Radiation Safety After a Permanent Implant
Frequently Asked Questions about Permanent Radioactive
Seed Implants for Prostate Cancer
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Prostate Cancer Issues
Logistics of a Permanent Implant
Side Effects of a Permanent Implant
Potency after a Permanent Implant
Radiation Safety after Permanent Implant
Pros and Cons of Implants
Follow-up after a Permanent Implant
How long will each kind of seed be radioactive after implantation?
How long will the patient be radioactive?
The radioactivity in any quantity of radioactive material continually
undergoes radioactive "decay", resulting ultimately in radioactive
atoms becoming non-radioactive ones. The rate at which this decay is usually
expressed by radioactive "half-life" of the material. A half-life
is the interval of time it takes the amount of radioactivity in a sample
of material at a given point in time to reach half that amount. While half-life
varies from isotope to isotope, the half-life for each isotope is unique
to that isotope and unchanging. For example, I-125 decays with a half-life
of 60.5 days and Pd-103 with half-life of 17 days. If we start with a certain
amount of material, in one half-life afterwards there will be half as much
activity; after 2 half-lives, there will be one quarter remaining; 3 half-lives,
one eighth; and so on. Most implants done with I-125 will utilize about
45 mCi, while most with Pd-103 will start with about 90 mCi (see below for
explanation). Thus, for I-125 about 22.5 mCi will remain 2 months after
the implant was done; about 11 mCi after 4 months; 5.5 mCi after 6 months,
and so on. Pd-103, with a 17 day half-life, will decay away even faster
(one half remaining after 17 days, one quarter after 34 days, one eighth
after 51 days, and so on). Please note that the radioactivity is continuously
decaying - with every passing moment there is a little less than there was
the moment before. Note also that the radiation levels around the patient
will also proportionally decrease as the amount of radioactivity decreases.
Since the radioactivity from I-125 will irradiate the treatment area
for a longer total period of time than from Pd-103, initial implant activity
used for Pd-103 is higher than for I-125 in order to deliver the same amount
of radiation to the treatment area over the months following the implant.
The reduction due for the decay of I-125 and Pd-103 is illustrated graphically
in the following figures.
As you can see, the bulk of the implant activity is gone within the first
few half-lives - 90% is gone in a little over three half-lives, nearly 99%
is gone after six half-lives. We can calculate, however, that ever decreasing
amounts of implanted radioactivity will still remain in the body for some
time. Remember, the radiation levels around the patient immediately after
the implant are already low, so from a practical point of view the patient
will cease to be even a small "radiation hazard" to those around
him within the first few half-lives.
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Percentage of Radioactivity Remaining After Time
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What about sleeping with someone? What about having sex? Do these
present any special hazards?
In general, patients can maintain their usual sleeping arrangements,
and there is also no significant radiation safety reason to refrain from
sexual activity. As discussed earlier, for reasons of anatomy, the patient's
body provides a great deal of shielding in nearly all directions from the
prostate's location. The highest (but still low) radiation levels will be
directly below the perineum (directly underneath the patient and between
his legs), but it is unlikely that another person would be in this location
and in close proximity to the perineum for any significant length of time.
Most likely during normal sleeping positions, the person sleeping with the
patient will be shielded by the bones and/or soft tissue of the patient's
hips, legs, spine, buttocks, and lower abdomen or back. Even during sexual
intercourse, the pubic bone will provide adequate shielding. Taking shielding
into account along with time and distance considerations, sleeping with
someone and sexual activity should not pose any realistic risk to the patient's
partner.
What about risks to children and pregnant women? Are there any special
precautions which should be taken about them?
The risks to unborn children or small children is exceedingly small.
Still, the developing fetus and young children are known to be more sensitive
to possible radiation effects. This is so because they are growing and therefore
the cells of their bodies are dividing more rapidly than adults. In general,
the more quickly cells divide, the more sensitive to radiation they are
(this same fact is what makes the cancer cells being treated more sensitive
to radiation.) Pregnant women and parents of young children are also likely
to be apprehensive about proximity to any amount of radiation, no matter
how small. Consequently, we usually recommend additional precautions to
reduce already low risks even further. Radiation exposures can be reduced
or eliminated by maintaining distances from the patient and spending less
time at close distances, and/or by using shielding. In general, maintaining
an average distance of more than 2 or 3 feet between patient and child or
pregnant woman is more than enough precaution. Note that brief encounters,
for example, to kiss or hug in greeting, are acceptable. Some patients may
have a child or grandchild who enjoys sitting in the patient's lap. If this
occurs frequently, a thin, flexible vinyl coated lead shield can be used
for the child to sit on. Such a shield would for all practical purposes
absorb all the radiation. Patients should discuss their own situation with
the Radiation Oncology physician for specific advice.
Can I take extra precautions to reduce the radiation exposures of
others around me?
Yes, while potential exposures are well within levels considered safe
for the general population, radiation protection practice commonly employs
a philosophy of taking reasonable steps to reduce already low potential
risks even further, whenever practical. This concept was employed in the
above recommendations about small children and pregnant women, for example.
