Answer to Question #3593 Submitted to "Ask the Experts"

I'm a radiologist. I forgot my lead apron while performing fluoroscopy. I used two minutes and 13 seconds of fluoroscopy time. I used pulsed fluoroscopy at three pulses/sec. I was positioned by the patient's head. The image intensifier tube was centered over the patient's epigastrium with one factor of magnification. The tube was likely 12 inches from the patient. I am 5 foot 1-1/2 inches tall. There is a chance I may be one to two weeks pregnant. What is the estimated exposure to my uterus and chest (breast and thyroid) without my lead apron?

When determining the entrance dose to the patient from a fluoroscopic x-ray beam, the technique has to be known. This includes model of unit, placement of the x-ray tube, the milliamperage (mA), the energy of the x ray (kVp), and the time the fluoroscopy was employed. Since a number of these parameters are unknown, I’ll assume a typical technique that produces a skin entrance dose of 5 cGy/min. Pulsed fluoroscopy usually reduces the radiation dose, and magnification will tend to increase the radiation dose. Therefore, I’ll assume no change from the 5 cGy/min. With a fluoro-time of 2.22 min, the entrance radiation dose would be 11.1 cGy. The radiologist would be exposed to the scattered radiation at a point by the patient’s head. This position is ~1 meter from the area of skin receiving the direct x-ray beam (assume x-ray tube is below the table).

The following calculation is performed based on a published report by King, et al., AJR, 178(1):153, 2002. This manuscript reports personnel radiation exposures at 30 cm from the table top at waist and thyroid levels during fluoroscopy. At waist level the exposure was reported to be 8.96% of the primary entrance exposure. For the present situation, the exposure at the same distance would be 0.994 cGy. Assuming the radiologist to be at 1 meter away (by the patient’s head), the exposure, after applying the inverse square law, would be 0.0894 cGy at the radiologist's waist. The thyroid measurement was reported as 0.284% of the entrance exposure. At 1-meter, the radiologist’s thyroid dose would be 0.00283 cGy. I will assume the breast dose to be between the waist and the thyroid, as 0.0461 cGy. The fetal dose would be less than the waist dose due to tissue absorption of the low-energy scattered photons, which I assume to result in 20% of the entrance dose reaching the uterus. The fetal (uterus) dose would then be 0.00179 cGy.

The Nuclear Regulatory Commission allows a nine-month fetal dose for occupational exposed individuals to be 0.5 cGy at 0.05 cGy/month. The breast dose, 0.0461 cGy, is less than the mean glandular dose limit of 0.3 cGy per single view, and the thyroid exposure is comparable to the radiation dose received from a plane trip across the United States. The image intensifier, if close to the patient, would have attenuated a portion of the scattered photons, thus reducing the reported exposures. The stature of the radiologist may have increased the exposure. However, based on the assumptions made, additional corrections are not warranted. In conclusion, the derived radiation doses represent a minimum risk.

Frank P. Castronovo, Jr