Trends in Radiation Sterilization of Health Care Products

Transcription

1 Trends in Radiation Sterilization of Health Care Products

2 TRENDS IN RADIATION STERILIZATION OF HEALTH CARE PRODUCTS

3 The following States are Members of the International Atomic Energy Agency: AFGHANISTAN ALBANIA ALGERIA ANGOLA ARGENTINA ARMENIA AUSTRALIA AUSTRIA AZERBAIJAN BANGLADESH BELARUS BELGIUM BELIZE BENIN BOLIVIA BOSNIA AND HERZEGOVINA BOTSWANA BRAZIL BULGARIA BURKINA FASO CAMEROON CANADA CENTRAL AFRICAN REPUBLIC CHAD CHILE CHINA COLOMBIA COSTA RICA CÔTE D IVOIRE CROATIA CUBA CYPRUS CZECH REPUBLIC DEMOCRATIC REPUBLIC OF THE CONGO DENMARK DOMINICAN REPUBLIC ECUADOR EGYPT EL SALVADOR ERITREA ESTONIA ETHIOPIA FINLAND FRANCE GABON GEORGIA GERMANY GHANA GREECE GUATEMALA HAITI HOLY SEE HONDURAS HUNGARY ICELAND INDIA INDONESIA IRAN, ISLAMIC REPUBLIC OF IRAQ IRELAND ISRAEL ITALY JAMAICA JAPAN JORDAN KAZAKHSTAN KENYA KOREA, REPUBLIC OF KUWAIT KYRGYZSTAN LATVIA LEBANON LIBERIA LIBYAN ARAB JAMAHIRIYA LIECHTENSTEIN LITHUANIA LUXEMBOURG MADAGASCAR MALAWI MALAYSIA MALI MALTA MARSHALL ISLANDS MAURITANIA MAURITIUS MEXICO MONACO MONGOLIA MONTENEGRO MOROCCO MOZAMBIQUE MYANMAR NAMIBIA NETHERLANDS NEW ZEALAND NICARAGUA NIGER NIGERIA NORWAY PAKISTAN PALAU PANAMA PARAGUAY PERU PHILIPPINES POLAND PORTUGAL QATAR REPUBLIC OF MOLDOVA ROMANIA RUSSIAN FEDERATION SAUDI ARABIA SENEGAL SERBIA SEYCHELLES SIERRA LEONE SINGAPORE SLOVAKIA SLOVENIA SOUTH AFRICA SPAIN SRI LANKA SUDAN SWEDEN SWITZERLAND SYRIAN ARAB REPUBLIC TAJIKISTAN THAILAND THE FORMER YUGOSLAV REPUBLIC OF MACEDONIA TUNISIA TURKEY UGANDA UKRAINE UNITED ARAB EMIRATES UNITED KINGDOM OF GREAT BRITAIN AND NORTHERN IRELAND UNITED REPUBLIC OF TANZANIA UNITED STATES OF AMERICA URUGUAY UZBEKISTAN VENEZUELA VIETNAM YEMEN ZAMBIA ZIMBABWE The Agency s Statute was approved on 23 October 1956 by the Conference on the Statute of the IAEA held at United Nations Headquarters, New York; it entered into force on 29 July The Headquarters of the Agency are situated in Vienna. Its principal objective is to accelerate and enlarge the contribution of atomic energy to peace, health and prosperity throughout the world.

4 TRENDS IN RADIATION STERILIZATION OF HEALTH CARE PRODUCTS INTERNATIONAL ATOMIC ENERGY AGENCY VIENNA, 2008

5 COPYRIGHT NOTICE All IAEA scientific and technical publications are protected by the terms of the Universal Copyright Convention as adopted in 1952 (Berne) and as revised in 1972 (Paris). The copyright has since been extended by the World Intellectual Property Organization (Geneva) to include electronic and virtual intellectual property. Permission to use whole or parts of texts contained in IAEA publications in printed or electronic form must be obtained and is usually subject to royalty agreements. Proposals for non-commercial reproductions and translations are welcomed and considered on a case-by-case basis. Enquiries should be addressed to the IAEA Publishing Section at: Sales and Promotion, Publishing Section International Atomic Energy Agency Wagramer Strasse 5 P.O. Box Vienna, Austria fax: tel.: sales.publications@iaea.org IAEA, 2008 Printed by the IAEA in Austria July 2008 STI/PUB/1313 IAEA Library Cataloguing in Publication Data Trends in radiation sterilization of health care products. Vienna : International Atomic Energy Agency, p. ; 24 cm. STI/PUB/1313 ISBN Includes bibliographical references. 1. Radiation sterilization. 2. Medical instruments and apparatus. 3. Medical care Equipment and supplies. I. International Atomic Energy Agency. IAEAL

