By Darren Orbach, MD, PhD, director, Neurointerventional Radiology, Boston Children’s Hospital
This article is a condensed version of a paper by Orbach and colleagues published October 24, 2013 in the American Journal of Neuroradiology.
Neurointerventions have dramatically improved the clinical outlook for children with previously intractable cerebrovascular conditions, such as vein of Galen malformations and complex arteriovenous fistulae. Preoperative embolization, even in the youngest patients, can make surgical resection of arteriovenous malformations and vascular tumors dramatically safer. However, these complex, sometimes lengthy neurointerventional procedures are performed under fluoroscopic guidance, exposing vulnerable pediatric patients to the effects of ionizing radiation.
What is the risk?
In predicting the likely side effects of medical radiation, such as x-rays, physicians and scientists have traditionally relied on data gathered from Japanese survivors of the atomic bomb explosions in Hiroshima and Nagasaki during World War II. Survivors living some distance away were exposed to low radiation levels comparable to what patients might receive during medical imaging. We know from these data that children, particularly infants, are most vulnerable to such radiation. While the Japanese survivors had whole-body exposure, the ionizing radiation used in medical imaging is focused on one body part, and there has been interest in tracking these patients’ long-term outcomes.
Recent epidemiologic evidence from a U.K. national registry of children undergoing CT scans of the head suggests a higher incidence of secondary tumors than would have been expected from the Japanese survivor data. Given that neurointerventional procedures involve x-rays, which typically deliver higher doses of radiation than CT scans, we sought to calculate the risk for secondary tumors in a large cohort of pediatric neurointerventional patients seen at Boston Children’s Hospital. We tabulated the radiation dose delivered to the skin and calculated the range of likely doses absorbed by the brain, using previously developed mathematical models. We then generated a predicted risk for secondary tumor development in our cohort as a function of brain-absorbed dose, using the U.K. registry findings.
Minimizing radiation exposure
Fortunately, several operator-dependent variables can dramatically reduce radiation dose. As a dedicated pediatric facility with expertise in imaging and neurointerventions in children, our fluoroscopic systems have been highly optimized. Our in-house physicist worked closely with the equipment vendors to minimize radiation dose and to demonstrate that we could achieve high-quality imaging at low doses. We adjusted every possible technical factor, using pulsed fluoroscopy, aggressive filtration and automated dose rate control; minimizing the air gap between patient and detector; making maximal use of image-hold technologies and using age- and sex-specific lead shields.
With these adjustments, our study found that the maximal skin- and brain-absorbed doses in our cohort were several-fold lower than those previously described. In practice, we often find that patients who are referred to us after undergoing procedures at primarily adult facilities have received a radiation dose several times higher than it would have been in our hands.
Despite decreased radiation exposure, however, we found an increased lifetime predicted risk of tumors in the cohort of young children undergoing neurointerventions, based on the U.K. head CT registry data. The increase was greatest in the youngest patients and in those undergoing repeated procedures.
Weighing risks and benefits
All children undergoing neurointerventions face conditions that are life-threatening or pose a risk of severe neurological impairment, and thus the risk-benefit ratio impels treatment. However, when there are few data to buttress multiple repeated interventions, the lifetime risks of radiation exposure in children must be added to the procedural risks of vascular injury. Particularly in the youngest patients, practitioners should consider the risk of radiation dosage in comparing treatment alternatives and in determining the number of acceptable rounds of neurointerventional procedures. Our data support the treatment of children at specialized centers, where fastidious attention to radiation dose can assure minimal needed exposure.