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Radiation Source Use and Replacement: Abbreviated Version SUMMARY Several thousand devices containing nearly 55,000 high-activity1 radiation sources are licensed for use today in the United States. The devices are used for cancer therapy, sterilization of medical devices, irradiation of blood for transplant patients and of laboratory animals for research, nondestructive testing of structures and industrial equipment, and exploration of geologic formations to find oil and gas deposits. These radiation sources and devices are licensed and regulated by the U.S. Nuclear Regulatory Commission (U.S. NRC) or by state agencies with authority to regulate materials covered by agreements with the U.S. NRC, called Agreement States. Because the array of applications of these radiation sources is so broad and the applications are essential to securing health, safety, and prosperity, the devices are licensed for use and found in every state in the nation. After the terrorist attacks on the United States on September 11, 2001, concerns about the safety and security of these radiation sources and devices grew, particularly amid fears that terrorists might use radiation sources to make a radiological dispersal device or “dirty bomb.” As part of the Energy Policy Act of 2005, the U.S. Congress directed the U.S. NRC to take several actions, including requesting a study by the National Research Council to identify the legitimate uses of high-risk radiation sources and the feasibility of replacing them with lower risk alternatives. The committee appointed by the National Research Council to carry out the study was tasked to provide a review of radiation source use, potential replacements for sources that pose a high risk to public health or safety, and findings and recommendations on options for implementing the identified replacements. To do that, the committee met with practitioners and researchers in the relevant fields, examined scientific research and trade literature, and visited facilities that use the radiation sources. In carrying out its charge, the committee has focused foremost on hazards and risks,2 feasibility of replacements, and options for implementing the replacements. This study is not the first effort to examine the uses for radionuclide radiation sources and prioritize among them based on certain kinds of risk. A number of studies (see, e.g., Ferguson et al., 2003; Van Tuyle et al., 2003) describe the system of supply of radionuclide radiation sources and their applications. The Department of Energy (DOE) and the U.S. NRC issued a joint report identifying risk-significant radiation sources and quantities of radioactive material (DOE/U.S. NRC, 2003). The IAEA, in a similar but broader effort, revised its Code of Conduct on the Safety and Security of Radioactive Sources (2003), which provides guidelines for countries in the development and harmonization of policies, laws, and regulations on the safety and security of radioactive sources. The IAEA Code of Conduct includes a categorization system for radionuclide radiation sources that provides a risk-based ranking of radioactive sources based on their potential for harm to human health under specific scenarios and for grouping of source use practices into discrete categories. The radiation sources in Category 1 are those that, if not managed safely or securely, could lead to the death or permanent injury of individuals in a short period of time. Similarly, Category 2 sources are those that could lead to the death or 1 Activity is the number of radioactive decays per second. Specific activity is the activity per gram of material. The high-activity sources cited here are Category 1 and 2 sources, as defined in the International Atomic Energy Agency’s (IAEA’s) Code of Conduct on the Safety and Security of Radioactive Sources, and described in this summary. 2 For clarity, and to be consistent with the standard scientific definitions, the committee uses the term hazard to denote the potential to cause harm and the term risk to describe a hazard linked to a context of exposure or possibility of an event leading to exposure. Gasoline is hazardous; gasoline stored where an open flame or spark might ignite it poses a high risk.