6 FOREWORD Over the past decades, radiation processing has been used in many sectors of national economies. For example, sterilization, polymer cross-linking (tapes, tubes, cables, etc.), tyre belt vulcanization, and the irradiation of certain food items for hygienization, are well established technologies. Either gamma radiation from isotopic sources or high energy electrons from accelerators are being applied in these processes. The IAEA, through various mechanisms, including its technical cooperation programme, coordinated research projects, technical meetings and conferences, is promoting the peaceful use of nuclear and radiation technologies. With IAEA support, several gamma and electron beam irradiation facilities have been built in developing countries, and some new technologies have been developed and transferred to Member States over the past ten years. Commercial radiation sterilization has now been used for more than 50 years. During this period, the market for disposable medical products has undergone enormous growth, and with it the use of ionizing radiation as a method of sterilization. Currently, 40 50% of disposable medical products manufactured in developed countries are radiation sterilized. There has been no major IAEA publication on the subject of radiation sterilization since 1990, when Guidelines for Industrial Radiation Sterilization of Disposable Medical Products (Cobalt-60 Gamma Irradiation) (IAEA- TECDOC-539) was published. In response to Member State requests that a similar publication reviewing recent developments in this field be prepared, the IAEA organized a meeting in Cairo, Egypt (15 18 May 2005) to survey the entire field of radiation sterilization. This report summarizes the basic aspects of radiation sterilization applied in routine commercial services in many developed and developing countries, including essential elements of dosimetry control, new developments in radiation sources and electron beam facilities. It will be of value to those working in the field of radiation technology development and applications. Developing Member States with radiation technology programmes will benefit from the rich experience in this area worldwide. The IAEA wishes to thank all the participants of the meeting, as well as other contributors to this publication for their valuable contributions, especially K. Mehta for compiling and reviewing the report. The IAEA officer responsible for this publication was M. Haji-Saeid of the Division of Physical and Chemical Sciences.

7 EDITORIAL NOTE This publication has been prepared from the original material as submitted by the authors. The views expressed do not necessarily reflect those of the IAEA, the governments of the nominating Member States or the nominating organizations. The use of particular designations of countries or territories does not imply any judgement by the publisher, the IAEA, as to the legal status of such countries or territories, of their authorities and institutions or of the delimitation of their boundaries. The mention of names of specific companies or products (whether or not indicated as registered) does not imply any intention to infringe proprietary rights, nor should it be construed as an endorsement or recommendation on the part of the IAEA. The authors are responsible for having obtained the necessary permission for the IAEA to reproduce, translate or use material from sources already protected by copyrights.

8 CONTENTS 1. INTRODUCTION Background Scope Summary GAMMA IRRADIATORS FOR RADIATION STERILIZATION K. Mehta 2.1. Introduction Gamma radiation sources Gamma irradiation facility Gamma irradiators Design principles Source design capacity and installed activity Process dose and delivered dose Radiation processing throughput Types of irradiators Self-contained irradiators (IAEA Categories I and III) Panoramic irradiators (IAEA Categories II and IV) Commercially available panoramic irradiators Product overlap irradiators Source overlap irradiators Pallet irradiators Batch irradiators Novel designs Some special features Computerized control systems Irradiator selection criteria Radiation safety Conclusions References

9 3. ELECTRON ACCELERATORS FOR RADIATION STERILIZATION A.G. Chmielewski, T. Sadat, Z. Zimek 3.1. Introduction Accelerator classification Service centre accelerators Direct transformer accelerators Single resonant cavity accelerators Linear accelerators Output and beam scanning devices Process control In-line systems X ray irradiators Criteria of accelerator selection Acknowledgement References RADIATION STERILIZATION CENTRES WORLDWIDE A.G. Chmielewski, A.J. Berejka 4.1. Introduction Regions and countries Europe Americas Asia Pacific Other countries Africa Conclusions Acknowledgement References ELECTRON BEAM STERILIZATION SERVICE CENTRE IN A DEVELOPING COUNTRY: POLAND Z. Zimek, I.M. Kaluska 5.1. Introduction Construction of a radiation facility Multipurpose facility Radiation sterilization facility Food product processing facility

