Answer to Question #8867 Submitted to "Ask the Experts"

Q

I work as a radiographer in a maternity hospital. There is concern from staff members and parents about the scatter/secondary radiation risks both to staff and babies (in adjoining incubators) on the newborn intensive care unit (NICU).

We x ray neonates from 500 g (approximately 24 weeks gestation) to 4,000 g (term babies) on the unit. Our general exposures vary from 60 kVp/0.7 mAs to 63 kVp/1.4 mAs (depending on size). The babies in the adjoining incubators are a minimum of eight feet from each other and all adults are required to either leave the room or to move at least eight feet from the primary beam before exposure.

We only provide lead aprons and gonad shielding to staff or parents who are holding infants. No lead shielding is placed between incubators as it is felt that dose and distance mean that there is negligible/no exposure to infants in the adjoining incubator or to adults who adhere to the requested distance.

A study has not been done, but as the national guidelines for people close to the primary beam for adult mobile x rays is six feet, we feel we are being cautious with our local guidelines.

Could one assume that there is no need for radiation shielding due to negligible secondary dose/scatter from the doses we use and the distance, or should we increase the distance and/or provide lead shielding between incubators?

Also, would you recommend that we do an internal study to confirm this or is there a study that we could use to confirm this?

A

Your assumption that there is no need for radiation shielding for the reasons stated is valid. Here are a few calculations that can be used to substantiate that assumption. Understand that without specific information about the x-ray machine and the imaging procedure being performed, these calculations include a number of assumptions.

According to Figure 4 in the “CRC Handbook of Radiation Doses in Nuclear Medicine and Diagnostic Radiology” (Authors - James Kereiakes and Marvin Rosenstein), the exposure rate in air from a three-phase x-ray tube, operated at a tube potential of 70 kV with a total filtration of 2.5 mm of Al is about 8 milliroentgen/mAs (mR/mAs) at 40 inches. This is a rough estimate of the exposure to the patient. The figure in the reference does not quite go down to 60 kV and the mR/mAs value would be lower for that tube potential, so for this illustration, using the 70 kV value is conservative. Thus, the estimated exposure to the patient being x rayed would be 8 mR/mAs x 1.4 mAs or 11.2 mR.

Of course, the question relates to the scattered exposure to other babies and staff around the patients. Table B-2 of NCRP Report No. 49 – “Structural Shielding Design and Evaluation for Medical Use of X Rays and Gamma Rays Up to 10 MeV” indicates that the ratio of the 90 degree scattered to incident exposure for 70 kV x rays at 1 meter is 0.0005 for a 400 cm² (about 8" x 8") field size. There is no value provided for 60 kV x rays and the ratio for 50 kV x rays is 0.00035, so the 70 kV value is a conservative estimate. The amount of scatter would increase or decrease proportionally with the change in field size.

Thus, the estimated scattered exposure at 1 meter from the x-rayed patient (assuming a 90 degree scattering angle) would be 11.2 mR x 0.0005 or 0.0056 mR. You indicated that the distance between the baby being x rayed and the adjacent babies is no less than eight feet (that’s about 2.4 meters). So, correcting for distance using the “Inverse Square Law,” the estimated exposure at eight feet (2.4 meters) would be 0.0056(1²)/2.4² or 0.00097 mR.

To put that exposure in the correct context, consider that the average member of the United States receives about 3.11 mSv (311 millirem) from background radiation exposure each year (Ref: NCRP Report No. 160 – “Ionizing Radiation Exposure of the Population of the United States”). In the aforementioned example, we can consider a milliroentgen and a millirem to be numerically equal. Therefore, 311 mrem/yr corresponds to 0.85 mrem/day, 0.036 mrem/hour, or 0.00059 mrem/minute. Thus, in your example, the estimated exposure to a baby positioned eight feet from another baby being x rayed is equivalent to between one and two minutes of natural background radiation, which is not something anyone should be concerned about.

Mack L. Richard, MS, CHP
IUMC/IUPUI Radiation Safety Officer