillus pumilus is recommended for biologic control of radiation sterilization as B subtilis is for ethylene oxide and B stearothermophilus is for steam sterilization.' Since many organisms can be killed by either heat or radiation, recent data c o n k n that properly chosen combinations of heat and radiation act synergistically to kill cells.* Studies using dry B subtilis spores show that increases in temperature can lead to an appreciable acceleration of radiation sterilization cycles. Similar synergism has been observed in a variety of biological systems, including active proteins, viruses, spores, bacteria, yeasts, mammalian cells, and human cancers.9 Two potential benefits of this synergism are (1) because many radioresistant organisms are relatively heat labile, sufficient heat alone will tend to destroy these organisms and (2) organisms that are both radioresistant and heat resistant can be eliminated by designing the proper heat-radiation cycle to inactivate the organisms synergistically.10 Preliminary data also suggest that combinations of heat and radiation may produce fewer mutations, one of the major disadvantages of radiation sterilization. It has been demonstrated that mutations induced by radiation may be as high as 15%. However, the mutation rate among survivors of heat-radiation cycles is near the natural level expected for the organisms investigated .l1 Types of commercial radiation plants
670
Types of ionizing radiation Table 1
Cobalt 60 gamma rays Advantaoes Highly penetrating; products can be sterilized in shipping cartons Large volumes can be irradiated simuttaneously Simplicity and reliability of radiation source Dose easily controlled Penetrates liquids and solids, including bulk powders Plants operate continuously Disadvantages a Exposure times are long (8 to 24 hours) a Plants are elaborate and expensive to build Stringent personnel safety standards because of increased risk of radiation exposure More expensive than electron accelerators to operate
Electron accelerators Advantages Sterilizationachieved in seconds Plants require less space and are cheaper to build and easier to operate Required safety precautions not as stringent as with gamma emitters Disadvantages Less penetration capability: penetration dependent on acceleration of electrons Only small packages can be processed Maintenance of constant dose is more difficult
in use. For irradiation of medical products, gamma-ray emitters and high-energy electron accelerators are the only methods in common use. There are approximately 65 gammaray facilities in use throughout the world as compared to approximately
AORN Journal, October I977,V o l 2 6 , No 4
Materials suitable for ionizing radiation Table 2
Thermoplastics (most common) polystyrene polyethylene polyamide (nylon) polyester (Dacron, Mylar) polyurethane polymethyl methacrylate (Perspex, Plexiglas) ionorner (Surlyn-A) polypropylene cellulose esters polyvinyl chloride (discolors) Thermosetting resins epoxy phenolics melamine urea Rubbers natural latex segmented polyurethane butadiene-styrene neoprene (discolors) butyl (discolors) Other materials wool silk
cellulose metals glass (discolors) ten accelerator systems.12 Even though gamma-ray emitters are more expensive to operate than accelerator systems, 20 years ago when irradiation plants were first being built the majority of radiation sources being used were the cobalt 60 plants. The accelerators now available have undergone considerable technical improvement, resulting in a highly reliable process at a substantially lower cost than cobalt 60 emitters. For this
reason, electron accelerators will probably gain in popularity in years to come. Sterilization of medical products is only one use of commercial radiation plants, which are also used for processing plastics, vulcanization of rubber, curing paint, preserving food, and other industrial uses. Irradiation processing requires less energy than other processing methods, a major consideration in dealing with energy shortages. Also, irradiation processing is less expensive. For example, steam sterilization of medical disposable products is estimated to cost 2$ per pound, whereas radiation sterilization is 0.05c per pound. Cobalt 60 plants consist of four basic parts: source of radiation, protective shield, product conveyor or transfer mechanism, and dosimeter. The plants may be either automatic (continuous product flow) or batch. Since gamma rays are highly penetrating, they are more versatile in that they allow for complete freedom of packaging. The rate of energy absorption or absorbed dose is much slower with gamma-ray sources than with accelerators, requiring as much as 8 to 24 hours to deliver the standard 2.5-Mrad dose. Because the rate is slower, it is easier to monitor, thereby ensuring constant levels of exposure. Particle accelerators may be either an AC generator, such as the Van de Graaff accelerator, or a linear accelerator. In both models there is an electron gun, an acceleration tube, and a scanning device to spread the electrons over the desired target area. In contrast to the gamma-ray emitters, the penetration capability depends on the speed of the electrons, which can vary, but the absorbed dose is obtained within seconds. The rapidly absorbed dose makes monitoring of the process
AORN Journal,October 1977,Vol26, No 4
679
onizing radiation is virtually devoid of thermal reaction.