10 5.3. Quality assurance and quality control at the INCT radiation sterilization facility Economic aspects Conclusions and recommendations References GAMMA RADIATION STERILIZATION SERVICE CENTRE IN A DEVELOPING COUNTRY: EGYPT El-S.A. Hegazy 6.1. Introduction Feasibility and implementation Feasibility study Implementation Operational phase Laboratories, safety and quality Radiation technology section Other central laboratories Radiation safety standards implemented at the NCRRT Conclusion References DOSIMETRY AND THE RADIATION STERILIZATION PROCESS K. Mehta, A.A. Abdel-Fattah 7.1. Introduction Need for dosimetry Applications of dosimetry Total quality through dosimetry Determination of dose Product qualification and process validation Product qualification Process validation Process control Process parameters Routine product dosimetry Product control Product release and certification Dosimetry laboratory and dosimetry systems Dosimetry laboratory

11 Classification of dosimetry systems Selection criteria for routine dosimetry systems Characterization of a dosimetry system Standards and quality assurance programmes Experience at NCRRT Conclusions Appendix to Chapter References MICROBIOLOGICAL ASPECTS OF RADIATION STERILIZATION A.A. Hammad 8.1. Introduction Principles of radiation sterilization Direct and indirect effect of radiation Radiation resistance of microorganisms Advantages of radiation sterilization Determination of sterilization dose Methodology Technical requirements Transfer of sterilization dose Microbiology laboratory Conclusions References REGULATORY ISSUES FOR RADIATION STERILIZATION CENTRES J. Meissner 9.1. Introduction History of quality management systems Requirements for radiation sterilization Application of ISO Application of ISO 11137: Issues of ISO revision and harmonization General Part 1: Requirements and guidance Part 2: Establishing the sterilization dose Part 3: Guidance on dosimetric aspects Process control

12 Computer and software requirements E-beam irradiators Gamma irradiators X ray irradiators Dosimetry and measurement uncertainty Conclusions References RADIATION SAFETY AT IRRADIATION FACILITIES J. Meissner, K. Mehta, A.G. Chmielewski Introduction Gamma sources and activated accelerator parts Manufacturing Transport and disposal Safety design principles Dose rate limits Biological shield design Regulatory approval Personnel safety system Radiation measurements Personnel dosimetry Accident prevention Safety procedures Training Conclusions References MATERIALS USED IN MEDICAL DEVICES A.J. Berejka, I.M. Kaluska Introduction Polymers used in medical devices Flexible thermoplastics Low cost rigid thermoplastics Premium rigid thermoplastics Thermoplastic summary Elastomers Adhesives Backings Packaging

13 11.4. Conclusions References RADIATION STERILIZATION OF PHARMACEUTICALS: AN OVERVIEW OF THE LITERATURE D. Ražem Introduction General reviews Specialized reviews Radiation sterilization as the method of choice Radiation sterilization as one of the alternative sterilization methods Radiation sterilization not considered Conclusions Acknowledgements References RADIATION STERILIZATION OF DRUGS B. Marciniec, K. Dettlaff Introduction Sterilization methods approved by European Pharmacopoeia Microbiological quality of pharmaceutical preparation according to European Pharmacopoeia Methods of analysis of drugs subjected to radiation sterilization Analytical methods Microbiological tests Biological tests Clinical tests and tests for bio-availability in vivo Effect of radiation sterilization on drugs Penicillins Cephalosporins Other antibiotics Sulphonamides Steroids Other drugs Conclusions References

14 14. RADIATION STERILIZATION OF HUMAN TISSUE GRAFTS A. Dziedzic-Goclawska, A. Kaminski, I. Uhrynowska-Tyszkiewicz, J. Michalik, W. Stachowicz Introduction Risk of infectious disease transmission with tissue allografts Sterilization of tissue grafts with ionizing radiation Processing of tissue grafts with gamma rays and high energy electrons Mechanisms involved in the inactivation of pathogens by ionizing radiation Effectiveness of radiation sterilization Factors affecting the effectiveness of radiation sterilization and resistance of microorganisms (pathogens) to irradiation Choice of radiation dose Interdisciplinary research Studies on the identification, origin and stability of paramagnetic entities produced by ionizing radiation in bone grafts Effects of various preservation procedures and radiation sterilization conditions on the osteoinductive potential of bone grafts Effect of preservation procedures and radiation sterilization conditions on in vitro solubility of rat bone collagen Effect of various preservation procedures and radiation sterilization conditions on the degradation of collagen, a major constituent of connective tissue grafts Evaluation of in vitro susceptibility to pepsin digestion of fresh and lyophilized human bone samples irradiated at room temperatures with a dose of kgy Alternative methods used at the Central Tissue Bank in Warsaw Cytotoxicity testing of polymers used for tissue graft packaging Cytotoxicity of bone medullary lipids after irradiation