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Radiation Source Use and Replacement: Abbreviated Version permanent injury of individuals who may be in close proximity to the radioactive source for a longer period of time than for Category 1 sources. Based on direction and authority in the Energy Policy Act of 2005 (P.L. 109-58), the U.S. NRC limited the radiation sources within the study scope to Category 1 and 2 sources. Data from the U.S. NRC show that out of the thousands of manufactured and natural radionuclides, americium-241, cesium-137, cobalt-60, and iridium-192 account for nearly all (over 99 percent) of the Category 1 and 2 sources. The features of these and some other key radionuclide radiation sources are summarized in Table S-1. TABLE S-1 Summary of Radionuclides in Category 1 and 2 Radiation Sources in the United Statesa Radionuclide Half-life Radioactive Emissions and Energies Typical Specific Activity (TBq/g) [Ci/g] Total Activity in U.S. Inventory (TBq) [Ci] Major Applications Typical Activity (TBq) [Ci] Physical or Chemical Form Americium-241 432.2 yr α–5.64 MeV, γ-60 keV, principal 0.13 [3.5] 240 [6,482] Well logging 0.50.8 [13–22] Pressed powder (americium oxide) Californium-252 2.645 yr α–6.22 MeV, fission fragments, neutrons, and gamma rays 20  0.26  Well logging 0.0004 [0.011] Metal oxide Cesium-137 (Ba-137m) 30.17 yr β-518 keV max with γ-662 keV (94.4% of decays) or β-1.18 MeV max 0.75  104,100 [2.8 million] Self-contained irradiators 75 [2,000] Pressed powder (cesium chloride) Teletherapy 50 [1,400] Calibrators 15  Cobalt-60 5.27 yr γ-1.173 and 1.333 MeV 3.7  7.32 million [198 million] Panoramic irradiators 150,000 [4 million] Metal slugs Self-contained irradiators 900 [24,000] 11  Teletherapy 500 [14,000] Metal pellets Industrial radiography 4  Iridium-192 74 d β-1.46 MeV max with 2.3 γ-380 keV avg, 1.378 MeV max (0.04% of decays) 18.5  5,436 [146,922] Industrial radiography 4  Metal Plutonium-238 87.7 yr α–5.59 MeV, and γ-43 keV (30% of decays) 2.6  34.7  RTG 10  Metal oxide Pacemakers (obsolete) 0.1  Fixed gauges 0.75  Selenium-75 119.8 d γ-280 keV average, 800 keV max 20–45 [530–1200] 9.7  Industrial radiography 3  Elemental or metal compound Strontium-90 (Yttrium-90) 28.9 yr β-546 keV 5.2  64,000 [1.73 million] RTG 750 [20,000] Metal oxide aNuclear decay data for this table and throughout the report are from Firestone and Shirley (1996).
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Radiation Source Use and Replacement: Abbreviated Version Consideration of technological alternatives to radionuclide radiation sources has been recommended by the Health Physics Society, the IAEA, and others. The replacement options may include replacing the radionuclide-based technology with a technology not involving radiation or with x-rays, an electron beam, or neutrons from a radiation generator (a particle accelerator device). Finally, the radionuclide or the chemical and physical form of the radionuclide may be changed to a less hazardous one. In the body of the report the committee discusses origins, forms, and applications of radionuclide radiation sources (Chapter 2), risks associated with radionuclide radiation sources (Chapter 3), accelerator and detector technologies (Chapter 4), each of the major applications of radionuclide radiation sources (Chapters 5 through 9), and options for implementation of application-specific replacement technologies, including the various kinds of incentives that might be applied (Chapter 10). The major findings and recommendations are described below and are discussed in detail in the body of the report. FINDINGS AND RECOMMENDATIONS Finding 1: The radiation sources examined in this study are used in applications that are important to the nation’s health, safety, and economic strength. High-activity radiation sources are used in the United States and other modern societies in a variety of ways: They are used in devices that improve the success of medical procedures—ensuring that medical devices and implants are sterile, preventing fatal complications from bone marrow transplants, and providing noninvasive techniques for treating brain lesions; they are used in devices for inspecting the integrity of buildings, bridges, and industrial equipment; and they are used to seek out oil and gas resources deep in the ground. These applications are immensely valuable to the United States. The question is not whether these activities should continue, but whether lower risk replacements for the radiation sources are feasible and practical, and what steps should be taken to implement replacements for the sources that pose a high risk to public health and safety. Recommendation 1: Replacement of some radionuclide radiation sources with alternatives should be implemented with caution, ensuring that the essential functions that the radionuclide radiation sources perform are preserved. As the nation seeks to improve safety and security, the value and benefits of current practices should be recognized and, where possible, the services the devices provide should not be compromised. Some replacements do entail trade-offs with respect to safety, security, costs, convenience, and performance, as discussed in Chapters 3 through 9. These trade-offs should be considered carefully. A reduction in the performance of a device may be acceptable if it provides sufficient benefits in safety, for example. Replacement should preserve acceptable performance of these applications to preserve the benefits that these applications provide, on many of which the United States has come to rely.
