Dear Colleagues:
In my message re: radiation from nuclear reactors, I goofed in
converting 9 kBq to over a half million disintegrations per second - I
obviously meant per minute. I meant only to suggest that a few hundred or
thousand cpm on a GM counter is trivial.
LUDWIK KOWALSKI <KOWALSKIL@alpha.montclair.edu>
wrote: "As a medical physicist, John, you should be able to specify the
dose (in rems)
that a typical patient receives after 20 mCi of Am-99m is injected. This
question was raised on this list about a month ago but was never answered.
The question as phrased does not have a definite answer since the dose to
various parts of the body will vary depending on the distribution of the
radioactivity. I'll assume that the radioactivity is distributed uniformly
and that none of it is excreted. I will also assume that about half of the
140 keV gamma rays escape from the body and that the body has a mass of 70
kg.
I'm sure Ludwig meant Tc-99m rather than Am-99m. Technicium
(=3Dartificial) is the only one of the original "92" elements which is not
found in nature. Tc -99 has a half life of about a million years and long
since disappeared. Metastable Tc-99m has a half-life of 6 hours and decays
into Tc-99 after emiting a 140 keV gamma ray. It is the "work-horse" of
nuclear medicine. It is obtained as the progeny (formerly "daughter") of
molybdenum-99 which has a 2.5 day half-life. Nuclear medicine labs buy
Mo-99 generators every week and elute the Tc-99m using sterile saline
solution.
I left the nuclear medicine field about 1965 before Tc-99m became
available. It was a wonderful discovery. Before that time Iodine-131 with
a half-life of about 8 days was popular. However I-131 emits a beta and a
few gammas. The beta is absorbed locally and contributes most of the dose.
=46or example, a thyroid uptake measurement with I-131 typically used 10
=B5Ci. The dose to the typical thyroid was about 1 cGy (=3D1 rad =3D 1 rem) =
for
each microcurie. (I measured my own thyroid 13 times but I used old I-131
capsules with an activity of 0.1 =B5Ci - only 1% of the usual dose. I had
to count my thyroid for 10 minutes.) For a scan of the thyroid the typical
dose was 100 =B5Ci and the dose to the thyroid was about 1 Gy or 1 Sv. This
is a significant dose. There is no evidence that these I-131 studies
increased the incidence of thyroid cancer. The dose to treat
hyperthyroidism (Graves disease) is typically 5 - 10 mCi. To treat thyroid
cancer the dose is about 100 mCi.
A refresher on radiation quantities before I answer the question. In
1928 an international committee (now known as the Int. Comm. for Radiation
Units - ICRU) defined the Roentgen (r) in terms of 1 esu of charge of
either sign produced in 0.001293 g of dry air (this is the mass of 1 cc of
air at STP). They did not give the quantity a name. Many years later the
quantity was named the "exposure dose" and later "exposure" The rad unit
for absorbed dose was defined (in terms of specific energy) as 100 ergs/g
of any material around 1950. The temperature rise from a whole body lethal
dose (500 rads or 5 Gy) of radiation cannot be detected. In a 70 kg person
that is only 350J or less than 100 calories or 1/10 food calorie.
Calorimetric studies in the laboratory can measure this quantity.
About WW II when it became obvious that some types of radiation
(e.g., alpha particles and fast neutrons) produced more biological damage
per rad than x-rays the Int'l. Comm. for Radiological Protection (ICRP)
defined a special quantity for radiation protection called the "dose
equivalent" with the unit "rem". You obtain the dose equivalent by
multiplying the absorbed dose by the "quality factor", Q where Q =3D1 for
x-rays, gamma rays and beta rays. The Q for fast neutrons was assigned a
value of 10 and the Q for alphas was assigned a value of 20.
In 1977 the ICRP/ICRU switched to SI units and defined the absorbed
dose unit gray (Gy) as 1 J/kg (=3D 100 rads) thus you often see the unit cGy
(which conveniently equals 1 rad). The dose equivalent unit was sievert
(Sv) resulting from the product of the absorbed dose and Q.
