CHICAGO – Applying the key principles for limiting radiation exposure for vascular surgeons and staff – not to mention patients – during endovascular procedures involves a thorough understanding of dose metrics as well as risk factors for high-dose interventions, according to recent findings reported at a symposium on vascular surgery sponsored by Northwestern University.
Melissa Kirkwood, MD, of the University of Texas Southwestern Medical Center in Dallas, reported on her institution’s experience with limiting radiation exposure in typically high-dose cases. “What we found was that even though we had a substantial amount of dose in these cases – they included a significant proportion of 5- to 10-Gy and greater than 10-Gy cases – there were still no skin injuries detected in these patients,” she said, referencing retrospective and prospective analyses (J Vasc Surg. 2014;60:742-8; J Vasc Surg. 2015;61:902-6).
Dr. Melissa Kirkwood
Vigilance regarding these principles for vascular surgeons is paramount, Dr. Kirkwood said, noting that the National Council on Radiation Protection and Measurements threshold for substantial radiation dose is 5 Gy or greater. “When you’re doing complex endovascular work, your doses can get that high,” she said.
As a means of measuring dose, Dr. Kirkwood called fluoroscopy time a “universally poor indicator” because in current practice vascular surgeons use digital acquisition mode in addition to fluoroscopy. “The digital acquisitions generate 10-100 times more dose than fluoroscopy, so if you’re only looking at fluoroscopy time, your potentially missing the majority of the dose for that case,” she said.
More applicable dose measures, she said, are kerma area product that measures total radiation beam output from the x-ray tube, which she called “a better reflection of operator exposure,” and reference air kerma (RAK), a measure of the dose at a reference point 15 cm along the beam axis toward the focal spot from the isocenter, which she said is the best approximation for patient peak skin dose exposure. However, the latter does not account for angle of the x-ray tube or patient position, which can vary based on the type of procedure or the patient’s size.
Dr. Kirkwood’s work at UTSW also determined that operator exposure during an endovascular procedure depends on where they stand. “Doubling the distance from the source can decrease the radiation level by a factor of four,” she said. For femoral access in the right groin, the operator is at greatest risk for exposure followed by the assistant when the assistant is standing to the right of the operator. The left brachial access site carries an even higher exposure for the operator, she said.
The table-mounted lead skirt plays a key role in limiting operator exposure, Dr. Kirkwood said. “It can be cumbersome, but it is very important in lowering your lower-body dose,” she said, because it will block radiation scatter coming off the bottom of the table.
At UTSW, the endovascular operators had a tutorial with the staff medical physicist on best practices to limit radiation exposure. “What we found was that we were significantly able to decrease the dose across all cases by simply going over a few principles,” she said.
Among those principles: “Always be aware when you’re on the fluoroscopy pedal and always use the lowest fluoroscopy mode possible,” she said. However, she noted that in difficult-to-visualize cases, a short-duration boost in fluoroscopy level might reduce overall fluoroscopy time and hence limit exposure. To limit digital acquisition mode, the use of fluoroscopic looping can allow for review of images during the procedure with a fraction of the dose that would be needed for a digital acquisition run.
Limiting magnification and using collimation can be complementary, Dr. Kirkwood said. “If you really have to magnify to see the area of interest, make sure you have tight collimation to try to decrease the scatter to you and your colleagues in the OR,” she said.
Dr. Kirkwood noted that raising the angio table as high as is comfortable and decreasing the distance between the source and image detector can limit patient exposure. Operators should avoid steep angulations of the x-ray tube, she said, but when angulations are necessary, operators should stand on the opposite side of the x-ray tube. “The best operating practice if you know you’re going to have a high-dose case with a lot of gantry angulation would be to tightly collimate to the area of interest and minimize the magnification,” she said.
Though not necessarily a principle, keeping up with software advances for imaging devices can also prove valuable for limiting radiation exposure, Dr. Kirkwood said. “It’s important to know about them because if you are purchasing new equipment, they are not necessarily included if you’re institution is looking to hold down costs,” she said.
Dr. Kirkwood had no relevant financial disclosures.