Researchers at the University of Washington in Seattle and Seattle Children’s Hospital have developed low-dose pediatric PET/CT protocols for 11 patient-weight categories, according to a study published in the October issue of the Journal of Nuclear Medicine.
According to Adam M Alessio, PhD, from the department of radiology at University of Washington and the department of radiology at Seattle Children’s Hospital and colleagues, the proposed protocols “offer an initial set of acquisition parameters for pediatric PET/CT” that will allow for the refinement of dose-reduction parameters while maintaining image quality “across the range of pediatric patient sizes.”
PET/CT is growing in importance in pediatric imaging, according to the authors, and adult PET/CT protocols need to be modified for the pediatric population. The authors pointed out that an analysis of World War II atomic bomb survivors suggests that the lifetime risk of cancer for those exposed to radiation at the age of five is 1.7 to 2.1 times greater than those exposed to radiation at the age of 20.
The authors came up with a set of whole-body pediatric PET/CT protocols based on their experience of acceptable image quality and “current, conservative estimates of radiation risk from PET/CT acquisitions.”
The patients underwent identical preparations and were scanned one hour after the injection of F18-FDG. The authors performed the PET portion of the scan first to reduce the potential for respiratory and other motion mismatch, and, in the case of contrast CT, to reduce the amount of scan time after the injection of the contrast material. Shorter patients were scanned head to toe, while taller patients were scanned from the bladder up. All images were acquired on a 64-slice PET/CT scanner (Discovery VCT; GE Healthcare).
For CT, the authors proposed the use of techniques designed for attenuation correction and anatomic correlation of the PET findings, and that resulted in different image noise levels—with a trend toward requiring less noise for smaller patients.
“The tube voltage requires careful selection to optimize dose efficiency. If the tube current remains fixed, as the tube voltage is reduced, the dose to the patient can be reduced substantially,” the authors reported, adding that if the image noise is fixed, as the tube voltage is reduced (and current is increased to maintain the same noise level), the dose to the patient changes only slightly and is a function of patient size.
Their results suggest that for the smallest patients dose could be reduced by, at best, 7 percent using 100 kVp and maintaining the same image noise. Larger patients would have the best dose efficiency at 140 kVp (15 percent dose savings compared with 120 kVp).
The proposed PET/CT protocols result in lower estimated effective doses for younger patients, but the authors also reported that their risk estimation suggests that despite the decreased effective dose for the younger patients, the effective cancer risk remains relatively constant across weight categories.
“The coefficient of variation across categories for the risk of cancer incidence is 18 percent, and for the risk of cancer mortality it is 12 percent,” they reported. “Referencing the highest risk estimates and acknowledging the numerous assumptions inherent in defining risk factors, one could assert that the risk of cancer incidence in a patient's lifetime from one of the proposed PET/CT exams is approximately 38 in 10,000. The corresponding risk of mortality from a cancer due to the proposed PET/CT exams is approximately 15 in 10,000."
Their proposed dose and risk estimates, the authors said, are intended to provide a frame of reference for comparing radiation dosimetry in PET/CT with other imaging procedures across the population. The risks presented, they said, are conservative and “are potentially higher than true values.”
But, even with those conservative estimates, “the risks from PET/CT are generally far outweighed by the benefit of this procedure when used appropriately. Furthermore, pediatric PET/CT is commonly used to diagnose, stage, and monitor already-present cancer, so there is a clearly favorable benefit-to-risk ratio."