If you stop to think about units of energy/mass - it is not a
logical way to measure radiation risk. It is a convenient way to specify
the dose to cancer tissue. A typical radiation therapy treatment involves
about 60 Gy to the cancer over a six week period.
In 1977 the ICRP also defined a radiation protection quantity,
"dose equivalent effective" (now called "effective dose") to estimate the
risk from localized radiation. The effective dose is the amount of
radiation to the whole body that has the same risk of inducing a fatal
cancer as the localized dose. To calculate the "effective dose" the dose to
each organ is multiplied by the Q factor and then by the "organ weighting
factor", WT. (The sum of the WT for all the organs =3D 1.0). This is not a
trivial calculation. Monte Carlo techniques are used to estimate the organ
doses.
An example: Calculate the annual effective dose from radon progeny
in the lungs: The absorbed dose is about 1 mGy/y to about 1 kg of lung
tissue. The Q of alphas is 20 therefore the equivalent dose is 20 mSv/y. To
get the effective dose this is multiplied by the WT for lungs which is
assumed to be 0.12. The effective dose is thus 2.4 mSv/y. All other
background radiation is about 1 mSv/y. Thus radon is assumed to be the
major contributor to our background.
Radiation protection quantities were redefined slightly in 1991 by
the ICRP. (Note that neither the equivalent dose or effective dose can be
measured.)
Prof. Harald Rossi, a distinguished radiation scientist and a
member of the NCRP and ICRU, published an article in 1996 Health Physics
in which he stated that radiation protection quantities are in a state of
chaos. Part of the reason is that the biological constants Q (now called
WR) and WT cannot be accurately determined. In 1990 the US National
Council for Radiation Protection and Measurement (NCRP) published the
results of a study by their Scientific Committee # 46 as NCRP Report No.
104. This report was to determine more accurate values of Q (now WR) for
radiation protection purposes. It concluded that Q cannot be determined.
Nevertheless, the NCRP published NCRP Report No. 116 (1993)
continuing to use WR (formerly Q) and not even mentioning that Report No.
104 said it couldn't be determined. There is little doubt that WT also
cannot be determined but they have carefully avoided doing an evaluation of
its scientific validity. Biology is not simple!
A logical approach is to use the imparted energy as the basic
quantity for radiation protection. This was proposed about 1960 but has not
been considered by either the ICRP or NCRP. At the present time we must
continue to use these unsatisfactory units as they are built into
regulations.
The average equivalent dose to 70 kg person given 20 mCi of Tc-99m
can be estimated by taking the mean life (=3D 1.44 x half-life) =3D 31,104 s=
=2E
times the initial activity (=3D 7.4 x 10^8 s^-1) x the average energy
deposited (assumed to be about 7 x 10^4 eV) times the conversion factor
from eV to J (I don't remember it) divided by 70 kg to get J/kg or Gy.
Since the Q or WR for gamma rays is 1.0 and the radiation is whole body,
the equvalent dose snf effective dose are numerically equal to the Gy. To
get rem, divide Gy by 100.
Since there is good evidence to suggest that short term doses less
than 200 mSv are hormetic (i.e. beneficial) we should not attribute any
risk to this relatively small dose.
Sorry to use so much space but the above information about
radiation protection quantities is not readily available. Radiation is not
a serious carcinogen. A study of radiation induced cancer in A-bomb
survivors showed that a cumulative imparted energy to those with over 500
mGy (50 rem) resulted in one fatal radiation induced cancer for about 3.5
kJ or ten times more than the acute (i.e. short term) dose to be fatal!
Best wishes for 1998! John
John R. Cameron, we are at our winter home at 2678 SW 14th Dr.,
Gainesville, FL 32608 phones : 352/371-9865; Fax 352/371-9866 until about
May 15, 1998 We then revert back to our summer home:
2571 Porter Rd., P.O. Box 405, Lone Rock, WI 53556-0405 Phones:
608/583-2160; Fax: 608/583-2269