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Radiation Source Use and Replacement: Abbreviated Version Finding 2a: The U.S. NRC ranks the hazards of radiation sources primarily based on the potential for deterministic health effects (especially death and severe bodily harm) from direct exposure to the radiation emitted by the bare (unshielded) sources. The U.S. NRC’s analyses that support the commission’s security requirements for nuclear materials licensees are based only on these potential consequences. The U.S. NRC has ranked radiation sources in terms of hazard using the IAEA system of five source categories, determining that the Category 1 and 2 sources are “high-risk sources.” The IAEA analyses supporting its source categorization system consider only deterministic health effects (such as early fatalities) from direct exposure to ionizing radiation from the unshielded source under different exposure scenarios. The initial DOE/U.S. NRC analysis used the same consequences and added a contamination threshold criterion that does not account well for the differing potential for area denial or economic consequences of dispersal attacks with different radiation sources. The U.S. NRC also carried out security analyses of each type of facility licensed to use Category 1 and 2 sources, but these analyses were confined to examining the potential for deterministic health effects caused by attacks involving the Category 1 and 2 sources. The U.S. NRC staff told the committee that this was seen as a first step, and that the commission was considering whether to include other factors. Finding 2b: Factors other than the potential to cause deterministic health effects are important when evaluating hazards from radiation sources, especially the potential to cause contamination of large areas resulting in economic and social disruption (area denial). A radiological incident (an accident or especially an attack) could have its most long-lasting and far-reaching effects as a result of contamination of land, buildings, and infrastructure in densely populated regions, partially or completely disabling those assets for human use for long periods of time. This is illustrated by the radiotherapy source incident in 1987 in Goiania, Brazil, and the Chornobyl nuclear reactor accident in the Ukraine. Although an event like the Chornobyl reactor fire is not possible with radiation sources and the scale of the contamination from an incident with radiation sources would inherently be smaller, that 1986 accident showed that radioactive contamination can create sizeable areas that are deemed uninhabitable for extended periods of time. The economic and social disruptions caused by such incidents can be difficult to quantify, but they are critical to understanding the scope of the impact beyond the fatalities and severe bodily injuries caused by these events. Recommendation 2: For prioritizing efforts to reduce risks from malicious use of radiation sources, the U.S. NRC should consider radiation sources’ potential to cause contamination of large areas resulting in economic and social disruption (area denial) to determine what, if any, additional security measures are needed. Having taken an essential first step in considering deterministic health effects from possible radiation exposure from an incident involving radiation sources, the U.S. NRC should now include economic and social disruption in its risk analyses of radiation sources. These impacts can vary significantly depending on the scenarios considered, but that variability does not make them less important. Further, even with such variability, certain factors emerge as important in other analyses of these issues (e.g., Van Tuyle et al., 2003). In carrying out its analyses, the U.S. NRC should not confine itself to the numeric source-activity cutoffs defining the lower limits for Category 1 and 2 sources because the source categorization system itself is
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Radiation Source Use and Replacement: Abbreviated Version based on deterministic health effects. For example, many self-contained irradiators are Category 2 devices, but are near the Category 1 threshold, and most americium-beryllium well logging sources have activities near but below the Category 2 limit. Review may show that each set of these devices should be regulated similarly. After 2001, the U.S. NRC imposed enhanced security requirements on its materials licensees: Compensatory Measures for panoramic irradiators, Additional Security Measures for its manufacturers and distributors, and Increased Controls for licensees with Category 1 and 2 devices and sources. Compensatory Measures include fairly robust access controls and alarms with response by armed security personnel, along with other measures. Increased Controls include access controls and alarms with response by security personnel, and other measures. After review of the risks associated with some sources and devices considering more fully the potential for contamination from an attack, the U.S. NRC might conclude that more stringent measures are needed for some Category 1 and 2 sources and devices. The committee did not examine these security matters in detail and so cannot prejudge the outcome of such analyses. The committee does note, however, that such measures could improve the security of the devices and create a disincentive for owning them. Finding 3a: Because of its dispersibility, solubility, penetrating radiation, source activity, and presence across the United States in facilities such as hospitals, blood banks, and universities, many of which are located in large population centers, radioactive cesium chloride is a greater concern than other Category 1 and 2 sources for some attack scenarios. This concern is exacerbated by the lack of an avenue