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CONSIDERATIONS IN DETERMINING WHETHER OR NOT

By Gloria Morgan,2014-05-07 12:10
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CONSIDERATIONS IN DETERMINING WHETHER OR NOT

    CONSIDERATIONS FOR RELEASE OF PATIENTS ADMINISTERED

    RADIOPHARMACEUTICALS

Since 1997, members of the public have been limited to 5 mSv (500 mrem) Total Effective Dose

    Equivalent (TEDE) from exposure to appropriately released patients who have been administered

    radiopharmaceuticals, pursuant to 10 CFR 35.75. Many Agreement States have adopted the

    Nuclear Regulatory Commission’s approach to patient release.

The educational review material in this tutorial was compiled by Carol S. Marcus

    (csmarcus@ucla.edu), Jeffry A. Siegel (nukephysics@comcast.net) and Michael G. Stabin

    (michael.g.stabin@vanderbilt.edu). Comments, criticisms, and additions are welcome. Some of

    this material has already been posted on the internet at the request of the California Radiological

    Health Branch (RHB). These educational materials, posted by RADAR members, may be used by

    all Authorized Users (AUs), Radiation Safety Officers and other professionals involved in the

    administration of radionuclide therapy in nuclear medicine.

There are three major elements involved in successfully meeting a performance standard of

    maintaining exposure of members of the public to released nuclear medicine patients to under 5

    mSv (500 mrem):

    ? The first is an evaluation of the patient’s living and working conditions to ascertain

    whether or not a given patient can be safely released.

    ? The second step is the appropriate performance of a patient-specific dose calculation to

    ensure that no individual member of the public will likely be exposed to a dose in excess of

    5 mSv (500 mrem).

    ? The third is to provide verbal and written instructions that are simple in order for the

    patient to limit the radiation dose to others to as low as reasonably achievable (ALARA).

    This requires patient education and an assessment by the AU physician that patient

    compliance with these instructions is highly likely.

PATIENT EDUCATION

Physicians will need to tailor advice to the patient's educational level, cultural background, and

    socioeconomic circumstances. If for any reason the patient cannot be appropriately educated, or

    there is a reasonable expectation of patient non-compliance, the AU physician should consider

    hospitalizing the patient.

     131I-NaI and to a lesser extent with Control of body fluid contamination is of some concern with 1318915332I-Bexxar, but is of minor concern with Sr-Cl(Metastron), Sm-EDTMP (Quadramet), P-2 3290chromic phosphate, P-sodium phosphate, and Y-Zevalin. External radiation exposure is an

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    131153I and Sm. Bremsstrahlung radiation important consideration for the photon emitters

    associated with pure beta-emitting radionuclides is not a significant radiation protection concern

    as a source of external radiation absorbed dose to others.

     131Patients who receive I and who will probably cook when they go home should be given several

    pairs of disposable gloves and taught how to wash and dry them, recalling that it is the inside,

    rather than the outside, which is of contamination concern. The usual caveats about kissing and

    sharing eating utensils and drinking vessels apply. The only new development here is the common

    installation of NaI radiation detectors in transfer stations and sanitary landfills which are able to 131153pick up very small quantities of I and Sm, often undetectable with conventional nuclear 131153medicine survey equipment. For this reason, patients should be told that whatever I and Sm

    cannot go out via the plumbing (toilet, sink, dishwasher, clothes washer) should be collected for a

    week and stored in a double plastic bag and returned to the physician administering the 1radiopharmaceutical. Alternatively, the materials may be collected for a week or so and then 2discarded. Concerns about the radiation risks of such discarded materials are not generally

    proportional to the actual level of hazard.

     131Issues concerning return to work are generally important only with I patients, but can affect 153Sm patients as well. Obviously, the type of work and contact with and proximity to others are

    major concerns here, and need to be dealt with on a case-by-case basis. It is courteous to take

    precautions if patients work in an environment in which they are monitored (e.g. nuclear power

    plant workers, drivers of garbage trucks, visitors to certain federal buildings, travelers through

    certain airports) or in which the trash is monitored (e.g., hospitals which attempt to thwart

    sanitary landfill radiation detectors).

Urinary incontinence is a problem with most therapy radiopharmaceuticals, and needs to be

    ascertained and dealt with. Some patients may be depended upon to appropriately manage

    catheters at home, but others may not.

