ML20141C326
| ML20141C326 | |
| Person / Time | |
|---|---|
| Issue date: | 05/02/1997 |
| From: | Piccone J NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| To: | Trottier C NRC OFFICE OF NUCLEAR REGULATORY RESEARCH (RES) |
| References | |
| RTR-REGGD-08.039, RTR-REGGD-8.039, TASK-*****, TASK-RE NUDOCS 9705190014 | |
| Download: ML20141C326 (8) | |
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UNITED STATES 'Q(gy
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- NUCLEAR REGULATORY COMMISSION . -
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- WASHINGTON, D.C. 2006H001
%,, # Nay 2, 1997 l[g s.gawe m
- MEMORANDUM TO: Cheryl A.Trottier, Acting Chief L
Radiation Protection and Health Effects Branch l Division of Regulatory Applications, RES FROM: p Josephine M. Piccone, Chie 4 -
Operations Branch h Division ofIndustrial and
- Medical Nuclear Safety, MSS
SUBJECT:
PATIENT RELEASE LIMITS FOR PURE BETA EMITTERS I
Regulatory Guide 8.39, DG-8015, " Release of Patients Administered Radioactive Materials" contains a table that lists the difTerent isotopes used in diagnostic and therapeutic procedures and the corresponding administered activities at which patients may be released, The table lists an l activity of 100 mci for the three pure beta emitting isotopes "P, "Sr, and "Y. After reviewing the P
proposed guidance, the Advisory Committee on Medical Uses ofIsotopes (ACMUI) questioned '
- the basis for the release limits of 100 mci for these isotopes. The staff of the Health Physics l L Section of the Radiation Protection and Health Effects Branch, RES, discussed this matter with my *
- i. staff and requested assistance in establishing a technical basis for the proposed release limits for the pure beta emitters.
- The Operations Branch reviewed this matter and provided calculations (attached) that indicat:: that >
release limits may not be necessary for the three pure beta emitters considered in this study.
L Specifically, the calculations demonstrate that the administered quantities of these isotopes l resulting in a public dose of 0.5 rem are'of the order of several curies. Therefore, if the y administered dose of any of these three pure beta emitters does not exceed several curies, then it is recommended that the release of patients be based on co'nsiderations other than doses to members ,
of the public.
Please call the technical contact if you need further discussion or clarifications.
Attachment:
As stated ,,
l CONTACT: Sami Sherbini,IMOB
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PATIENT RELEASE CRITERIA ESTIMATE OF DOSES FROM PURE BETA EMITTERS
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The release of patients who have been injected with radioactive materials as part of diagnostic procedures is based, in pan, on the estimated dose to a member of the public who may be in close proximity to the patient after release. Release is contingent on this dose being less than 500 mrem. The assumptions used in estimating the doses from the gamma emitters are that the
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patient is reasonably well represented by a point source, the member of the public is at a distance of 1 meter from that source, and contact with the patient (presence within 1 meter of the patient), )
by any member of the public is not greater than 25% of the time after release. The dose is
, calculated over the period starting from the time of release until the activity is eliminated from l the patient both by radiological decay and biological elimination.
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Isotopes emitting gamma radiation present little difficulty in determining an estimated dose to a i member of the public. However, pure beta emitters are more difficult to assess because beta radiation does not escape from the patient's body, and does not therefore directly irradiate the member of the public. However, beta radiation produces x-rays, oc bremsstrahlung, in the i patient's body, and these x-rays then escape from the body and irradiate the members of the public. It is therefore necessary to estimate the dose that may result from such x-rays to ensure that the total dose from patients administered pure beta emitters does not exceed the 500 mrem value. Three pure beta emitters are ofinterest in this context: "P, "Sr. and "Y. Two methods were used to estimate the x-ray dose: an approximate method based on estimating the fraction of
- the beta energy that is converted to x-rays in a point source, and an more detailed estimate using transport calculations in an extended medium for the beta rays and the x-rays they produce using the Monte Carlo method.
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- 1. Relative Photon Intensity '
This method is based on estimating the fraction of the beta spectrum energy that is converted to bremsstrahlung in a point source. The fraction of the beta energy so converted is !
given by:
I ZE, V 3000 (I) l l
where, I = bremsstrahlung intensity, Mev/p disintegration j E = average beta energy, MeV E, = end-point beta energy, MeV Z = atomic number of absorber
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Attachment 1
Since the radionuclides of concern in this calculation are bone seekers, it wa.s be assumed that the beta radiations interact in bone. This gives a higher x-ray production rate than would be the case if the radionuclide is assumed to be in soft tissue. The effective atomic number of bone varies depending on the type of radiation interaction being considered, but a good estimate is 14, Z% = 14 I (2)
- =4.67x 10 ~3E, E
The energy fluence rate at distance r from a point source of bremsstrahlung at any time t is given by:
$= e 'h r
(3) where,
$ = Energy fluence rate, MeV/hr/cm2/Ci K = conversion constant
= 2.54 x 10" I = bremsstrahlung emission rate, MeV/p disintegration r = distance from the source, cm t = decay time, days 1 = decay constant for removal from bone, day d 2
The energy fluence T, in MeV/cm /Ci at distance r from the point source is obtained by integration of Equation (3), and is given by:
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T= e'hdt (4) r
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l Integrating this expression, and substituting 100 cm (the distance from the source to the member l
l of the public) for the distance r, the expression for fluence becomes, '
1 1 l T == 3.67 x 1OS I Tv, (5) where Tv, is the source halflife in days. The radionuclides ofinterest in this calculation are l retained in bone for long periods of time, and their radiological half-lives are relatively short ;
compared with the biological halflives. Elimination will therefore be controlled by radiological decay. The isotopes, some of their relevant characteristics, and the calculated bremsstrahlung emission rates, are shown in Table (1) below. .
j Table 1. Bremsstrahlung emission rate from the three beta emitters l calculated using Equation (2) l Isotope E, E Tv, I !