15 Clinical efficacy of radiation sterilized bone allografts Conclusions Acknowledgements References CONTRIBUTORS TO DRAFTING AND REVIEW

16 1. INTRODUCTION 1.1. BACKGROUND Worldwide, over 200 gamma irradiators are in operation for a variety of purposes in 55 countries; 120 of these plants are located in Europe and North America. Syringes, surgical gloves, gowns, masks, sticking plasters, dressings, tetrapacks, bottle teats for premature babies, artificial joints, food packaging, raw materials for pharmaceuticals and cosmetics, and even wine corks are gamma sterilized. An increasing number of electron accelerators are also being used, although they currently radiation sterilize a minority of products. The use of electron beams (e-beams) as a radiation source has many attractive features, such as nearly instantaneous dose delivery, scalability for different throughput and the capability to integrate into an in-line process. However, e-beams would seem to suffer from processing inflexibility due to penetration limitations. On the other hand, the gamma irradiator has an advantage in processing nonuniform and high density products; however, it suffers from the fact that it uses a radioactive material. Consideration of these characteristics would seem to identify the use of X rays as the technology of choice. The use of high energy X rays for sterilizing medical devices was proposed during the 1960s, and implemented during the 1990s. X ray processing is now practicable for these applications because high energy, high power electron accelerators and large area targets for converting electron beams to X rays are available, and the unit cost for processing is comparable to other treatment methods. For sterilization processes, process validation plays a very important role in quality assurance and quality control as emphasized by various documents of the International Organization for Standardization (ISO); for example, ISO For example, process validation is understood as a documented procedure for obtaining, recording and interpreting the results required to establish that a process will consistently yield product complying with predetermined specifications. 1 INTERNATIONAL ORGANIZATION FOR STANDARDIZATION, Sterilization of Health Care Products Radiation, Part 1: Requirements for Development, Validation and Routine Control of a Sterilization Process for Medical Devices, Part 2: Establishing the Sterilization Dose, Part 3: Guidance on Dosimetric Aspects, ISO 11137, ISO, Geneva (2006). 1

17 INTRODUCTION There is increasing interest in computer modelling. Modelling has always been used in irradiator design, but now it is being moved into the irradiator operating environment, where it can be used to optimize processing. Using modelling and a reliable database of dosimetry measurements, the performance of an irradiator can be predicted and optimized. In addition, the impact of process variables on the distribution of dose can be simulated at a level that would not be practical with actual dose measurements alone. Computer modelling can also predict the distribution of dose due to source replenishment, prior to the actual placement of a single new pencil in the rack. Even though the focus of this publication is on the sterilization of disposable medical products, three other important fields of technology application are also summarized here: pharmaceuticals, tissue grafts and medicinal plants (herbs). Radiation processing techniques have evolved so that radiation sterilization has become the first choice for thermosensitive solid state drugs as described in the decision trees of the European Medicines Agency (EMEA) for the selection of sterilization methods ( For pharmaceuticals, the geometry of the vials as well as the nature of the packaging material (glass or plastic vial, stopper, sealing) can affect the distribution of absorbed dose. In addition, in the case of electrons, the angular divergence of the e-beam from the central axis influences dose uniformity. The IAEA has been a catalyst in bringing about tissue banking development, by advancing the technology of radiation sterilization of tissues and promoting standards to meet strict medical specifications. In addition, the technical infrastructure, such as irradiators (gamma and e-beam) or dosimetry systems, is very important for process implementation. In many cases, there are common facilities used for several different applications, thus, two sections address these subjects as well. Medicinal plants (herbs) and spices hygienization are very close, from logistical and technical points of view, and for this reason some aspects of food irradiation are included here. The most significant regulation impacting on the switch to radiation from ethylene oxide (EtO) was pollutant release and transfer registers (PRTF), which was recently proclaimed in many countries. For example, in Japan in 2001, ethylene oxide (EtO) gas was included in the list of poisonous materials and its phase-out was enforced in SCOPE There are many purposes for preparing such a document for radiation sterilization application; however, two principal objectives are: (a) to assemble 2

