Answer to Question #11569 Submitted to "Ask the Experts"

Category: Radiation Basics — Radiation Effects

The following question was answered by an expert in the appropriate field:

Q

I have been studying radionuclides for a couple of weeks, specifically carbon-11 (11C). I have found out so much information on it, but the one piece of information I really need and want to know is the risks involved in using 11C. I have surfed the Internet, and I can't find anything on the risks involved with 11C use. Even if they are just minor risks, I would still really appreciate knowing what those risks are.

A

The radionuclide 11C, as I expect you have learned in your pursuit for information, is a radionuclide with a quite short half-life, about 20.4 minutes. The radionuclide decays with the emission of a positron (equivalent to a positively charged electron), which quickly disappears as the positron loses its kinetic energy and combines with a conventional electron to produce two annihilation photons. The two annihilation photons go off in opposite directions, each with an initial energy of 511 kiloelectronvolts (keV). This characteristic has made 11C desirable for particular imaging procedures in nuclear medicine diagnostic procedures, especially for imaging certain types of cancers. The procedures fall under the category denoted as positron emission tomography (PET).

The ultimate risk of major concern in handling many radioactive materials is possible increased likelihood of cancer from exposure to the radiation. The exposure pathways may be external (i.e., the 11C is outside the body and the external radiation exposes an individual) or internal (i.e., the 11C is taken into the body, thus exposing internal organs to radiation directly from the 11C which may distribute in various tissues). The organ expected to receive the largest dose from internally deposited 11C is the pancreas. For individuals who handle the radionuclide closely in unshielded form, there is also risk of skin dose from the positrons emitted during decay, although significant impacts, such as skin reddening and skin ulceration, are very unlikely. The exception would be if one were to be extremely negligent in handling appreciable amounts of 11C or if one transferred relatively large amounts to one's skin, possibly through an accident, and did not decontaminate the affected skin. Because of the very short half-life, such incidents are extremely unlikely because in most cases the activity would decay away before sufficient dose accrued to cause a problem.

The level of potential risks associated with the use of 11C depend, in part, on what one's role is regarding the radionuclide. I do not know whether you are directly involved with handling the radionuclide, might be a patient being administered the 11C, or possibly might be a family member of a patient who has received 11C in a diagnostic test, so I will attempt to consider the likely possibilities.

The radiation risk to individuals involved with the preparation and application of the radionuclide comes primarily from the annihilation radiation produced when the positron combines with an electron. The annihilation photons are quite energetic compared to the photons from typical radionuclides used in nuclear medicine and are more difficult to reduce in intensity by the use of local shielding. The greatest potential radiation risk would likely be to those involved in the production and preparation of the radionuclide for use. The 11C is typically produced in a particle accelerator, usually a cyclotron located within or very close to the occupancy area of the end users of the 11C. The proximity is necessary because of the short half-life of 11C. The individuals involved in the production and preparation for use may be required to handle considerably larger amounts of radioactivity than is ultimately used in any given procedure and may be doing so on a rather frequent basis. This provides occasion for them to receive more external dose, especially from the annihilation radiation. There is also some potential for skin exposure from the positrons when materials are being processed for use, although this can be avoided through proper shielding and handling.

A nuclear medicine technologist or physician would be handling one dose at a time, and with due diligence, his/her doses should be well below recommended limits for occupational workers in a medical arena even when multiple procedures are performed. The major difference between administering typical nuclear medicine radionuclides, such as technetium-99m (99mTc) and 11C, is that the energies of the 11C photons are higher so that the use of shielding devices, such as syringe shields, is not as effective, and more attention may be required to improve efficiency to reduce exposure time. The external photon radiation dose constant for 11C is 1.908 × 10-4 millisievert per hour per megabecquerel (mSv h-1 MBq-1) at 1 meter (m) from a point source (source whose dimensions are much smaller than the distance between the source and the dose point). One could use this to estimate external doses from handling small volume sources. As an example, suppose a technologist spent five minutes at an effective distance of 0.6 m drawing up, calibrating, and administering a dose of 740 MBq to a patient. We could estimate the external dose to the technologist (neglecting effects of decay during the five minutes) as [(1.908 × 10-4 mSv h-1 MBq-1 m2)(740 MBq) (1 h/60 min)(5 min)]/(0.6 m)2 = 0.032 mSv. Multiplying by the likely number of such procedures per month would yield an estimate of the added monthly dose from performing this procedure.

Technologists and physicians, abiding by the rules and protocols appropriate to their tasks, should not experience any significantly increased risk because of the use of 11C. Accrued dose naturally varies with the number of procedures being conducted.

A patient who receives a dose of 11C for diagnostic purposes will typically be receiving an internal dose that most radiation protection professionals would consider of no risk significance. For example, if a patient received a dose of 370 MBq of 11C (as labeled choline) for a particular prostate imaging test, the typical expected effective dose to that patient would be about 1.6 × 103 microsieverts (µSv), based on an effective dose conversion factor of 4.4 µSv MBq-1 (dose conversion factor from the FDA Prescribing Information). This would represent about one-half of the dose any one of us typically receives annually from exposure to normal background radiation. Such a dose would produce no expected adverse effects in an individual.

Because of the short half-life of the 11C, there should be no concern about the annihilation radiation emanating from the patient’s body producing any dose of concern to anyone, such as a family member, in the vicinity of the patient.

In summary, patients and others associating with patients after they have received 11C would not be expected to receive radiation doses that would produce any measurable negative effect, most notably cancer. Occupationally exposed workers, especially those working with larger amounts of activity and/or being exposed for longer durations have a potential for receiving higher doses, but as long as they abide by regulations and protection recommendations, their doses should not be sufficiently high as to yield noticeably higher risks of cancer or any other adverse effects of radiation exposure.

I should finally note that, while we in the radiation protection community choose to apply a very conservative philosophy that assumes that any added radiation dose produces an increased risk of cancer, the risk being proportional to the dose, in reality we have no actual data to verify this assumption for low doses. There do exist considerable data that indicate that low doses of radiation do not present any increased risk and may even induce a protective effect against future exposures. In fact, the Health Physics Society issued a formal position paper in 2010 recommending that we should not attempt to quantify any radiation-induced health risks to an individual who has received less than 50 mSv in a given year or to an individual who has received less than 100 mSv over a lifetime.

George Chabot, PhD

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