MeV MeV Days MeV/Disint. l 32P 1.71 0.70 14.2 0.0057 "Sr 1.49 0.58 50.5 0.0040 "Y 2.284 0.93 2.7 0.0099 II. Monte Carlo Estimates This method generates electrons from the source atoms and follows their paths until they are absorbed. Any bremsstrahlung phctons generated alon'g the paths of the electrons are also followed through the various scattering reactions until they are absorbed. The method was implemented in this case using the MCNP-4A code, a general purpose Monte Carlo code developed at Los Alamos National Laboratory.
The geometry selected for this calculation is the ICRU (International Commission on Radiation l Units and Measurements) sphere, which is a 30-cm spherical phantom made of tissue-equivalent material. The sphere simulates the human torso, and is used to define many of the dosimetric quantities currently in use. This geometry was chosen because it provides a reasonably good phantom for the purpose of this calculation, and it is also relatively simple to model
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l The beta emitting material was assumed to be concentrated in bone, which in this geometry was l l simulated by a spherical shell of bone material I cm thick and with the outer surface of the shell I i 1 em below the surface of the ICRU sphere. The rest of the sphere was filled with tissue equivalent material. The bremsstrahlung dose rate was estimated at a distance of 1 meter from
_ the surface of the sphere. The beta emission spectra for the three isotopes were generated using l data published in National Bureaus of Standards Publication 13 of the Applied Mathematics Series," Tables for the Analysis of Beta Spectra", June,1952. A plot of the spectra generated for l
l the three isotopes considered in this calculation is shown in Figure (1). l III. Results j l
The energy fluence from the point source beta emitters (Method I) and from the ICRU sphere !
(Method II) are shown in Table 2. The quantities shown in the column for Method I were calculated using Equation (5). i l
l Table 2. Energy fluence per curie administered, at 1 meter from the source. i l
i Energy Fluence, MeV/cm2 /Ci l Isotope Method I Method II 32P 3.0 x 10' 7.7 x 108 i "Sr 7.4 x 10' 2.0 x 10' I i
"Y 9.8 x 108 2.9 x 105 !
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4 The energy fluence is converted to dose using the following expression:
D = 1.6 x 10-8 Y p,/p (6) where, D = dose, rad /Ci T = energy fluence, MeV/cm2 /Ci l ,/p = mass energy absorption coefficient, cm2 /gm l l l The doses at 1 meter from the source were calculated by substituting the data in Table (2) into
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1-5 Equation (6), and the results are shown in Table (3). The mass energy absorption coefficients used in these calculations were weighted mean values, with the we!ghting functions being the bremsstrahlung energy spectra emitted by the beta sources. These bremsstrahlung spectra were obtained from the Monte Carlo calculations. An example of such a spectrum, for 32P, is shown in Figure (2).
Table 3. Doses at I meter from the source, and the corresponding administered activities for a dose of 0.5 rem to a member of the public, assuming an occupancy factor of 25%.
Isotope Dose, rad /Ci Administered activity, Ci at 1 meter from source for 0.5 rad at 1 meter Method 1 Method 2 Method 1 Method 2 32P 1.6 0.4 1.2 5.0 8'Sr 3.9 1.1 04 1.8 "Y 0.5 0.2 4.0 10.0 IV. Conclusions The doses obtained by the Monte Carlo method are lower than those ebtained by Method I by a factor of approximately 3 - 4 for the three isotopes considered. This is not unexpected in view of the different geometries used in the two methods, namely a point source in air for Method I and a j spherical shell source embedded in a sphere filled with tissue-equivalent material in Method II. '
The Monte Carlo calculation was repeated for "Y using the same ICRU sphere but with the j source in the form of a spherical shell ofinner radius 1.0 cm and outer radius 2.0 cm. This I converted the geometry to one that is much closer to the point source geometry used in Method I. ]
The calculated energy fluence in this modified geometry was within 20% of the fluence j calculated using Method I. Therefore, the differences in results between the two methods may be attributed mainly to differences in geometry.
The geometry used in the Monte Carlo method is a more realistic representation of the generation of bremsstrahlung in a patient than a point source, and should therefore be used in establishing the patient release limits, even though Method I results in the lower release limits. As indicated in Table (3) above, the limits for a public dose of 0.5 rem based on Method II, assuming a 25% !
occupancy factor, extend from about 2 Ci for Sr to 10 Ci for "Y. If the doses administered to j patients during diagnosis and therapy are always less than these levels, then we recommend that i I
release of patients administered one of these three isotopes be based on considerations other than the administered dose.
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