Similarly, if the measures are simple to take and do not unduly interfere
with a person's lifestyle and quality of life, a patient may wish to employ
steps such as maintaining some additional average distance from those around
him, if this is not too burdensome.
How long should precautions be employed?
This is very much up to the individual patient and his family. An y precautions
related to reducing exposures to those around the patient should be continued
for as long as maintaining the precautions is not burdensome, but no longer
than 2 to 3 half-lives after the implant procedure (120-180 days for I-125,
34-51 days for Pd-103). Beyond this time special precautions are probably
not worth the effort. Remember, even if no special precautions are taken,
radiation risks to people around the patient are still very low. In any
case where a surgical or other invasive medical procedure is to be performed
on or near the prostate, some precautions or arrangements for special handling
procedures may be necessary for somewhat longer. While the amount of time
depends on the originally implanted activity and the isotope used, in nearly
all cases involving Pd-103 any concerns after 6 months would be unwarranted,
while in nearly all I-125 cases, 2 years would be sufficient.
How dangerous are these levels of radiation?
The basic answer is, "not very dangerous at all". We've already
discussed the basic levels of radiation around the patient, in terms of
the radiation dose rate (in units of millirems per hour) and how long it
would take to get exposures similar to what one gets from a chest x-ray.
But what is the nature of the biological hazard, what is the level of risk,
and just why are we concerned about radiation exposures at all? The truth
is that for low doses (like 20 millirem) given over a period of hours or
days, we don't know the exact level of risk, but we do know that the risk
must be very small. Our chief concern is the induction of cancer occurring
many years after exposure.
Why can't we pin the answer down precisely?
Well, to make a long story short and not to get into all the statistical
details, we would need to take a very large population of human beings (in
excess of tens of millions of people) and deliberately expose them to a
known small amount of radiation. Then we would need to follow them for 20
to 30 years, along with an equally large control population of unexposed
people, all the while trying to adjust for the variation in natural background,
occupational, or other medical radiation exposures to both groups and for
variations in exposure to all other potential cancer causing agents as well.
This sort of study is, in reality, impossible to perform (and would cost
more than any nation or group of nations could afford even if it were technically
possible).
So, what do we do?
Essentially, we devise a "model" to allow us to predict or
extrapolate from the data from highly exposed populations to those at low
exposures. One of three basic things are likely to occur as we go from high
to low doses. These possibilities are: 1) the cancer risk per millirem per
person will be the same at low doses as it is at high doses, and there will
be some small proportional risk associated with any level of exposure, 2)
the risk per millirem per person will decrease gradually as the dose goes
down, but there will still be some small additional risk with any exposure,
or 3) risk will decrease gradually as dose decreases, and there will be
a dose level below which there is no additional risk (that is, there is
a threshold below which there is no hazard). There is actually a great deal
of scientific evidence that there is a practical threshold below which no
radiation effects would ever be expected to be seen. We are all familiar
with many examples of similar thresholds - take one aspirin a day and you
may reduce your involvement with coronary disease, but take a whole bottle
at once, and you'll increase your chance of involvement with a coroner instead.
We know that the body has mechanisms for repairing radiation damage, so
there is good reason to suspect that there are levels of radiation which
represent no risk whatsoever.
Can you describe the radiation levels in another way?
Let's look at radiation exposure another way. Everyone is continually
irradiated from naturally occurring radioactive materials in the soils and
rocks around us, and in our own bodies, and from cosmic rays. The amount
from natural radioisotopes within our bodies gives us each about 20 millirem
per year. The amounts from cosmic rays and sources in the earth vary with
altitude and the type of rocks and soils in an area. In southeastern Pennsylvania,
Delaware, and New Jersey for example, we each get about 40 millirem per
year from cosmic rays (sea level) and 60 millirem per year from rocks and
soils. Residents of Colorado, by comparison, get an average of 120 millirem
per year from cosmic rays and 100 millirem from the rocks and soils of that
region. That means a resident of Denver gets about 10 millirem more than
a Philadelphia resident every month! And no one has identified higher rates
of cancer or other illness among residents of Colorado than, for example,
New Jersey. The lucky Philadelphian who flies to Hawaii for vacation will
pick up about 10 extra millirem on the flight there (and another 10 on the
way back). So the radiation doses anyone could get from a prostate patient
are well within the range of variation that people are exposed to naturally,
a level at which no ill effects have been observed.
For what period of time is it possible to pass seeds through urination
or sex?
If seeds are going to be passed, it will happen in the majority of circumstances
during the first few urinations or climaxes. It is extremely rare for a
seed to be lost in this way beyond this time. You need not worry about "lost
seeds". After completion of the implant a urologist will perform a
cystoscopy and recover any seeds that were inadvertently injected into the
bladder.
Who do I call for questions or requests for more information about
these issues?
Calls can be directed to Jefferson's Office of Radiation Safety (215-955-7813).
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