CALCULATIONS

    The methodology for calculation of absorbed dose from an external source, such as a patient or a 3spill on the floor, was described in 1970 in Appendix I of NCRP 37. Other methods were 4suggested in the more recent Appendix U of NUREG-1556 Vol. 9 (the NRC guidance associated 56with the patient release rule pursuant to 10 CFR 35.75), NCRP Report No. 155, a Society of 7Nuclear Medicine (SNM) guidance document, and other references at the end of this document. The method of NCRP Report No. 37 uses a relatively simple approach, and gives reasonable

    estimates of dose, better than the simple release criterion based on retained activity that was used 8for many years, but whose origin is uncertain. In a 2007 article in Health Physics, we argued that licensees rely too heavily on the overly conservative approach in NUREG-1556, and should 9thoughtfully adopt more realistic methods in most cases. The recently issued NRC guidance

    addressing the post-radioiodine therapy release of patients who might come into contact with

    infants and/or young children was discussed in a Journal of Nuclear Medicine Newsline article in

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    10, which concluded that there are no data to support such guidance. 2008

The method can certainly accommodate increasing complexity. For example, current research

    with more realistic body phantoms provides dose factors that do not rely on treating patients as 11,12point sources, which is somewhat conservative in many cases. The specific gamma ray

    constant as used in most dose calculations assumes that the patient is an unshielded point source.

    Patients are of course not unshielded point sources, and if biodistribution and shielding can be

    measured or calculated, a more appropriately determined or modified gamma ray constant should

    be used.

When the external source is a fixed radioactive source, like a spill on the floor, the physical half-

    life of the involved radionuclide will obviously apply and should be used in any dose scenario.

    When the external source is a patient with a radiopharmaceutical, one should always combine the

    biological half-life with the physical half-life to get the effective half-life. The effective half-life

    should be used in the dose equation instead of the physical half-life when the radiation source is a

    patient.

     T?Tbiologicalphysical T?effective T?Tbiologicalphysical

THE USEFUL DOSE EQUATION

The equation that can be used in most cases to estimate dose to others from a released patient is:

    ???0.693?? ??34.6?0.25?A?T??1?e 1/2?? T1/2?? D(?)?

    2 (100cm)

     ?4 D(?)?8.66?10A?T??1/2

See Appendix I for the derivation of this expression. Appendix II shows a comparison of a dose

    calculation for a thyroid cancer patient using this expression and using the NUREG 1556 equation.

    Since the T or T is representative of the clearance from the radioactive patient’s total body, 1/2effthis version of the equation has one component with respect to half-time, e.g., if there is a

    monoexponential total body clearance of radiopharmaceutical. If total body clearance follows

    more complex kinetics, e.g., biexponential or triexponential clearance, then additional components

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    ) or (T) must be replaced by ? f (T) where f is the uptake are necessary. In this case (T1/2TBeffTBiiieffifraction of the i-th exponential component and (T) is the effective half-time of the i-th ieff

    exponential component.

One can use a model with fixed parameters to facilitate dose calculation, but of course actual

    measurement of such parameters for individual patients will further improve the accuracy of the

    dose estimate. For example, in the case of radioiodine treatment of thyroid cancer and

    hyperthyroidism, a three-component model with a set of assumed uptake fractions and effective 4half-times has been advanced in NUREG-1556, Vol 9, representing an eight-hour non-void

    period with an occupancy factor of 0.75, and non-thyroidal and thyroidal compartments. Thus,

    after the non-void period, the total body clearance is biexponential in this case. In Appendix II we

    show an example of this approach, but we also note that there are no scientific data to support 9this non-void component.

    The examples given in this document employ certain assumptions (e.g. in Example 5, “Assume that a person spends one fourth of all time at 1 meter from the subject (very conservative)”) that are needed to complete the calculations. As with model parameters (e.g. effective half-times), in

    the absence of any information, default values may be used, but it is always best to apply

    reasonable values for each individual case, based on information gleaned in discussions with each

    patient. Assigning an occupancy factor of 0.25 (one fourth of all time after patient release) at 1 m

    from the patient is probably conservative for most situations, and lower values may be applied if

    appropriate. Of course, one can also envision family situations in which this is quite reasonable, or

    in which even higher values of occupancy might reasonably be applied. Use good judgment in

    each case. When there is uncertainty, being conservative is always the preferred approach, but

    being unreasonably conservative may place burdens on patients and their families, as well as the

    treating institution.

