Recently, the online radiology communities have been debating the problems of removing the fluoroscopy spacer meant to keep the distance of the X-ray source consistent on each patient and each procedure.
Many surgeons are asking for removal of the spacers to facilitate the surgical field. We asked our radiation safety expert, Louis Wagner, Ph.D., DABR, FACR, FAAPM.
Question for Dr. Wagner:
What are the practical consequences of removing the fluoroscopy spacer cone?
Dr. Wagner’s Answer:
Removing the spacing device has advantages in that it allows for easier positioning of the fluoroscope when the c-arm must be rotated to different angles around the patient. It also makes procedures safer by reducing the likelihood of a collision with the patient or some surgical or other medical apparatus.
The purpose of the regulation requiring spacer devices is to avoid the rapid build up of dose in a patient’s skin during long procedures.
For example, let’s consider a fixed 100-cm SID mobile c-arm. By regulation the maximum output at 70 cm from the focal spot under standard operation is slightly less than 90 mGy/min. For High Level Control (HLC), the limit is under 180 mGy/min at 70 cm. Under this condition using the inverse-square-law, the fluoroscopy output rate at 20 cm from the focal spot would be about 2,205 mGy/min. With backscatter considered, the skin dose rate might be about 2,800 mGy/min or 2.8 Gy/min.
Theoretically, hair loss and transient erythema can be induced in a little over a minute of fluoroscopy under that worst-case condition. After about 2 – 3 minutes, a delayed erythema might be induced. The potential effects get worse as time increases. Using the fluoroscope with the source close to the patient on a regular basis is terrible practice and runs the risk of causing an injury when that rare prolonged case comes along and nobody thought about the dose rates at short SSDs.
Under many common circumstances, the dose rates are not at maximum output and there is at least some space between the port and the skin. Skin dose rates at lowered output for 20-cm probably are more typically around 300 – 600 mGy/min. That’s still very high. Using the spacing device for a 30-cm minimum distance, the rates would be reduced by a factor of 2.25. For example: 1.2 Gy/min would be the worst case fluoro skin dose and 100 – 300 mGy/min a more typical skin dose.
Skin effects are not likely to occur using a mobile device where fluoroscopy is almost exclusively employed, but there is always that perfect storm situation. Removing the spacing device for angiographic and cardioangiographic machines is more of a concern because not only can the fluoroscopy rates be high, the dose accumulation from imaging sequences are also at an extreme. Further, those procedures are often lengthy. Risk for skin injury is greatest following procedures where oblique imaging or cranial-caudal angulation is employed in large patients and where the beam angulation places the x-ray tube in close proximity to the skin.
The percentage of obese patients is at an all-time high of 40% of adults and 20% of adolescents. This poses another question: What do you do when the patient is obese and skin contact is more likely? In these cases, the physician is responsible for understanding the dose increase and making a risk vs. benefit decision. Obviously, shorter exposure time will decrease the dose. Physicians with fluoroscopy and radiation safety training are able to more accurately make informed decisions.
The practical advice is that there is a reason the FDA is concerned about using short SSDs. Things can go wrong and the perfect storm of high dose rate at short SSD in a difficult case involving a large patient can result in serious injury. Make a practice of keeping a healthy gap between the x-ray source and the patient. And if a rare and unusual case arises where that is not possible, keep fluoroscopy time short and keep in mind how the inverse-square-law rapidly increases dose rate.