for permanent disposal of high-activity cesium radiation sources, which can result in disused cesium sources sitting in licensees’ storage facilities. As such, these sources pose unique risks. Radioactive cesium chloride sources are in the form of a steel-encapsulated, compressed powder. The salt is highly dispersible and water soluble. There are approximately 1,300 high-activity cesium chloride devices (each with an activity of tens to hundreds of terabecquerels [hundreds to thousands of curies]) across the United States, nearly all of which are self-contained irradiators. The number of these devices and sources appears to be increasing. Because it emits energetic gamma rays and its half-life is long enough that an irradiator does not need to be reloaded over the device’s expected lifetime, cesium-137 has been the key component of self-contained irradiators for blood irradiation and research for many years. Cesium chloride is the least expensive and highest-specific-activity form of cesium-137 available today. Because of the nature of the applications that employ these irradiators, they are most commonly located in hospitals, blood banks, and universities, many of which are located in cities, large and small. The presence of these sizable sources in areas that are potentially attractive targets is a major factor making radioactive cesium chloride such a concern to the committee. Finding 3b: In view of the overall liabilities of radioactive cesium chloride, the committee judges that these sources should be replaced in the United States and, to the extent possible, elsewhere. Finding 3c: In most (and perhaps all) applications, radioactive cesium chloride can be replaced by (1) less hazardous forms of radioactive cesium, (2) radioactive cobalt, or (3) nonradionuclide alternatives. However, not all of these alternatives are commercially
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Radiation Source Use and Replacement: Abbreviated Version available now, and all are currently more expensive than radioactive cesium chloride for the users. Some alternatives to radioactive cesium chloride include radioactive cesium glass and a mineral form (pollucite) loaded with radioactive cesium (described in Chapter 2). These alternative material forms use the same cesium-137 as radioactive cesium chloride; thus the gamma rays and the half-life are identical, but the specific activity of these sources is smaller and the pollucite is more difficult to fabricate, especially for high-activity sources. The committee judges that none of the current applications of high-activity cesium sources about which the committee was informed necessitates the higher specific activity afforded by cesium chloride. Accommodating the larger volume needed to achieve the same source activity would require redesign of some (but not all) devices. High-activity cesium sources are not, however, available in these alternative material forms today, and making them available may require the cesium source producer (the Production Association Mayak, in Russia) to modify its production process. Cobalt-60 may be substituted for radioactive cesium chloride for many applications (see the discussion in Chapter 5), although as much as twice the shielding thickness may be required for a source that achieves the same dose rate, and the half-life of cobalt-60 is shorter (5.3 years for cobalt-60 versus 30 years for cesium-137), thus lowering significantly the useful lifetime of the source. Shielding challenges can be addressed in part by switching from lead shields to more effective tungsten or depleted uranium shielding, but tungsten shielding is more expensive than lead and manufacturing depleted uranium shielding is a very specialized, expensive operation that requires U.S. NRC licensing for its whole life cycle. The shorter useful lifetime of cobalt-60 radiation sources requires that they be replaced periodically, which entails transportation of a fresh source and the used source, with the attendant risks associated with source transportation. X-ray generators are already commercially available as substitutes for applications that do not require the gamma rays with definite energies emitted by cesium-137 and cobalt-60. X-ray tubes can be expensive and require more maintenance for periodic calibration and replacement than radioactive sources require. Finding 3d: Government action is required to implement replacement of radioactive cesium chloride sources because the alternatives cost more and the liabilities or social costs of the sources currently are not borne by the end users. There is no indication that replacement of devices containing Category 1 and 2 radioactive cesium chloride sources with lower hazard alternatives will improve or worsen the performance of the devices in their standard and proper uses. The act of replacement incurs monetary costs, and the replacements themselves currently cost more in most cases than the radioactive cesium chloride devices. All of these costs would be borne by the end users (paying more for the alternatives) and the current device manufacturers (depending on the price elasticity of demand and potential loss of sales). The benefits of replacement are in reducing the liabilities and social costs (including the costs associated with the risk of terrorist attacks and, in some cases, the full costs of disposal, discussed in Chapters 2 and 10). Those social costs, including the risks, are shared by the public. Except in cases where the replacements prove to be cheaper, end users have little incentive to shift away from radioactive cesium chloride; and unless there is a demand for the alternatives, manufacturers are unlikely to invest in making the alternatives available. Government action can, however, provide the requirements or incentives to implement replacement.