Another component of dose considered by some is an internal dose component, from radioactivity

    released from the patient in saliva, sweat, or other body fluids and accidentally taken in by a

    person who may have also received an external radiation dose. In the 2007 HPJ paper, we

    evaluated this component and concluded that the internal dose component can be “considered to -6be negligible due to the use of an intake factor of 10and that for individuals exposed to either

    thyroid cancer or hyperthyroid patients the internal dose component does not have to be taken 9into account, as it will always be less than 10% of the external dose component..

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    SPECIFIC GAMMA RAY CONSTANTS

    FOR COMMON RADIONUCLIDES AS UNSHIELDED POINT SOURCES

     182 F 5.73 R-cm/mCi-h1312I 2.2 R-cm/mCi-h 99m2Tc 0.78 R-cm/mCi-h 672Ga 0.79 R-cm/mCi-h 2012Tl 4.7 R-cm/mCi-h 1372Cs 3.3 R-cm/mCi-h 602Co 13.2 R-cm/mCi-h 1112In 3.2 R-cm/mCi-h 1232I 1.6 R-cm/mCi-h 892Sr 0.00046 R-cm/mCi-h 1922Ir 4.8 R-cm/mCi-h 1532Sm 0.45 R-cm/mCi-h

     131In some cases, modified versions of these values may be used. For example, when I is dispersed in a patient, the value is on average approximately 0.6 times that of the unshielded specific gamma 2ray constant, (about 1.3 R-cm/mCi-h). This was measured in 157 cases with administration of 13113I-Bexxar. It should be noted that this factor was shown to be a function of patient mass. This 131factor will be applied to the extrathyroidal component of I-NaI subjects in the calculations

    below. For the thyroidal component, the activity is close to the surface of the body, and use of the

    unshielded point source value is appropriate.

    PHYSICAL HALF-LIVES FOR COMMON RADIONUCLIDES (days)

     18F 0.076 131I 8.04 99mTc 0.25 67Ga 3.26 201Tl 3.0 137Cs 11,000 60Co 1,920 111In 2.8 123I 0.55 89Sr 50.5 192Ir 73.8 153Sm 1.93

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EXAMPLE

     131I Here is an example dose calculation using for a thyroid cancer patient receiving a 100 mCi Na

    activity treatment prescription:

Occupancy Factor = 0.25

    F = 0.95 (extrathyroidal fraction) 1

    T = 0.32 d (effective half-time for extrathyroidal fraction) 1eff

    F = 0.05 (thyroid fraction) 2

    T = 7.3 d (thyroid effective half-time) 2eff

     ??hrem?cm2??34.62.2(100mCi) ??dmCi?h??D(?)??0.25{0.6?0.95?0.32d?0.05?7.3d}

    (100cm)2

    ??rem?cm??D(?)?8.66x10?100mCi?2.2?{0.6?0.95?0.32d?0.05?7.3d}2???4mCi?h??

    DDE(?)?0.104rem?104mrem

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    EXAMPLE PROBLEMS

     67Ga citrate for imaging infection. Three days later a surgeon, 1. A patient has been given 5 mCi

    wishing to operate, hesitates to do so fearing radiation exposure. The surgical procedure

    would take about 3 hrs. Estimate the radiation dose to the surgeon, and compare it with

    common radiation exposure sources.

     67The T of Ga is 78 hrs. Very little is excreted over the 3-day period, so use of the physical 1/2

    half-life is reasonable. Let us assume 2.5 mCi is still in the patient. Assume that the average

    distance between the patient and the surgeon is 2/3 meter (this is conservative; 1 meter might ??also be considered) ??3??0.693rem?cm24??????234.6(2.5mCi)(3.26d)0.791?ed?????3.26??mCi?h????d??

    ??D?

    (67cm)2

    D?0.0013rem?1.3mrem

    For comparison, background = 1 mrem/day on average in USA, about 1.6 mrem/day in

    Denver. In airplanes, one receives about 1 mrem/1000 mi flown. Natural background averages

    300 mrem/y average, + about 300 mrem/y currently from man-made radiation. So one could

    say that this exposure could have resulted from exposure to (1.3/300) x 365 ? 2 days of 14natural background radiation.

