The revised 10 CFR 35.75 allows the licensee to release from
its control any individual who has been administered radiopharmaceuticals
or permanent implants containing radioactive material if the
total effective dose equivalent (TEDE) to any other individual
from exposure to the released patient is not likely to exceed
0.5 rem (500 mrem). Licensees may demonstrate compliance with
this dose limit by either: 1) using a default table provided
in Regulatory Guide 8.39 for activity (e.g., <33 mCi for I-131)
or dose rate at 1 meter (e.g., <7 mrem/h for I-131) or, 2)
performing a patient-specific dose calculation (NRC, 1997b).
The licensee must:
1. Provide written instructions to the released patient
on actions recommended to maintain doses to other individuals
as low as reasonably achievable (ALARA) if likely to exceed 0.1
rem (100 mrem). These instructions should include guidance on
interruption or discontinuation of breast-feeding and information
on the consequences of failure to follow this guidance if the
dose to a breast-feeding infant or child could exceed 0.1 rem.
Written instructions are required for I-131 patients at time
of release if 1) the administered activity is greater than 7
mCi, 2) the dose rate is greater than 2 mrem/h, or 3) a patient-specific
dose calculation is performed.
2. Maintain records (for 3 years after the date of
release) documenting the basis for patient release, if release
was determined through dose rate or patient-specific dose calculation.
If patient release is based on administered activity and this
activity is below the value given in the default table in Regulatory
Guide 8.39 (e.g., <33 mCi for I-131), there is no recordkeeping
requirement.
3. Maintain records (for 3 years after the date of
release) documenting that instruction was provided to breast-feeding
women if radiation dose to infant or child from continued breast-feeding
could result in a TEDE exceeding 0.5 rem.
Note: To optimize radionuclide therapy, regulations must be
based on sound dosimetric and radiobiologic principles. The dose
received by an individual from a patient administered I-131 therapy,
for example, will be governed by iodine distribution, the rate
of iodine clearance and the time spent in close proximity with
the person. Guidelines to limit radiation exposure should ideally
make use, therefore, of iodine retention and instantaneous dose
rate measurements in combination with data on social contact
times. It is clear that measurements of administered or retained
activity (the old 30 mCi or 5 mrem/h at one meter release criteria),
without reference to patient pharmacokinetics and behavior patterns,
are insufficient to formulate adequate advice on restrictions
to limit the dose to members of the public (as we will see later).
The new NRC regulations are dose-based rather than activity-based;
patients can now be released regardless of how much administered
activity they received, as long as the total dose to an individual
from exposure to the released patient is less than 500 mrem.
Licensees can demonstrate compliance by either (1) using default
values for activity or dose rate, or (2) performing a patient-specific
dose calculation. The latter method takes into account the patient-specific
pharmacokinetics and should be the preferred method for patient
release.
Before we go on, a review of dose rate measurements is in
order. The dose rate is measured with a gas-filled detector.
Gas-filled detectors (ionization chamber, proportional counter,
GM counter) quantitate the amount of radiation in terms of ionizations
produced in air. Ionizations occur when the incident radiation
transfers enough energy to "knock free" an electron
from an atom or molecule. The electron has a negative charge,
and the atom or molecule that is now missing an electron is left
with a positive charge. The electron and atom/molecule together
are referred to as an ion pair. These ionization events are detected
by applying a voltage across two electrodes (one positively charged
and the other negatively charged) which attract the ion pairs
formed (electrons and the positively-charged air atoms). The
electrons are collected by the positive electrode and the positive
atoms are collected by the negative electrode. The basic concept
of this is depicted in the drawing below:
Depending upon the voltage applied, the number of ion pairs
that are collected by the electrodes will change dramatically.
The higher the applied voltage the more ion pairs will be collected
for the same amount of incident radiation as shown in the Figure
below:
As can be seen, there are four main regions to the above curve:
1. Recombination - voltage is low and most of the ion pairs recombine
and do not get collected by the electrodes. This is because the
ion pairs are created close together and their natural attraction
to each other (unlike charges attract) is greater than their
attraction to the electrodes at this low voltage. As voltage
is increased more ion pairs are collected.
2. Ionization chamber (saturation) region - in this region
there is a plateau, i.e., no change in the number of ion pairs
collected with increasing voltage. In this region, all the ion
pairs created by the incoming radiation are collected. Calibrated
ion chambers are energy independent and will give true dose rate
readings regardless of the radionuclide being measured.
3. Proportional region - as voltage is increased the number
of ion pairs collected will be increased by a factor of one thousand
to one million through the phenomenon of gas amplification known
as the Townsend effect. The voltage is now high enough to cause
initially created electrons to cause ionizations of their own.
4. Geiger - Muller (GM) region - the number of ion pairs collected
is further increased to a factor of 1E+08 to 1E+10. A quench
gas is added to stop ionizations from occurring continually.
GM counters are energy dependent and will only give true dose
rate reading if the radiation to which they are exposed is the
same as that used for calibration. If the voltage is increased
beyond the GM region (referred to as the continuous discharge
region), atoms of the counter will be spontaneously ionized.
Care must be taken to operate below this voltage or permanent
damage to the detector can result.
Because of the above, calibrated ionization chambers should
be used to make all dose rate measurements from a radionuclide
therapy patient.
The measurement distance, r, at which a survey meter is used
is also very important. The measured rate from a source will
drop off roughly as a function of the measurement distance squared
assuming that attenuation of the radiation by the medium is small.
This only holds exactly for a point source, but the general principle
of dose being reduced at a distance applies to the patient exposing
others as well. In the case of measuring the radiation emitted
by the patient, the medium through which the radiation travels
after leaving the patient is air. The attenuation of gamma radiation
emitted by the patient in air is very small. Thus there will
be some relationship between the measurement value and measurement
distance for patients, even though it will not exactly follow
the distance squared formulation.
This general relationship of the effect of distance on measure
dose can be better understood by considering a point source.
Assuming a given number of emissions leaving the source per unit
time, the number of emissions passing through a given area at
any distance will be proportional to the measurement reading
at that distance. Since the surface area of a sphere is proportional
to the radius squared, as the measurement distance increases
by r, the area of the sphere increases by r squared. Thus the
number of emission per unit area is reduced as a function of
the measurement distance squared. The drawing below illustrates
how the radiation "field" is "denser" closer
to the target, and how a detector of the same size will detect
more radiation the closer it is to the radioactive source. (It
appears that the second detector is shorter than the first, but
this is merely optical illusion, measure them and see for yourself
if you want.)
For example, if a reading of the same patient is taken at
2 meters instead of at 1 meter, the reading will be only one
forth as high. Even a relatively small measurement distance can
result in a relatively large reading error. The table below shows
the error as a function of measurement distance, assuming the
desired value is read at one meter.
|
Measurement Distance (cm) |
Error Compared to
1 Meter Measurement |
|
80 |
56% |
|
90 |
23% |
|
95 |
11% |
|
105 |
-9% |
|
110 |
-17% |
|
120 |
-31% |
|
