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For questions about Radiation Safety and the Clinical Use of Radioactive
Materials contact the Hospital Health Physicist at 206.543.3190, e-mail:
radsaf@u.washington.edu,
or the Radiation
Safety Office at 206.543.0463.
Most nuclear medicine studies are done with Technetium 99m, which has a
6-hour half-life and emits fairly low energy gamma rays. Typical patient
doses are in the 1 - 30 millicurie (mCi) range. In the unlikely event that
a patient is injected for a bone scan and blood is immediately drawn, a 10 cc
blood tube will contain a little less than 50 microcuries (uCi) and while the
radioactivity would be very easily detected, a phlebotomist holding the tube
continuously until it decayed away would receive a hand dose that would be
less than 1% of the annual dose limit. A rather more likely scenario is a
blood draw 3 hours after injection. The activity in the blood tube would
be about 24 uCi, the dose to the hands of someone handling the tube for 10
minutes would be about 5 millirem (the annual dose limit for hands is
50,000 millirem), and the dose to his or her body would be about 0.02
millirem per hour at 1 foot, or 0.002 millirem per hour at 1 meter (the
annual dose limit for workers is 5000 millirem, 100 millirem for babies
and other members of the public). The annual ingestion limit (the quantity
the would have to be ingested to give an internal dose equivalent to the
annual external dose limit) for Tc-99m is 80 mCi. The total activity in
the blood tube is much less than 0.1% of the intake limit.
Other radioactive materials are used, but the patient doses are smaller and
the worker radiation doses are smaller as well.
In many cases nuclear medicine procedures are done to localize tumors, lymph
nodes or infection. These patients are likely to have subsequent biopsies or
surgical procedures, which can result in radioactive samples going to pathology
for examination. The most familiar of these procedures is lymphoscintigraphy
followed by sentinel node biopsy. Approximately 0.5 - 1 mCi of Tc-99m colloid
is injected around the tumor site to trace the drainage to lymph nodes.
Radiation exposure rates near the injection site immediately after injection
are about 50- 100 millirem/hr at an inch (2.5 cm), 4 - 8 millirem/hr at 4
inches (10 cm), and background levels at two meters. The fraction of the
administered dose that reaches the sentinel node is only about 1%, or 10
microcuries. The dose rate in near contact with the sample is less than
10 millirem per hour and at 10 cm is less than 0.1 millirem per hour.
The dose to a pathologist's eyes would not be able to reach 10% of the
annual eye dose limit (15,000 millirem) in one year if continuous exposure
to samples fresh from surgery were viewed at 10 cm from the eye. Viewing
through a microscope will provide significant shielding from any radioactive
materials in these tissues. After 24 hours, the sample will have decayed
to 1/16 of its initial activity. (1 mCi would decay to 0.016 mCi for Tc-99m)
Patients are not considered to be radiation hazards, and they are not
restricted or tracked. The quantity of radioactive materials that may be
present in samples is below the level for which labeling ("Caution -
Radioactive Material") is required. Because the potential for hand and body
doses is much less than 10% of the annual dose limit, which is the criteria for
requiring the use of dosimeters, the wearing of whole body dosimeters or finger
dosimeters is not necessary. Contamination control measures taken for body
substances are more than adequate for these levels of radioactivity.
Infectious waste disposal contractors routinely monitor for radioactivity
in waste. They are unable to identify permitted disposals from unauthorized
radioactive waste disposal. Tissue from sentinel node procedures should not
be shipped for disposal for at least two days after the procedure.
Radioactive "seeds" are sometimes implanted permanently in patients with
prostate cancer or non-resectable tumors. The radiation dose to the treatment
site is delivered slowly over a period of weeks, so the radiation levels near
the patient are very low. Permanent implant seeds are made from short-lived
radioactive materials, which decay to background radiation levels in a period
of months. The radioactive materials used are low energy gamma emitters, so
the radiation is not very penetrating. The seeds are silver in color and are
approximately 0.5 mm in diameter and 5 mm long. Seeds are visible on x-rays.
Seeds, which are still radioactive, are easily detected with a sensitive geiger
counter.