18 INTRODUCTION in a single document nearly all aspects of the technology, as is evident from the list of contents; and (b) to provide assistance to developing Member States in establishing this technology. With these two as clear objectives, an attempt was made to cover all elements necessary for the application of this technology, as well as some future directions in the use of radiation for sterilization. The discussion in this publication may be roughly divided into two groups: irradiation facility related and products related. In the first group, the discussion topics include: the types of radiation sources that are suitable and available (gamma rays, electrons and X rays); a description of various types of irradiation facilities; the establishment and operation of service centres for industrial sterilization; QA procedures, including regulatory aspects and ISO guidelines for process validation; process control, including dosimetry and microbiology requirements; and radiation safety. In the second group, the discussion topics cover: materials compatible with radiation; and status and problems related to the sterilization of pharmaceuticals, cosmetics, allografts and medicinal plants SUMMARY Based on the discussion of various topics, several conclusions can be drawn: Radiation sterilization is a well established technology; radiation sources can be used safely, and hundreds of facilities using gamma rays and high energy electrons are operating well. Sterilization is a special process and thus process validation is essential for its success; the ISO procedure should be followed meticulously. Unlike other methods of sterilization, this technology allows sterilization of the final packaged product. Thus, there is no need of an aseptic room for packaging sterilized products. Since medical products are subject to strict health regulations, it is important to have a quality management system in place during all the phases of the irradiation facility. So far, gamma rays have been dominating this technology; however, the size of the medical industry as well as accelerator manufacturing have come to a point where electrons can be used efficiently for this process. High power accelerators are suitable for commercial service centres, while low power ones would fit more easily into in-line production. Dosimetry plays a crucial role at various stages of the process, namely, dose setting, process validation and process control. Thus, every radiation 3

19 INTRODUCTION sterilization facility must have a well equipped and adequately staffed dosimetry laboratory. Medical device material is essential for an efficient radiation sterilization process. There are already several radiation resistant materials available. Radiation safety of workers and the general public is important. Relevant safety standards have been developed by international bodies and are implemented at the irradiation facilities. Such facilities have been operating safely for several decades, which shows that this technology is safe. 4

20 2. GAMMA IRRADIATORS FOR RADIATION STERILIZATION K. MEHTA Vienna, Austria 2.1. INTRODUCTION The radiation processing industry gained significant impetus with the advent of nuclear reactors, which have the capability to produce radioisotopes such as 60 Co. These gamma ray emitters became popular radiation sources for medical and industrial applications. Many gamma ray irradiators have been built, 200 of which are estimated to be currently in operation in Member States of the IAEA. In recent times, the use of electron accelerators as radiation source (and sometimes equipped with an X ray converter) is increasing. However, gamma irradiators are difficult to replace, especially for non-uniform and high density products. Currently, 60 Co is used almost solely as a gamma radiation source for industrial use now, mainly because of its easy production method and its non-solubility in water. Based on the total cumulative sale of 60 Co by all suppliers, it can be estimated that the installed capacity of cobalt is increasing at the rate of about 6% per year. It is interesting to note that the worldwide use of disposable medical devices is growing at approximately the same rate (5 6%), which seems to be driving the growth in cobalt sale GAMMA RADIATION SOURCES The most suitable gamma radiation sources for radiation processing are 60 Co and 137 Cs because of the relatively high energy of their gamma rays and fairly long half-life (30.1 years for 137 Cs and 5.27 years for 60 Co). However, the use of 137 Cs has been limited to small, self-contained dry storage irradiators, used primarily for the irradiation of blood and for insect sterilization. Currently, all industrial radiation processing facilities employ 60 Co as the gamma radiation source. 5

21 MEHTA FIG Decay scheme of radionuclide 60 Co. Cobalt-60 ( 60 Co 27 ) decays (disintegrates) into a stable (non-radioactive) nickel isotope ( 60 Ni 28 ), principally emitting one negative beta particle (of maximum energy MeV) with a half-life of about 5.27 years (Fig. 2.1). Nickel-60 thus produced is in an excited state, and it immediately emits two photons of energy 1.17 MeV and 1.33 MeV in succession to reach its stable state. These two gamma ray photons are responsible for radiation processing in the 60 Co gamma irradiators. With the decay of every 60 Co atom, the strength or the activity level of the cobalt source is decreasing, such that the decrease represents 50% in about 5.27 years, or about 12% in one year. Additional pencils of 60 Co are added periodically to the source rack to maintain the required capacity of the irradiator. Cobalt-60 pencils are eventually removed from the irradiator at the end of their useful life, which is typically 20 years. Generally, they are returned to the supplier for reuse, recycling or disposal. In about 50 years, 99.9% of 60 Co would decay into non-radioactive nickel. The radioactivity level is the strength of a radiation source, which is defined as the number of disintegrations of radioactive nuclides per second. The special name of the SI unit is becquerel (Bq) [2.1]. However, this is a very small amount of activity and, traditionally, activity is measured in units of curie (Ci). Thus: 1 becquerel (Bq) = 1 dis/s = 1 s 1 1 curie (Ci) = Bq For a gamma irradiator, source power depends on the source activity, such that 1 million Ci of 60 Co emits about 15 kw of power. 6