I
more difficult, and because the rays are less penetrating, the versatility of packaging is somewhat restricted. Table 1 lists advantages and disadvantages of gamma-ray and accelerator plants. In 1967, the International Atomic Energy Agency, in collaboration with the World Health Organization, prepared a ‘?Recommended code of practice for the radiosterilization of medical products,” which was reviewed and updated in 1974.13 Major categories discussed in the code include conditions for the manufacture of items to be radiosterilized, microbiological control of radiation facilities, and operation of radiation sterilization facilities. These recommendations will be subject to updating as new technical information and experiences are accumulated. Current applications. Originally used only for such items as sutures, needles, syringes, and other disposable hospital products, radiosterilization is now used for pharmaceuticals, biological tissues, prostheses, highly specialized surgical instruments, hospital-administered fluids, and numerous industrial uses. Although many of these uses are still in the research and development stage, current studies are establishing their safety and feasibility. Table 2 lists some of the materials used in medical products that are suit-
680
able for ionizing radiation sterilization.14 Pharmaceuticals: Dry pharmaceuticals and ointments are mostly stable when subjected to radiation, but pharmaceuticals in solution are mostly un~tab1e.l~ For example, dry antibiotics may discolor but generally suffer no appreciable change, whereas insulin shows severe degradation. Internally administered drugs that require excessive testing for safety and efficacy are being cautiously investigated as to their suitability for radiation sterilization. Serum albumin: Lyophilized human serum is stable to irradiation even in doses greater than 2.5 Mrad; in aqueous solution, however, human serum undergoes simultaneous polymerization, degradation, and denaturation. Biological tissues: Tissues, including fascia, cancellocortical bone grafts, cartilage grafls, dermoepidermal grafts, chorioamnionic grafts, and tendons, are being studied for feasibility of radiosterilization. Definitive studies concerning the use of radiosterilization for tissues and other biological substances will enhance the handling and management of these materials in the future. Advantages and disadvantages of ionizing radiation. With ionizing radiation gaining more prominence in commercial and industrial steriliza-
AORN Journal,October 1977, Vol26, No 4
tion, it is important to determine benefits derived as well as limitations encountered. Advantages 0 There is freedom of choice for packaging materials, and packages may be hermetically sealed prior to sterilization, allowing for indefinite shelf-life under proper storage conditions. 0 The power of penetration into liquids and most solids eliminates the diffusion problems encountered with steam and ethylene oxide processing. 0 Since ionizing radiation is virtually devoid of thermal reaction (heat may increase zero to four degrees), heat-sensitive articles may be safely processed. 0 It is easy to predict constant and reliable penetration doses by using reasonably uncomplicated monitoring controls. 0 Since radiation is usually a continuous process, the monitoring of the absorbed dose is relatively simple compared to other methods of sterilization. 0 Once plants are established, irradiation is more economical than other means of sterilization. Disadvantages 0 Radiosterilization requires sophisticated equipment, expensive to build and operate and difficult to house, making hospital use impractical at this time. 0 Some organisms are radioresistant, and doses applicable to most products will only reduce the number of organisms rather than eliminate them, as would steam or gas sterilization.
