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    St. Jude researchers shed light on measuring bone density

    Sue C. Kaste, DO

    Clinicians treating young males who have brain cancer might be able to make therapy safer for the developing bones of these patients thanks to the results of a new St. Jude study. 

    The St. Jude team showed that in young males being treated for brain cancer who have not yet reached puberty (ages 11 to 15 in the study), using bone age rather than the chronological age of the child gives a more accurate measure of bone mineral density.

    Bone mineral density (BMD) is a measure of how well the bone is absorbing minerals and becoming dense and strong. 

    The finding is important because failure to reach normal peak BMD during adolescence increases the risk of osteoporosis during adulthood, according to the researchers. Osteoporosis is a disease where BMD is reduced and the risk of fracture increased. BMD is reduced by brain irradiation used to treat cancer, by disruption of release of certain hormones caused by the cancer and its treatment, and by the limited mobility of these patients, among other factors. Therefore, it is important for clinicians to monitor these children to minimize long-term impact on bones.

    Clinicians have traditionally compared BMD in young patients to standards established from studies of healthy children of the same age. Specifically, researchers use a type of CT scan called quantitative CT to generate images of bones. Then they use this information to calculate measurements called BMD Z-scores. BMD Z-scores enable clinicians to compare bone development in their male and female patients with those previously calculated from normal, growing males and females, respectively, of the same age.

    However, the rate at which children of the same age mature can vary, regardless of whether there is underlying disease, said Sue Kaste, DO, Radiological Sciences and Oncology. Therefore, endocrinologists usually use estimations of a child’s bone age rather than chronological age to evaluate and monitor abnormal bone growth, she explained.

    The bone age of a specific child is compared to the average age at which children normally reach the stage of bone development observed in the patient, Kaste said. For example, if the bones of a 14-year-old child are more similar to those of an 11-year-old child than to a 14-year-old, then the bone age is 11. Kaste is senior author of a report on this work that appears in the advanced online issue of Pediatric Radiology.

    “Many clinicians have suggested following endocrinologists’ methods of using bone age rather than chronological age to determine bone mineral density, even though it’s more difficult to determine bone age,” Kaste said. “Since St. Jude treats many children who are at risk for abnormally low bone mineral density, we decided to study whether chronological age or bone age was the best tool for interpreting studies of bone mineral density.”

    The St. Jude researchers enrolled 163 children with various types of brain tumors who had a bone age assessment done within six months of the study using X-rays of the hand and wrist. The team then determined the difference in BMD Z-scores using chronological age and compared the results to the BMD Z-score using bone age. 

    The investigators found that there was no significant difference between BMD Z-scores based on chronological age or bone age for most of the children. This suggests that clinicians can usually use a child’s age, rather than the bone age, to accurately help to determine how dense the bone structure is.

    “Using chronological age to determine Z-scores is straightforward because you know the child’s age,” Kaste said. “But clinicians have to figure out what the bone age is. The ability to use the easier chronological age is convenient.”

    There was no difference between the Z-scores based on chronological age and bone age among females 3 to 17 years of age. Among the males, however, 18 children aged 11 to 15 years exhibited significant differences between these two measures. Specifically, boys in the study aged 12 to 13 years had less bone mineral buildup than was suggested by the chronological age and quantitative CT study; boys aged 14 to 15 years had more buildup than these measurements predicted.

    “The bones of boys this age are rapidly assimilating minerals into bones and building bone mineral density,” Kaste said. “That means a small change in difference in maturity greatly affects the expected BMD. So for these children, using bone age is a better strategy.”

    Other authors of this report include Brannon Morris, MD, and Amar Gajjar, MD, both of Oncology; Matthew Smeltzer of Biostatistics; Nicole Thomas and Thomas Merchant, DO, PhD, both of Radiological Sciences; John Shelso, MD, of Endocrinology; and Chin-Shang Li and E. Jane Karimova (both formerly of St. Jude).

    November 2008

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