22 GAMMA IRRADIATORS FOR RADIATION STERILIZATION FIG Slugs (small cylinders) of 60 Co, which are the building blocks of the radiation source rack (courtesy of MDS Nordion, Canada). Production of radioactive cobalt starts with natural cobalt (metal), which is an element with 100% abundance of the stable isotope 59 Co. Cobalt-rich ore is rare and this metal makes up only about 0.001% of the Earth s crust. Slugs (small cylinders) or pellets made out of 99.9% pure cobalt sintered powder and generally welded in zirconium alloy capsules are placed in a nuclear reactor, where they stay for a limited period (about months), depending on the neutron flux at the location. While in the reactor, a 59 Co atom absorbs a neutron and is converted into a 60 Co atom. During the two years in the reactor, a few per cent of the atoms in the cobalt slug are converted into 60 Co atoms. Specific activity is usually limited to about 120 Ci/g of cobalt (about Bq/g). After irradiation, the capsules containing the cobalt slugs are further encapsulated in corrosion resistant stainless steel to finally produce the finished source pencils in a form such that gamma radiation can come through but not the radioactive material ( 60 Co) itself, with subsequent quality tests (bubble, helium leak, wipe) (Fig. 2.2) [2.2]. The required source geometry is obtained by loading these source pencils into predetermined positions in source modules and distributing these modules over the source rack of the industrial irradiator (Fig. 2.3). Even though 60 Co is the most popular radiation source, caesium sources are also used for some applications. Caesium chloride (with 134 Cs content of between 1% and 3% as an impurity) with specific activity of about 22 Ci/g is used as an active material for the manufacture of finished radiation sources [2.3]. 7

23 MEHTA cobalt-60 slug cobalt-60 source element (an inner capsule containing slugs of cobalt-60) source pencil (contains 2 source elements) source module (contains up to 48 source pencils) source rack (contains modules in different configurations, depending on irradiator design) COBALT-60 SOURCES FOR INDUSTRIAL USE FIG Buildup of a typical cobalt source rack from slugs, pencils and module (courtesy of MDS Nordion, Canada). Total quality management of radiation sources by a supplier requires a life cycle approach. This covers various aspects, including design, manufacture, installation, field inspection, source surveillance and return at the end of their useful life in compliance with ISO 9000, which is the current international standard describing requirements for quality management systems [2.4] GAMMA IRRADIATION FACILITY In a large irradiation facility, the irradiation room where the product is treated with radiation is the focal point of the facility (Fig. 2.4). Other major components of a commercial facility include: Shielded storage room (dry or wet) for the radiation source rack ( 60 Co); Source hoist mechanism; Radiation shield surrounding the irradiation room; 8

24 GAMMA IRRADIATORS FOR RADIATION STERILIZATION FIG A typical panoramic, wet storage gamma irradiation facility (courtesy of MDS Nordion, Canada). Control console (room); Product containers (totes); Product conveyor system through the shielding maze; Control and safety interlock system; Areas for loading and unloading products; Supporting service equipment. The radiation source is either in the irradiation room (during irradiation of the product) or in its shielded storage room (generally located under the irradiation room), which could be dry or wet. There is enough shielding provided by solid wall (dry storage) or water (wet storage) so that staff can work in the irradiation room (for example, for maintenance) when the source is in the storage room. Water has several desirable characteristics when used as a shielding material, including that it is an easily available liquid, it is convenient to circulate for heat transfer and it is transparent. For a wet storage facility, nearly all materials used to construct the source rack, guide system and source containers are made of stainless steel so that galvanic corrosion is eliminated. Surrounding the irradiation room is the radiation shield (also referred to as biological shield ), generally consisting of a concrete wall thick enough (usually 2 m) to attenuate the radiation emanating from the source so as to maintain the radiation level at the location of the control console at natural background level. The concrete wall is constructed as a maze (labyrinth) in 9

