Why scanning peds is necessary.
Healthy bone grows throughout childhood and adolescence unless a genetic or acquired disorder disrupts the process. Otherwise, bone mineral density in healthy children increases until it peaks in the mid-20s. But it is the bone growth that complicates DXA interpretation and diagnosing osteoporosis in children.
"Changes in bone size over time, as occur in normal pediatric development, confound DXA interpretation," stated Larry A. Binkovitz, M.D., and Maria J. Henwood, D.O., in January's issue of Pediatric Radiology.
The causes of low bone density in children include anorexia nervosa, inhaled steroids used to combat asthma, chronic liver and kidney diseases and cystic fibrosis. These are just a few of the common disorders and medication that can lead to low bone density and fragility fractures in children, according to the National Osteoporosis Foundation.
"I've worked in bone densitometry since 1991 and the biggest change I've seen is in the age of the children scanned with DXA, down to age 5," said Pam Johnson, B.S., R.T.(R)(BD).
Many of the children that Ms. Johnson scanned with DXA in New York were enrolled in clinical studies at the same time they were undergoing a physicianordered exam. Based on her experience, Ms. Johnson emphasized that pediatric DXA imaging requires extensive knowledge of pediatric needs and the right pediatric database and equipment.
"You have to make sure that the software used in scanning has a database that applies to children," said Ms. Johnson.
When osteoporosis is misdiagnosed in children, it's due largely to misinterpreting DXA scans, a 2004 study found. The most common error was use of the T-score, which compares values to appropriate normal controls (the Z-score) with peak adult BMD, rather than using the Z-score alone, according to Rachel I. Gafni, M.D., and Jeffrey Baron, M.D., in their study "Overdiagnosis of Osteoporosis in Children Due to Misinterpretation of Dual-energy X-ray Absorptiometry," published in the February 2004 issue of The Journal of Pediatrics. Other errors included using a reference database that did not consider gender or ethnic differences, using incorrect bone maps and inattention to short stature. After correcting for the errors, 53 percent of the pediatric patients had normal BMD and only 26 percent retained a low BMD diagnosis.
"Because the T-score is a measure of bone density loss since early adulthood, its use in children whose BMD has yet to peak will always yield a low result," said Drs. Binkovitz and Henwood.
Delayed growth and maturation also can complicate interpretation of DXA findings in chronically ill patients, according to Laura K. Bachrach, M.D., professor of pediatrics at the Stanford School of Medicine.
For a diagnosis of osteoporosis to be assigned to a patient younger than 20 years, the International Society for Clinical Densitometry recommends that the following measures be taken:
* Use Z-scores instead of T-scores and don't include T-scores in reports or on DXA printouts.
* Base a diagnosis of osteoporosis in children on more than DXA criteria alone.
* Interpret Z-scores in the light of the best available pediatric databases of age-matched controls. The reference database should be cited in the report.
* Measure the spine and total body.
* Clearly state in the chart that all adjustments are made to account for bone size, pubertal stage, skeletal maturity and body composition. This is necessary because there is no agreement on standards for adjusting BMD or bone mineral content for such factors.
* When appropriate, perform serial BMD studies on the same machine using the same scanning mode, software and analysis. Changes might be required with growth of the child.
* State any deviation from standard adult acquisition protocols in the report, such as use of low-density software and manual adjustment of region of interest.
Ms. Johnson emphasized proper positioning and the need for pediatric patients to stay still for the recommended exams.
Last, she pointed out the ISCD position on the type of scan. "Some orders in pediatrics call for a femur and hip scan, which is a wasted scan," according to Ms. Johnson. "You only get results in scanning children with a total body and lumbar spine scan."