Answer to Question #13943 Submitted to "Ask the Experts"

I am considering a dental hygienist position at a dental office. I am concerned that the office has a digital combo machine of a panoramic and Cone-Beam Computed Tomography (CBCT) scanner. The operatory that I will be using is approximately 1.5 m away and only has a partial cabinet system separating the space. The state has tested the machine; however, since I will be working right there, not just taking the x ray, I want to know if that distance is adequate. It was my understanding that a CBCT machine had to be in a room with lead-lined or brick walls and/or a minimum of 4.6 m distance. Can you please provide me some insight on the relative safety of this type of machine and the distance provided?

Without actually seeing the layout of the facility, it is difficult to determine precisely what your potential exposure would be in the adjacent operatory where you would be working. You mentioned that the state had tested the machine and presumably they may have taken some "scatter" measurements around the machine during that test. Such measurements could also have been taken by either the device manufacturer or possibly a consulting physicist. You could ask if those scatter measurements were taken and if the results indicated the estimated annual radiation doses, based upon assumptions of the number of CBCT or panoramic exams performed, meet the state requirements for either "restricted" or "unrestricted" areas. The acceptable annual radiation dose requirements are lower for unrestricted areas compared to restricted areas. If the estimated annual radiation doses meet the higher restricted area requirements, individuals working in that area would likely be provided a personnel monitoring device (i.e., a dosimetry badge) to assure that radiation levels are within occupational dose limits.

Typically, states require that some type of shielding evaluation be performed to demonstrate compliance with the aforementioned state requirements (i.e., for restricted or unrestricted areas). A few years ago, I performed a shielding evaluation for a CBCT device and utilized specific radiation scatter data provided by the manufacturer to determine radiation levels at given distances around the unit and if any shielding was needed. Using the scatter data from that evaluation, your distance of 1.5 m, and assuming 20 CBCT scans per week, I came up with an estimated annual equivalent dose of 9.51 millisieverts (mSv). A reference from the UK titled Recommendations for the Design of X-Ray Facilities and the Quality Assurance of Dental Cone Beam CT (Computed Tomography) Facilities recommends using an estimated scatter equivalent dose per exam that is about two times higher than the above if there is no data provided by the manufacturer. Doing so would result in an estimated annual equivalent dose of about 19 mSv. To put those numbers in perspective, most US states limit radiation dose levels to between 1 and 5 mSv annually for unrestricted areas and 50 mSv for restricted areas. Assuming even the higher dose value per scan and an estimated 20 scans per week, the estimated annual dose equivalent would fall somewhere between the limits for an unrestricted and restricted area.

Don't forget that the aforementioned estimate assumes you were present at 1.5 m from the CBCT unit for every scan that is performed. Of course, if more than 20 scans per week were performed, the scattered radiation dose would increase proportionally. However, if you were to spend some of your time further away from the CBCT device (e.g., on the opposite side of a dental chair), the radiation dose would be lower. You also mentioned that there is a "partial" cabinet system separating the space. It isn't clear what is meant by "partial." If the cabinet extends above the "line of sight" to the CBCT scanner, then the cabinet would provide some degree of shielding. Even though the cabinet is probably not made of lead, even a couple of inches of wood or a few millimeters of metal will reduce the scattered radiation dose. The amount of reduction would depend upon the type and thickness of the cabinet material.

In conclusion, from the information provided, it isn't possible to determine the scattered radiation level from the CBCT unit to a great degree of accuracy. Based upon the assumptions used to calculate the above estimates, it appears the worst-case scenario would be that the adjacent operatory might be considered a restricted area. The best way to determine what the true radiation exposures would be in that operatory area would be to review any radiation measurements that were made in that area, review the documentation of the necessity (or lack of the need) for shielding, and request some type of personnel monitoring device you could wear when working in the adjacent operatory to actually measure the radiation exposures that you might receive.

Mack L. Richard, MS, CHP