25 MEHTA order to permit movement of the product and yet significantly reduce the scattered radiation reaching the control console, from where the operator can control or monitor the movement of the source and the product [2.5]. The transport mechanism for the product can be simple or quite elaborate, depending on the irradiator design. For continuous irradiation (as shown in Fig. 2.4), the product containers are moved around the radiation source on a conveyor bed which passes through the maze. For stationary irradiation, the radiation source is moved into the irradiation room after the product containers have been arranged there for irradiation. The irradiation facility also provides areas for storage of the unprocessed product as well as the processed product. It is a requirement that the design of the facility is such that these two types of product cannot be mixed inadvertently (note the separating fence in Fig. 2.4). Also, all facilities have laboratories suitable for carrying out dosimetry measurements. Some facilities also have a microbiology laboratory or a materials testing laboratory GAMMA IRRADIATORS Design principles There are several types of irradiators available commercially. A potential developer of an irradiation facility would have an easy task of selecting one that is best suited to the intended application. The design of an irradiator varies from being small and suited to radiation research, to very large and suited to the throughput of hundreds of tonnes of product per day. The main differences among the various irradiators are the activity level of the radiation source (that is, amount of cobalt) and the method of moving the products in the radiation field. In addition, the method of operation of the irradiator can be selected to suit a specific application. Manufacturers can and are willing to modify the design of an irradiator to suit more specific needs. The basic design principles for all irradiators are: Maximize radiation energy utilization; Provide relatively uniform dose in the product; Ensure safe and easy operation. These principles are addressed by incorporating the following elements in the design, which have been recognized since the early days of the industry and have worked well: 10

26 GAMMA IRRADIATORS FOR RADIATION STERILIZATION Double encapsulated 60 Co source pencils; Water storage pool; Several layers of product surrounding the source; Biological shield made of standard density concrete with a maze design Source design capacity and installed activity The product throughput depends to a large extent on the activity of the radiation source currently installed in the irradiator. The activity can vary from tens of thousands curie to several million curie. The installed activity should always be less than the maximum activity for which the irradiator is designed, which is referred to as the design capacity. Selection of the design capacity is based on the dose requirements for the intended application(s) and the expected maximum annual throughputs during the lifetime of the facility, including future needs. It is common practice to start an irradiation facility with less source activity installed (as required by current needs) than what it is designed for, and later, as higher throughput is needed, for more cobalt to be added. An irradiator is licensed to have no more source activity than the design capacity, since it is specifically designed for that, especially with respect to the shielding requirements. The dose rate in the product is directly related to the installed activity of the source, and the operator controls the absorbed dose delivered to the product by adjusting the time that it is exposed to radiation, either by selecting the irradiation time interval or by selecting the conveyor speed. The only variation in the source output is the known reduction in the activity caused by radioactive decay, which can have a significant impact on the operation of the facility (financial as well as scheduling) if not taken into account. The activity of a cobalt source decreases by about 12% annually. The irradiator operator compensates for this loss of activity (which decreases the dose rate) by incrementally increasing irradiation time (approximately 1% per month) to maintain the same dose to the product. Eventually, irradiation time becomes impractically long (reducing the throughput), requiring the addition of 60 Co pencils to the source rack (source replenishment) at regular intervals, depending on operational requirements. For the currently available commercial gamma irradiators, typically 30% of the energy emitted by the radiation source is usefully absorbed by the product. Thus, an irradiator with 1 MCi (1 million curie) of 60 Co would process about 0.65 t (Mg) of product per hour where the minimum dose requirement is 25 kgy (for the sterilization of health care product). If the dose were 4 kgy (typically for food), the throughput would increase to about 4 t/h. 11

27 MEHTA Process dose and delivered dose Process dose, that is, the dose needed to achieve a desired effect in the product, is determined through radiation research, which involves determining the dose effect relationship for the effect (such as sterility level versus dose and functional quality of the product versus dose). Generally, the outcome of such research is the identification of two dose limits: the lower dose limit sets the minimum dose that is required to achieve the desired sterility level of the product, and the upper dose limit is set to ensure that radiation will not adversely affect the quality of the product (for example, plastic components of health care products may become brittle). Usually, each product or process has a pair of these limits, and these values define the acceptable dose window, such that every part of the product should receive a dose within that range. The ratio of the upper dose limit to the lower dose limit may be referred to as dose limit ratio. During a radiation process, gamma radiation interacts with the product (any material) through several types of atomic interactions, such as Compton scattering, photoelectric effect and pair production. Through these and subsequent interactions, it imparts energy and thus radiation dose to the product. As radiation proceeds through the product, its intensity decreases with the result that dose also decreases with depth. This is referred to as depth dose distribution (Fig. 2.5, curve a or b ). The rate of decrease depends on the composition and density of the product and the energy of the gamma radiation. Besides the variation of dose with depth, there is also dose variation in the lateral direction. This variation depends on the geometry of irradiation. Both types of dose variation contribute to the non-uniformity of the dose delivered to the product. Variation in dose in the irradiated product is unavoidable. One accepted method of describing this non-uniformity of dose is the concept of dose uniformity ratio (DUR), which is the ratio of the maximum dose in a product container to the minimum dose in the container. This ratio increases with the density of the product as well as with the size of the container (Fig. 2.6). This ratio should be close to unity (for example, less than 1.05) for radiation research samples, where the research objective is to correlate radiation effect in the sample to the dose. This is generally achieved by reducing the size of the sample. For commercial operation, this is not possible for economic reasons. A typical product container can be 60 cm 50 cm 150 cm, and some irradiators are designed to irradiate entire pallets of product, for example, of 120 cm 100 cm 150 cm. The dose uniformity ratio would be significantly larger than unity for such large containers. 12

