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Nuclear Medicine - Patient Release Criteria
Read the cool stuff below, but please check out our slightly stupendous
patient release calculator, too!!!
(from Mackay Cartoons)
Dose from radioactive patients released after radionuclide therapy
Patients who receive therapeutic amounts of radiopharmaceuticals are a potentially significant source of radiation to their family members, members of the public whom they pass by on their way from the hospital to their homes, and others. Note - although we have addressed the specifics of NRC regulations and guidance applicable in the United States, our approach and the radiation protection principles involved are relevant and applicable to all other countries regardless of their regulatory specifics. Each country has its own specific approach to this problem.For many decades in the US, the release criterion for such patients was activity based irrespective of radionuclide; most patients were treated with 131I sodium iodide for hyperthyroidism (Graves’ disease) or thyroid cancer. The release limit (which no one knows how it was originally derived1) was that patients could be let go when their activity level was 1100 MBq (30 mCi), or the dose rate at 1 m from the patient was 50 microSv/hr (5 mrem/hr), although this dose rate limit was not officially issued by the NRC until 1987 when NRC revised 10 CFR 35.75 – until 1987, the official regulation did not even contain the 30 mCi rule, rather it was a license condition. In a more recent revision of 10 CFR 35.75, issued in 1997, the NRC changed the system, to be more objectively based on a purely dose-based criterion and to adequately cover the many more therapeutic radiopharmaceuticals in use. Licensees are now able to release patients regardless of how much administered activity they received, if the radiation dose to any individual from exposure to the released patient can be shown to not likely exceed 5 mSv (0.5 rem), integrated over all time after patient release. The rule states that the “licensee shall provide the released individual, or the individual’s parent or guardian, with instructions, including written instruction, on actions recommended to maintain doses to other individuals as low as is reasonably achievable…”2.
The NRC did not intend to enforce patient compliance with the instructions nor is it the licensee’s responsibility to do so. But hospitals do need to keep records showing that they have ascertained that the doses to the maximally exposed individual is “not likely” to be above the stated dose limit of 5 mSv. The NRC published a “Regulatory Guide”, NRC Regulatory Guide 8.39, superceded by NUREG-1556 Vol. 9 to help licensees comply with the revised 10 CFR Part 35 regulations. These “guides” do not carry any force of law, unless the licensee formally adopts them in his/her license as part of the facilities official procedures. The NRC has formally noted that other good methods can be used for these calculations and may be accepted by the commission if they can be shown to be sound. The method used in the guidance document was quite conservative in a number of aspects. First, the patient was treated as an unattenuated point source in calculation of external exposure rates; lower doses will be delivered if line or volume sources are considered rather than point sources. Patients will generally have activity distributed throughout their entire bodies, and some self-attenuation will occur, thus the use of a point source is quite conservative. Then, the decay of activity was assumed to be only by physical decay of the radionuclide; biological elimination by the patient was not considered. Actual measurements on patients’ family members by one group of authors indeed showed that the real doses received by people are significantly less than that assumed by this NRC guidance method3. The equation used was:
Here, D(¥) is the dose integrated over all time, G is the radionuclide specific gamma constant, Q0 is patient activity at time of release, Tp is the radionuclide half-life, OF is the assumed occupancy factor and r is the assumed average distance from a subject over the time of irradiation. For short-lived nuclides (Tp < 1 d), an OF of 1.0 was used, and for others an OF of 0.25 was used. The default average distance from a subject was assumed to be 1 m. The NRC provided a default table of activity levels and dose rates for various radionuclides at which they deem the dose criterion will be met. Here is a sample portion of that table:
The NRC guidance proposes essentially two different patient release methods:
1. Use of the above table based on physical half-life only; and
2. Peformance of a patient specific dose calculation in which they assume conservative values for biological clearance, internal intake, etc for oral administration of Na131I.
These use of the dose calculation as part of the release criteria is clearly an improvement over the use of the “default” table values. However, the method has some shortcomings, for example, being overly conservative in the continued use of a point source model and conservatively accounting for the biologic removal of the radiopharmaceuticals. The calculations have been shown to be conservative4,5,6 and the Society of Nuclear Medicine has even published its own guidance document for use by licensees.7
In a recent paper by Siegel et al. originally entitled “The Emperor Has No Clothes: Licensee Over-Reliance on NRC Guidance for Release of Patients Administered Radioactive Material Does Not Promote Public Health and Safety”8, the authors examined “the guidance methods to assess the external dose component, the internal dose component, and thus the TEDE, and by so doing, demonstrated that the guidance procedures are overly conservative and introduce an unnecessary regulatory burden not codified in NRC requirements. Alternative procedures are proposed to enable licensee compliance with 10 CFR 35.75, and these procedures should be used by all licensees, instead of automatic reliance on the NRC guidance document. Patients and their families share the largest burden when overly restrictive release criteria are enforced. Possible consequences of overly rigid adherence to the NUREG procedures include the under-treatment of patients, issuance of overly restrictive release instructions, and unnecessary confinement of patients to hospital beds. The significant and unjustified additional cost to patients and their loved ones, the requirement for hospitals to prepare and decontaminate unneeded rooms so that staff can receive unnecessary radiation exposures, and the adoption of substandard patient release policies associated with licensee adherence to NRC patient-release guidance, should be critically re-evaluated.
This subject has been of interest to RADAR members for some time now, and they have published extensively on it. The key focus should always be on the health and well being of patients and their families, not simply adherence to regulations. Use of the alternative RADAR promoted methods satisfies both goals and lessens the burdens both on patients and licensees. Further information, and specific instruction in and practice with the required patient release calculations is also available in a low-cost on-line short course at the CDE training site. So………………………