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Radiation Source Use and Replacement: Abbreviated Version Recommendation 3: In view of the overall liabilities of radioactive cesium chloride, the U.S. government should implement options for eliminating Category 1 and 2 cesium chloride sources from use in the United States and, to the extent possible, elsewhere. The committee suggests these options as the steps for implementation: Discontinue licensing of new cesium chloride irradiator sources. Put in place incentives for decommissioning existing sources. Prohibit the export of cesium chloride sources to other countries, except for purposes of disposal in an appropriately licensed facility. In Chapter 10, the committee offers several suggestions as its lead candidates for how to implement the replacement, and they are summarized here. First, to stop the addition of new Category 1 and 2 cesium chloride sources to the nation’s inventory, the U.S. NRC should discontinue all new licensing and importation of these sources and devices. This includes import of new sources from other countries and recycling of sources from decommissioned devices. Second, many licensees may need incentives to decommission their existing sources or devices because the devices still have use value. Indeed, there are now also disincentives to decommissioning beyond the loss of use, including the costs of decommissioning. Third, if the sources recovered from decommissioned devices (or the devices themselves) are simply sold outside the United States, then the sources are still potentially available for use in an attack on another country or even the United States. Therefore, disposition options are needed in the United States. These are discussed in more detail in Chapter 10. The overall policy could make exceptions based on unique needs that cannot be met with alternative technologies, but the threshold for creating exceptions should be set high, similar to what the U.S. NRC has done for panoramic irradiators. Finding 4a: Nonradionuclide replacements exist for nearly all applications of Category 1 and 2 radionuclide sources (not just radioactive cesium chloride). At this time, these replacements may not all be practical or economically attractive, but most of them are improving. Chapter 4 shows a variety of accelerator systems that can be designed to operate as radiation-generator replacements for radionuclide radiation sources. In Chapter 5, the committee explains that self-shielded irradiators can be operated with x-ray generators instead of radionuclides. Some x-ray-based irradiators are already commercially available and more companies that design and manufacture x-ray generators told the committee that they are considering entering the market. As described in Chapter 6, large companies in the business of sterilization of medical supplies and devices operate several kinds of facilities (ethylene oxide, gamma irradiation, and electron beam irradiation) to use the technology that is best suited to the sterilization contract. An x-ray irradiation facility can be a direct replacement for a cobalt-60 panoramic gamma irradiator, and may offer both electron-beam and x-ray irradiation in one facility. Some supporters of x-ray irradiation have concluded that larger x-ray facilities (several hundred kilowatts) would have economic advantages. The first of these larger scale facilities for x-ray irradiation is to be built soon in Belgium. It is unclear whether such facilities will be cost-neutral, more expensive, or less expensive per pallet irradiated than similarly sized gamma irradiators. As noted in Chapter 7, linear accelerators for radiotherapy have almost entirely replaced cobalt-60 teletherapy devices in the United States, except for the Gamma Knife®, the use of which is still growing. The Gamma Knife® is less versatile than a linear accelerator for radiotherapy, but offers features that some customers perceive to be advantages, which their competitors are trying to match with accelerators. The development of new technologies,
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Radiation Source Use and Replacement: Abbreviated Version especially in the areas of ultrasonics and x-ray sources, has provided several alternatives to gamma radiography in the field of nondestructive inspection. In some areas, it is likely that the use of some of the alternatives is currently limited by the availability of trained personnel and wider acceptance of the results as durable records of proper inspection, as noted in Chapter 8. Chapter 9 similarly explains that the neutron well logging tools that use americium-beryllium sources are beginning to see competition from accelerator fusion sources. Table S-2 summarizes the radiation source applications and replacements. Finding 4b: Neither licensees nor manufacturers now bear the full cost of liabilities related to misuse of Category 1 and 2 radiation sources, nor do they bear the costs of disposal of cesium and americium sources. Category 1 and 2 radiation source licensees are not required to be insured for the possible consequences of a malicious use of their radiation sources. This is no different than in other sectors of our society, but it means that the costs of some liabilities are not borne by licensees. In addition, licensees of Category 1 and 2 cesium-137 and americium-241 sources in the United States do not now bear the costs of disposal of their sources because the disposal facilities for these high-activity sources can only accept