Re: prostate implant source strength and rad. safety

[Ccja@aol.com]

The detectable exposure rate is quite instrument-dependent, though it's true
that, even using, e.g., a Victoreen 450P, in which the ion chamber is
pressurized to six atmospheres, and the resolution on the lowest range is 1
uR/h, it can be difficult to obtain a good reading at one meter. (Note that,
if I were to use that instrument, I would apply a multiplicative
energy-correction factor of 1.25 to measurements of iodine implants and one
of 2.0 to those of palladium implants.) For that reason, I normally make the
measurement at 50cm (sometimes, even 30cm), and normalize to 100cm by the
inverse-square. Usually, I use a Victoreen 450, which has a resolution, on
the lowest range, of 0.01mR/h, remove the 200mg/sqcm buildup cap, and do not
worry about energy-dependency. A typical exposure rate at one meter from
these patients is 0.05 mR/h. This is true for both iodine and Pd implants.
(As an aside, I believe that the effective E of iodine seeds is 28keV; that
of Pd seeds is probably around 20. One would think that there'd be a good bit
of Compton garbage, in the radiation emitted by the patient.)

As implied by Tyler's posting, this measurement is highly position-dependent.
Put another way, the effect is quite beam-like, and it's important to be
centered directly over the prostate, in an AP view. Postoperatively, when the
patient is still on the gurney, which is where he'll spend the majority of
the time, the only time that the RN's receive any readily measurable dose is
when they're immediately adjacent to the implant. Interestingly, I've found
that the exposure rate there, at a distance of, say, 15cm ("gurney-side"), is
about the same as that at a meter. Are others seeing this?

About the only real "worrying" we do about exposures to other people,
subsequent to discharge, and aside from the question of how to handle seeds
passed during urination, or sexually, is to ask our patients to deny their
small relatives, or friends, lapsitting privileges, for at least a half-life
(for Pd, to keep it simple, we say 3 weeks).

I imagine that the most sensitive measurement of the exposure rates around
these patients would be made with an end-window, or pancake, GM probe, or a
scintillometer, that had been calibrated to a standard seed.

I suggest that anyone who's interested in these issues obtain copies of
NUREG-1492, Regulatory Analysis on Criteria for the Release of Patients
Administered Radioactive Material, and NRC Reg Guide 8.39, Release of
Patients Administered Radioactive Materials. Also, consider consulting your
RSO for help. As I observed in an earlier posting, the question of what sort
of instructions to give patients, so as to maintain ALARA the doses received
by other people (whether occupationally, or not), is normally handled
("negotiated") at the time of licensure. Such instructions, and explanatory
remarks, in the form of handouts, to place in the patient's chart, for the
nursing staff, or to give to the patient and family, are usually a part of
the materials reviewed, and approved, by the licensing agency. Therefore,
that, or the equivalent, is what you should be giving them. These issues are
really problems for the RSO, not the radiological physicist (unless, of
course, they are identical people).

chris alston
ccja@aol.com