Answer to Question #2174 Submitted to "Ask the Experts"

Q

What is the average external (skin entrance) dose to a patient who is undergoing a radiographic procedure utilizing a c arm: 150 fluoro time, 70 kVp, 3 mA? The distance from the source to the skin is 30 cm. I am an educator with a school of radiography and my department has been called upon to perform this procedure utilizing this extreme exposure time.

A

Based on the above information, a number of additional assumptions have to be made. Typically, the desired information is termed the Entrance Skin Exposure (ESE) rather than a dose. Using a c arm as described is a fluoroscopy, not a radiographic, procedure. The techniques given, i.e., the kVp and mA values, may be typical values for this unit. The given time of 150 did not specify units. However, since the text of the question expressed concern for the “extreme exposure time,” it is assumed that the procedure time is 150 minutes instead of seconds. Since no measured ESE rate measured by a medical physicist or health physicist is given, a range typical for c-arm x-ray units in compliance with the requirements of your state could be used. It is not unreasonable to see many fluoro units with an ESE rate of 2–7 R/min, measured at 30 cm from the image intensifier. The patient is typically not placed 30 cm from the x-ray focal spot, since this increases patient exposure and decreases image quality. Many c arms have a source-to-image distance of 40 inches (~100 cm) and the patient is placed as close to the image intensifier as possible, i.e., as far away from the x-ray tube as possible, to obtain the optimal image quality and lowest patient dose. Since the type of x-ray procedure was not provided, a worst-case scenario is assumed with the x-ray beam not being moved from one area of the patient to another, i.e., x-ray unit and patient were stationary.

Given the above assumptions for this case, i.e., a very long fluoro procedure using an x-ray unit that is properly calibrated and operated by trained personnel, it is possible that the ESE could range from 300 R at 2 R/min to 1050 R at 7 R/min. Since there are numerous factors that may affect this result, this number should only be used to provide a "ballpark" range of ESE. If a physicist were requested to perform an actual ESE calculation for a specific patient and case, more information would have to be provided to properly assess the risk to the patient. It is also very important to note that it is possible to have a case with prolonged fluoroscopy in order to provide a desired medical diagnosis, intervention, etc. A facility performing a case of this length should have a program in place to educate the equipment operators, i.e., physicians and technologists, and the patients as necessary and to perform routine equipment testing to assure that x-ray equipment is used in a manner that provides the best medical results while minimizing radiation exposure to patients, workers, and the general public.

Ken "Duke" Lovins, MS, CHP
Health Physicist

Editor's note: Information on injuries that can result from fluoroscopic procedures can be found at the Food and Drug Administration Web site.