28 GAMMA IRRADIATORS FOR RADIATION STERILIZATION FIG Depth dose distribution in a product container irradiated from two sides with a 60 Co source. Curve a represents the depth dose distribution when the product is irradiated from one side only (source is at position a ). Similarly, when the source is at position b, the dose distribution is curve b. The total dose due to irradiation from two sides is then shown as curve a + b. Note that this total dose is much more uniform than that due to single sided irradiation (curve a or b ). FIG Dependence of dose uniformity ratio (DUR) on product density for two different irradiator designs (courtesy of MDS Nordion, Canada). 13

29 MEHTA Fortunately, for a large majority of products, there is a wide window of dose that is acceptable to achieve the desired level of sterility without detrimentally affecting the quality of the product. For such products, the dose limit ratio is between 1.5 and 3, and sometimes even larger. Thus, the guiding principle is: the measured dose uniformity ratio should be smaller than the dose limit ratio prescribed for the product. There are different ways to reduce the dose uniformity ratio (that is, for making dose more uniform) in a product container. The variation along the depth is easily reduced by irradiating the product from more than one side (as illustrated in Fig. 2.5). This can be accomplished either by rotating the product container during irradiation or for the product container to travel around a radiation source. All gamma irradiators use one of these techniques for the purpose. The lateral dose variation may be reduced in several ways, including placing the higher activity source pencils near the periphery of the source rack (source augmentation), and relative arrangement of the product containers and the source (source overlap or product overlap). Different irradiators apply different methods to improve dose uniformity Radiation processing throughput Processing throughput is the amount (mass or volume) of product processed per unit time (e.g. kg/h or m 3 /h) and is determined by the power (activity) of the radiation source, product density and the product absorbed dose. For a gamma irradiator, source power depends on the source activity, such that 1 MCi of cobalt emits about 15 kw of power. Figure 2.7 shows the effect of density on the throughput for two different irradiator designs TYPES OF IRRADIATORS Over the years, the manufacturers and suppliers of gamma irradiators have put significant effort into responding to the growing needs of the industry. The main elements which have been the focus of continuous attention include cost effectiveness of the radiation process, dose uniformity in product, turnaround time and operational reliability. These elements have seen steady improvement with time. Subsequently, these measures have resulted in a variety of sizes and designs of irradiators that are suitable for specific applications. Thus, commercially available irradiators could almost meet the current requirements of the industry. Besides, the designs can be modified to suit the more specific needs of a product. 14

30 GAMMA IRRADIATORS FOR RADIATION STERILIZATION FIG Dependence of product throughput on product density for two different irradiator designs (courtesy of MDS Nordion, Canada). Gamma irradiators may be divided into two broad types: Self-contained irradiators; Panoramic irradiators Self-contained irradiators (IAEA Categories I and III) Self-contained irradiators are specially designed for research and for applications that need small doses such as blood irradiation for preventing transfusion induced graft versus host disease (GVHD), and reproductive sterilization of insects for pest management programmes and relatively small throughputs (sterilizing tissue grafts). A large majority of these are dry storage irradiators and the source activity is limited to several kilocuries (e.g. about 25 kci for 60 Co) (Fig. 2.8). These irradiators house the radiation source (either 60 Co or 137 Cs) within a protective shield of lead or other material, and have a mechanism to move the sample from the loading position to the irradiation position. Such units can be placed very conveniently in an existing laboratory or a room without needing extra shielding. The principal advantages of such small irradiators are that they are easy to install and operate, and that they provide high dose rate and good dose uniformity, which is essential for radiation research. These characteristics are achieved by surrounding the sample with radiation source pencils, such that it receives radiation from all directions. Such a design arrangement places restrictions on the sample size, typically limiting it to 1 5 L. However, this volume is quite adequate for research and small scale irradiations. To irradiate, the sample is placed in the irradiation chamber while 15