Wikipedia says the unit Gy is J kg-1.Does this mean a mouse exposed to 1 Gy receives the same amount of
radiation per mass as an adult human who weighs 200 pounds exposed to the 1 Gy?
He might be exposed to greater joules, but his larger mass counteracts this, giving
him the same absorbed dose as a mouse?
I noticed that some new CT (computerized
tomography) scanners have a low-dose pediatric setting that has a CTDIvol
~ 4mGy for an abdominal/pelvic scan.
The dose from an adult scan of the
same region is ~ 25mGy.
I’m using CTDIvol because
in all the reports by the International Commission on Radiological Protection it
is stated that it is a close approximation of “absorbed dose.”
Understandably, a larger patient would
require more joules of energy to visualize his greater mass on a CT. But his
larger mass in kg should balance out so that he could still receive a safer
“pediatric” dose and still produce good images.
My question is, if they can produce
good images in children with only ~4mGy why can’t they do the same for adults
and reduce their radiation exposure?
I know you physicists like math, so to
I can put the same question in a mathematical perspective:
A child who weighs 35 kg, exposed to
4mGy CT receives 140 joules to her body.
An adult who weighs 77 kg, exposed to
a CT with CTDIvol of 25 mGy, would receive 1,925 joules.
That is 13.75 times as much energy
deposited in his body, even when we account for his much larger size.
So why do they expose adults to so
much more radiation when it is possible to use less and still obtain quality
images, as they do with a “pediatric” setting?”
Many of your inferences about the
definition of absorbed dose, often expressed in units of Gy or mGy are correct.
Two tissues or bodies, each receiving an absorbed dose, averaged throughout the
tissues or bodies, of 1 Gy, do receive an average of 1 J kg-1 of
tissue mass or body mass. If the body that received such a dose is that of a 35
g (0.035 kg) mouse, the total energy deposited throughout the mouse’s body mass
would have been 0.035 J, and if the body had been that of an 91 kg (200 pound) man,
the total deposited energy would have been 91 J. Dose is an intensive quantity,
and it is the magnitude of energy deposited per unit mass that is important in
determining effects. This idea of intensity as the effects-governing factor is
pervasive with respect to many other quantities as well. For example if we
quickly supply 1,000 calories of energy to one liter of water we would expect
the temperature of the water to increase by 1 degree centigrade, but if we
quickly supplied the same 1,000 calories of heat to 1 cc of water we would
expect the water to vaporize very quickly.
An important consideration in
performing medically useful CT scans is that the x rays used in the scan are
attenuated, more or less exponentially as they traverse the thickness of
tissues in their path. If the volume of the target object is large, the
projected x-ray pathlengths through the body are commensurately large and, in
order to obtain statistically reliable data, the integrated intensity may have
to be greater than might be the case when the target object is smaller. Thus,
it is appropriate to reduce pediatric doses by using techniques that favor
lower values of CTDIvol compared to what would be used with larger
adult patients. We should keep in mind, however, that the CTDIvol
values are not specific to a particular patient. They are based on the assumed
use of phantoms of specific dimensions for specific portions of patients (e.g.,
head or body) and for patients of different sizes (e.g., child vs. adult). The
individual patient, whether child or adult, may have dimensions that vary
appreciably from that assumed for the specified CTDTvol values, and
corrections to the CTDIvol number may be necessary to get the best
estimate of absorbed dose to the patient.
As you are likely aware, there have
been numerous instances in which the CTDIvol values for an adult
have been used in estimating doses to children, and such estimations may
underestimate doses by more than a factor of two because of the lower
attenuation of radiation in the child, thus leading to higher doses throughout
the irradiated tissues. As you noted, the pediatric setting for a CT abdominal
scan may indicate a CTDIvol value of 4 mGy. If the adult settings
had been used for the machine, the value may have been about three times as
great. Because of the smaller pathlengths traversed by the x rays in the child,
the intensity of x rays may be reduced considerably compared to an adult, and
suitable images may still be obtained.
Regarding your last example of a 35 kg
child and a 77 kg adult, your general conclusions are correct, although the
absolute values of the energies in joules that you calculated should be reduced
by a factor of 1,000 (you were using mGy not Gy, and 1 mGy is only 0.001 J kg-1).
Your factor of 13.75 for estimated energy deposited is still correct, but keep
in mind that the estimated adult-to-child dose ratio is 25 mGy/4mGy = 6.25.
This value is still significant but more than a factor of two less than the
factor for total energy deposited, and it is the dose value that is an
indicator of potential harm. Also, the value of 25 mGy for a 77 kg adult seems
rather on the high side for an abdominal CT scan.
I guess the bottom line is that you
can’t get something for nothing, and with the smaller child we can, and should,
use reduced x-ray intensities compared to what are necessary for a larger adult
in order to obtain acceptable images.
We should all be concerned with
minimizing unnecessary doses to patients and, to the extent practicable, we
should be implementing procedures that are consistent with that goal. Using
appropriate pediatric settings for CT scans is an important step in that
direction. Thanks for your question.
George Chabot, PhD, CHP
Editor’s Note: The Health Physics Society is a member of the Image Gently Alliance. See http://hps.org/media/documents/image-gently-press-release.pdf. This campaign emphasizes CT dose reduction in children.
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