How dangerous is it to eat a banana?
A discussion of the radiation dose of a bone age hand X-ray.
The hazard of ionizing radiation is of great concern in pediatric radiology, but there is a need for a better understanding of the huge differences in the effective doses of various examinations. The effective dose of a pediatric hand X-ray is remarkably small, 0.0001 mSv, equivalent to 1 minute of airline flight. Sometimes this is said to be equivalent to eating a banana, but as explained below, this comparison is flawed.
A typical example of how radiation doses are summarised is found in the recent overview from Radiology.
Standard radiographic examinations have effective doses (and potential detriment) that vary widely by over a factor of 1000 (0.01–10 mSv). CT examinations tend to be in a more narrow dose range but have relatively high effective doses (approximately 2–20 mSv), and doses for interventional procedures usually range from 5 to 70 mSv. Most nuclear medicine procedures vary in effective dose between 0.3 and 20 mSv. This can be compared with an annual effective dose from natural background radiation of about 3 mSv.
The most common yardstick is a PA chest X-ray of 0.02 mSv, or 20 µSv. Mammography yields 20 times more, namely 400 µSv.
This overview is typical in its inaccurate account of extremity exams. A pediatric hand X-ray gives 0.1 µSv, or 0.0001 mSv, which is a factor of 100 below the range indicated above, so the total dose range of radiographic exams is in fact a factor of 100.000. Such a wide span can be difficult to grasp, but charts like this can be helpful.
The dose 0.1 µSv is 200 times less than the PA chest X-ray dose and it corresponds to 30 minutes of the natural background radiation. Flying from Europe to US and back gives 80 µSv, so the pediatric hand X-ray is equivalent to just 1 minute of flight. This is the best way to explain the dose to the parents, because it puts the decision to take the image in perspective with other decisions in their life, and it suggests that they should care more about ways of transportation than about the radiation dose of a hand X-ray.
On a lighter tone, these small effective doses have been compared to the dose received from eating a banana. A banana contains ½ gram of Potassium, and 0.012% of this is as the unstable isotope 40K with a half-life of 1.25 billion years. As a result, a banana produces 15 radioactive decays per second, and in fact a few bananas can trigger the radiation sensors used to detect smuggling of nuclear material. The effective dose of eating a banana has been computed to be 0.1 µSv for an adult, so the boy in the image above would need to eat just half of the banana to get 0.1 µSv, the same as the dose of a pediatric hand X-ray. Although this comparison has gained some popularity, for instance it was used by the BBC, using this to inform parents would be inaccurate, because the banana dose was computed under the assumption that it takes on average 30 days for the body to get rid of Potassium in excess of what is dictated by the body’s homeostasis. This assumption has been questioned (see Wikipedia) and if homestasis works much faster, the net dose from eating a banana is close to zero.
It is therefore recommended to use the equivalence to 1 minute of airline flight to inform parents.
This discussion is also important in the context of research. The concern about radiation hazard has had the consequence that in recent years bone age X-rays have rarely been included in scientific studies where they would be relevant, for instance in longitudinal studies of normal children. It is important to study the effect on maturation and bone mass from the change in lifestyle, such as exercise, nutrition and physical and social environment, which are related to increased incidence of obesity and early puberty. Investigators seem to restrict themselves from including X-rays and this could be due to poor knowledge about the smallness of the radiation dose. Thus it is more frequent to include DEXA scanning, which seems to have a better ethical reputation, despite the fact that a pencil beam spine DEXA yields 0.2-0.5µSv, and a full body scan on a Hologic scanner yields 3.4-4.6 µSv, see page 11 in A practical guide to bone densitometry in children, National Osteoporosis Society.
The bottom line is that if there is a good clinical or scientific reason for recording a hand X-ray for bone age (and bone mass), there is no rational reason to avoid it due to the radiation dose.
The effective dose 0.1 µSv of a pediatric hand X-ray was derived in 1998 by Huda and Gkanatsios, rather late in the history of X-rays, 103 years after Röntgen’s young wife placed her hand in the beam. The original paper can be found here: Huda W, Gkanatsios NA (1998) Radiation dosimetry for extremity radiographs. Health Phys 75:492–999, and the following summarises their work.
The authors started by defining standard parameters for a pediatric hand X-ray at various ages
- 100 cm focus-object distance
- 55 kVp
- 5 mAs (age 10, body weight 32 kg)
This gives an adequate exposure on film-screen combinations with speed 80. Modern digital detectors are at least as effective as film, so the same mAs can be used today.
The authors then used a "hand phantom" in this geometry and derived the energy deposited ("imparted") in the "phantom" with such an exposure.
Finally the energy was converted to effective dose and listed in this table. The effective dose of a pediatric hand X-ray is seen to be in the range 0.10-0.12 µSv, i.e. approximately 0.1 µSv.
Author: Hans Henrik Thodberg (c). Published 24 Feb 2012