    2. An ultrasound technologist balks at performing a gallbladder procedure in a patient who has 99m99mjust had a Tc-disofenin ("HIDA") scan. The patient received 2 mCi Tc 3 hrs. ago. What

    is the approximate radiation dose to the technologist assuming that the ultrasound

    examination takes 30 minutes? Again, assume a distance of about 2/3 of a meter:

     3??0.693d ???24?? Activity remaining: 0.25dA?2mCi?e?1.4mCi

    7

    ??

    ??0.5??0.693rem?cm24??????234.6(1.4mCi)(0.25d)0.781?ed?????0.25??mCi?h????d??

    ??D?

    (67cm)2

    D?0.0001rem?0.1mrem

     123NaI for a thyroid study and soon vomits a small amount on the 3. A patient swallows 400 μCi

    floor. A calibrated ion chamber measures 0.2 mR/hr 50 cm above the vomitus. About how

    much activity did the patient vomit?

    1??0.693d?????24??2??0.55d??rem?cm??0??34.6(QmCi)(0.55)d1.661?e??mCi?h????????0.0002rem?2(50cm)

    ?Q0.33mCi?330?,Cisothepatientvomitedmostoftheadministeredactivity0

     99m4. A technologist inadvertently spills 1 mCi of Tc on the floor while injecting a patient with 25 99mmCi of Tc-MIBI. The technologist does not clean up the spill. What is the most radiation

    that could reasonably be absorbed by the most exposed person frequenting that area? Let’s assume an average distance of 1 m for this calculation and an exposure time of 4 hours: ??

    ?? 4??0.693rem?cm24??????234.6(1mCi)(0.25d)0.781?ed?????0.25??mCi?h????d??

    ??D?

    (100cm)2

    D?0.00025rem?0.25mrem

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    131I for a Graves' disease therapy. Her thyroid uptake is 55%, her 5. A patient requires 50 mCi

    thyroid biological half-life is 5 days, and her renal function is normal. If you treated her as an

    outpatient, what is the highest expected radiation dose to someone with whom she shares a

    household? Assume she sleeps alone and does not share eating utensils. Assume that a person

    spends one fourth of all time at 1 meter from the subject (very conservative). Activity not

    taken up by the thyroid is assumed to have a 0.25 d biological half-life, which works out to be

    a 0.24 d effective half-time.

     5d?8dT??3.1d 5d?8d effective

     ?? ??0.693??rem?cm2???? ??34.6(0.55?50mCi)(3.1d)2.21?e?0.253.1??d??mCi?h ????D? 2(100cm)thyroidal

     ?? 0.6932??????rem?cm0.24?? ??34.6(0.45?50mCi)(0.24d)1.31?e?0.25d????mCi?h ????D?2 non?thyroidal(100cm)

     D?D?D?0.162?0.006rem?0.17rem?170mremthyroidalnon?thyroidal

     2Note: We used the estimate of 1.3 R-cm/mCi-h for the non-thyroidal component, as

    described on page 4.

Note: Since we integrated to infinity, the (1-e) terms become 1.0, and could have been left out.

Note: The biological half-life of the non-thyroidal component in a person with normal renal

    function is about 8 hrs or 0.33 days, and the effective half-life is 0.32 days. In a hyperthyroid

    person with normal renal function it is about 6 hours or 0.25 days, and the effective half-life is

    0.24 days. In a hypothyroid patient with normal renal function it is about 12 hours or 0.5 days,

    and the effective half-life is 0.47 days.

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    131I for therapy. Assume that uptake in the 6. A thyroid cancer patient requires 200 mCi

    postoperative thyroid remnants is 1% and that the patient has normal renal function. If you treated him as an outpatient, what is the highest expected radiation dose to someone with whom he shares a household? The biological half-life of iodine in normal thyroid tissue is 80 days. If the patient is treated with Thyrogen (recombinant human TSH) assume a normal biological halflife of non-thyroidal iodine of 0.33 days. If the patient went naturally hypothyroid, assume a biological half-life of non-thyroidal iodine of 0.5 days. Here we assume rhTSH treatment.

    80d?8d T??7.3d 80d?8deffective

    ?? rem?cm2??34.6(0.01?200mCi)(7.3d)2.2?0.25?? mCi?h??D? 2(100cm)thyroidal

     2??rem?cm ??34.6(0.99?200mCi)(0.32d)1.3?0.25??mCi?h ??D?2 non?thyroidal(100cm)

     D?D?D?0.0278?0.0712rem?0.099rem?100mremthyroidalnon?thyroidal

    Note: we did not show the (1-e) terms this time, since they are equal to 1.0.

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