Radiation is mutagenic, and damaged but surviving organisms may become resistant or acquire other unpleasant characteristics. 0 Ionizing radiation is damaging to certain types of materials. Radiation may not be satisfactory for use with metals under certain circumstances, since particles become attenuated as they strike the outside of the metal and lose penetration capability. The problem of a toxic residue forming on certain items initially sterilized by radiation and then subjected to ethylene oxide has prompted a great deal of discussion and research within the past several years. Original warnings against the resterilization of gamma-irradiated items have recently been modified, and complete elimination of this practice is no longer recommended. In light of present-day information and “provided recommended aeration times for PVC following EO sterilization are rigidly adhered to, previous gamma-irradiation of the item is not a contraindication to resterilization with ethylene oxide.”16 There is some damage to materials exposed to radiation. With most products, no visible changes are seen, even though minor chemical changes probably do occur. Unless these changes alter the function or characteristics of the materials, they usually do not cause problems. Some thermoplastic materials will discolor when exposed to radiation; Teflon is one of the few materials that is damaged excessively when exposed to high doses of radiation. The chemical changes that take place in organic substances and biological tissues present a somewhat different picture; hence, it is necessary to test these products individually for 0
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biological characteristics, pharmacopeial quality, and toxicity to determine their suitability for radiosterilization. Conclusion. Since the first commercially irradiated sutures were marketed in 1956, the use of radiation for the sterilization of hospital products has grown into a business estimated soon to reach $3 billion. The availability of ionizing radiation through cobalt 60 and electron accelerator plants has made a tremendous impact on sterilization methods, both technologically and economically. Although not the panacea for all sterilization problems and not yet practical for on-site, inhospital processing of supplies, it is nevertheless safe to conclude that it will become an even more versatile adjunct to the armamentarium of sterilization techniques. Notea 1. K ti Morganstern, "Appraisal of the advantages and disadvantages of gamma, electron, and x-ray radiation sterilization," Radiosterilization of Medical Products, 1974, Proceedings series (Vienna: International Atomic Energy Agency, 1975) 271. 2. Pamela A Wills, et al, "Industrial and dosimetric aspects of radiation sterilization in Australia," Radiosterilization of Medical Products, 1974, Proceedings series (Vienna: International Atomic Energy Agency, 1975) 101. 3. 6 L Gupta, "Chemical and biological effects of radiation sterilization of medical products." Radiosterilizationd Medical Products, 1974, Proceedings series (Vienna: International Atomic Energy Agency, 1975) 181. 4. Charles Artandi, "Sterilization by ionizing radiation," International Anesthesiology Clinics 10 (Summer 1972) 125. 5. Eugene T O'Sullivan, Martin ti Stein, "Gamma radiation as a microbiological control process," Bulletin of the Parenteral Drug Association 30 (January-February 1976) 50. 6. Gupta, "Chemical and biological effects of radiation sterilization," 186. 7. OSullivan, Stein, "Gamma radiation," 50. 8. C A Trauth, ti D Sivinski, "Synergistic effects of heat and irradiation treatment (thermoradiation) in the sterilization of medical products." Radiosterilizationof Medical Products, 1974, Pro-
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ceedings series (Vienna: International Atomic Energy Agency, 1975) 26. 9. Ibid, 26. 10. lbid, 40. 11. Ibid, 41. 12. R N Mukherjee. H C Yuan, "Factors involved in planning radiation-sterilization practices and technology in the developing countries and the agency's promotional rde," Radiosterilization of Medical Products, 1974, Proceedings series (Vienna: International Atomic Energy Agency, 1975) 417. 13. "Recommendations for the radiation sterilization of medical products ," Radiosterilization of Medical Products, 1974, Proceedings series (Vienna: International Atomic Energy Agency, 1975) 513. 14. Altandi, "Sterilization by ionizing radiation." 127- 128. 15. Gupta, "Chemical and biological effects of radiation sterilization," 188. 16. Robert 6 Roberts, "X-rays + PVC + EO = OK," Respiratory Care 21 (March 1976) 223-224.
Congress deadline dates Nov 7-Up-tedate local membership roster information to be in at Headquarters Nov 22-Membership computer printout and delegate forms mailed from Headquarters to chapter treasurers Jan 28-Postmark deadline for Congress advance registrations Jan 30-Postmark deadline for forms listing delegates and alternates to be sent to Headquarters Feb 13-Written change of delegate status to be received at Headquarters Feb 20-Letters outlining duties mailed from Headquarters to delegates and alternates March 31-Postmark deadline for written request for refund of registration fee
AORN Journal,October 1977, Vol26, No 4