The isotopes used for permanent implant seeds emit very low energy gamma
radiation, comparable to mammography x-rays, which are easily absorbed in the
patient. Because the implant is permanent, the radiation dose near the patient
must be low enough to allow the patient to be discharged without presenting a
risk to other people.
Bare seeds may be a hazard. Close contact with the seeds themselves may
result in fairly high local radiation doses to the skin, so precautions are
taken to avoid handling seeds directly. Precautions are also taken to avoid
losing seeds. Because they are so small, seeds can be difficult to see, but
they are very easily detected with a sensitive geiger counter. Seeds are not
a radioactive contamination hazard. The radioactive material in the seed is
encapsulated in titanium, and will not leak out or make the patient's body
fluids radioactive unless the capsule is cut or heated to a very high
temperature. Old seeds are not a hazard. The isotopes used for seeds have
short half-lives and decay to background radiation levels in a period of months.
Iodine-125 seeds are essentially non-radioactive after 20 months. Palladium-103
seeds are at background radiation levels in less than six months. Active seeds
should not be disposed of as infectious or biohazard waste. The seed
encapsulation will not survive incineration, and the release of the radioactive
material in this way violates environmental regulations. Commercial infectious
waste disposal facilities check all incoming containers with sensitive radiation
detection equipment and will reject any packages, which appear to be
radioactive. All red bag waste from implant procedures must be checked with
a sensitive geiger counter. Suspicious seeds recovered from patients should
also be checked before they are disposed of.
Some patients who have implants may subsequently require surgery, or may
die and be autopsied. Tissue samples from these procedures may contain seeds.
- The seeds are silver in color and are approximately 0.5 mm in diameter and
5 mm long.
- Close contact with seeds that are still active may be a radiation hazard.
Seeds should not be directly handled.
- The time since the implant may have been long enough so that for many
patients, the seeds are no longer radioactive (20 months for Iodine-125,
170 days for Palladium-103). Samples from these patients require no precautions.
- Prolonged exposure to unshielded seeds is safe at distances of two meters.
- Active seeds may be easily identified with a geiger counter. Call Radiation
Therapy or Nuclear Medicine for assistance if seeds are found or suspected.
They will dispose of the seed.
Pathologists and other lab staff should be aware that any unmarked specimen
material from in- and outpatients may be radioactive. Radioactive materials
may include blood, urine, or tissue specimens from diagnostic nuclear medicine
patients or nuclear medicine patients who have been treated and released and
may include tissue containing radioactive permanent implants ("seeds").
More than 10 million nuclear medicine diagnostic procedures are done each year
in the United States. A much smaller number of radiopharmaceutical therapy
procedures are done on an outpatient basis, and a yet smaller number of
permanent implants are done, primarily to treat prostate cancer. The radiation
dose near patients is too low for them to be public health hazards, and while
their tissues and body fluids may be radioactive, the contamination that results
has also been determined not to be a public health risk.
A variety of diseases are treated with radioactive materials. These
patients are similar to diagnostic nuclear medicine patients although
the agents used are longer lived and more radiotoxic. Special instructions
are provided to each patient and routine blood work is not ordered for a
period after the radiopharmaceutical is administered. Patients being
treated for some cancers with radioactive drugs may be confined to the
hospital for radiation safety reasons, although many states allow
these patients also to be released. If an inpatient that is being treated
at these levels requires surgery or lab tests the attending physician or the
nuclear medicine department will inform the lab or the pathologist and take
any steps that are necessary to control the radiation hazard. Very rarely,
patients who have been released may be treated in the ER or admitted during
the period that they are still radioactive. It may not be possible to
identify the patient ahead of time, but since the patient was released under
strict federal guidelines, the radiation hazard represented by blood or
tissue is minimal. For example, a Grave's disease patient treated with 30
mCi of Iodine 131, an outpatient procedure, will have an initial blood
concentration of about 4.4 uCi/ml and the hand dose from handling a 10 cc
blood tube for 10 minutes could be almost 15 millirem (0.03% of the limit).
The clearance of I-131 from blood is so rapid that 24 hours after the dose
is administered the concentration is less than 1 uCi/ml and the sample
is comparable to a sample from a diagnostic patient.
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