sources that come from DOE or its predecessor, the Atomic Energy Commission. DOE has a program called the Offsite Source Recovery Project, which packages, transports, and stores high-risk radiation sources and devices without fee. Some licensees pay for the cost of packaging and transportation to effect the removal on their own schedule, but the cost is lower than the cost of disposal will be in an as-yet-unknown disposal facility for “Greater than Class C” low-level waste. Recommendation 4: In addition to actions related to radioactive cesium chloride, the U.S. government should adopt policies that provide incentives (market, regulatory, or certification) to facilitate the introduction of replacements and reduce the attractiveness and availability of high-risk radionuclide sources. The committee describes several options for implementation of alternatives in Chapter 10 of the report. Among these options are to make licensees bear the full life-cycle cost of radiation sources, particularly for disposal of cesium-137 and americium-241 sources; to revise the requirements for decommissioning funds for Category 1 and 2 devices to increase the up-front costs for higher hazard sources; enhance the DOE’s Offsite Source Recovery Project to include a buyback of devices that still have use value, provided that the devices are replaced with lower hazard devices. The government could impose charges on all sources, or just on new sources, based on hazards or risks. Finding 5: Accelerator neutron sources and californium-252 sources show promise as potential replacements for americium-beryllium sources in neutron well logging tools. However, there are technical obstacles for these replacement sources and they are at a disadvantage based on the extensive experience and data accumulated with americium-beryllium sources. Recommendation 5: The Society of Petrophysicists and Well Log Analysts should task an industry working group, called a Special Interest Group (SIG) such as the Nuclear Logging SIG, to address the technical obstacles to implementing replacements for the
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Radiation Source Use and Replacement: Abbreviated Version americium-beryllium sources used in well logging and the challenges of data interpretation. The group should decide what obstacles are most important, but the issues might include development of new reference standards for these replacement tools, examination of the response of these tools relative to the americium-beryllium tools, and exploration of any differences in response when the replacement tools are used in combination with other nuclear and nonnuclear well logging tools.
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Radiation Source Use and Replacement: Abbreviated Version TABLE S-2 Summary of Radionuclide Radiation Source Uses and Possible Replacements (a) Radiotherapy (b) Gamma Knife® Panoramic Irradiators Prevention of GVHD (Blood Irradiation) Radiography Well Logging Porosity Measurement Radionuclide Radiation Sources Radionuclide and activity Co-60: 500 TBq (15 kCi) Co-60: 220 TBq (6 kCi) Co-60: 100,000 TBq (3 MCi) Cs-137: 40–100 TBq (1–3 kCi) Ir-192: 4 TBq (100 Ci) Co-60: 0.25–4 TBq (5–300 Ci) Am-Be: 0.25–0.8 TBq (8–22 Ci) Primary device suppliers None currently sold in the United States. Elekta MDS Nordion MDS Nordion J. L. Shepherd CIS SPEC, QSA Global, and others Schlumberger, Baker-Hughes, Halliburton, and others Capital cost None currently sold in the United States $4M for machine, $2M for bunker Approximately $54 per TBq ($2 per Ci), or $6M–10M for a large facility $150,000–$225,000 $8,000 for system $0.4/GBq for source $30,000 for system $4/GBq for source $30,000–$80,000 for the source, depending on activity and encapsulation requirements Lifetime Reload about every 5 years Annual partial reload 30 years (a) 3 months, (b) 5 years Decades Radiation Source Possible Replacements Replacement technology Dedicated or specialized radiotherapy linac Electron accelerator to make electron beam or x-ray beam X-ray irradiation Tubes Linacs Cs-137 robust forms Co-60 Filtration Chemical treatment Pulsed x-ray Compact accelerator Phased-array ultrasonics D-T (fusionaccelerator source) or Californium-252 source Replacement device suppliers Elecktra, Siemens, Varian, and others Accuray, BrainLab IBA Varian Others could be interested MDS Nordion Many Not now available Not currently sold Not yet approved Not yet approved Many Schlumberger Pathfinder Capital cost $4M for machine, $2M for bunker, unless linac is shared for standard radiotherapy $10M for a large x-ray facility $150,000 $3M or $0 (if already in house) Approx. $50,000 or more Approx. $200,000 Ranges $50,000–$100,000 Estimated at $40,000–$50,000 based on other D-T sources $5,000–$6,000, replace more often than Am-Be Operating cost compared to radionuclide option Lower than Gamma Knife® Somewhat higher than gamma irradiator Higher than gamma irradiation Higher than gamma irradiation Higher than gamma radiography Higher than gamma radiography Higher, technician requires more training Similar to radionuclide source Lifetime 10–15 years (obsolescence of computer controls) Perhaps comparable to gamma irradiator Unknown 4–5 years 4–5 years Comments Some doctors prefer Gamma Knife® over linac options Viability not yet proven against contract gamma irradiators Currently only one model available Backup option only Some applications still require radionuclide radiography Commercially available today, but not yet widely adopted.