FORT LAUDERDALE, FLA. — The use of 18F-fluoride positron emission tomography may be useful in the follow-up of patients with monostotic forms of Paget's disease, Dr. Jean-Pierre Devogelaer said at a meeting sponsored by the Paget Foundation for Paget's Disease of Bone and Related Disorders.
In Paget's disease of bone, biochemical markers are used to monitor treatment response. However, in patients with limited bone involvement, these global indices often remain in the normal range, said Dr. Devogelaer, professor of rheumatology at Catholic University of Louvain and Saint-Luc University Hospital, Brussels.
Positron emission tomography (PET) using 18F-fluoride as an imaging agent appears to be of value in measuring regional skeletal metabolism, and therefore may be helpful in determining whether bisphosphonate therapy should be stopped or prolonged depending on the local level of pagetic activity, he said.
Twelve patients with monostotic Paget's disease of bone underwent 1-hour dynamic 18F-fluoride PET scans at baseline and at 1, 6, and 12 months after bisphosphonate treatment (intravenous pamidronate in nine, oral risedronate in two, and oral tiludronate in one). Biochemical markers were measured at the same time points. The affected areas were pelvis in three patients, tibia in three, femur in two, and humerus, vertebral body, skull, and scapula in one patient each.
Changes in bone metabolism as measured by the PET scans were assessed in two ways: via dynamic plasma clearance of 18F-fluoride to bone mineral, which requires arterial blood sampling; and with a standardized uptake value, a semiquantitative index that averages the tracer uptake with respect to the injected dose and the body weight. Calculation of the standardized uptake value does not require arterial sampling and therefore is a far more convenient method for measuring pagetic activity in a clinical setting, Dr. Devogelaer added.
The two values correlated with each other at all time points. Both showed huge activity prior to treatment and significant drops thereafter, by about 30% at 1 month, 40% at 6 months, and nearly 50% at 1 year.
In contrast, the biochemical markers correlated with the PET scan results at baseline but not after treatment: Total alkaline phosphatase dropped by about 25% at 1 year, but remained within the normal range throughout the study. Fasting levels of urinary N-terminal cross-linking telopeptide of type I collagen (NTX) decreased significantly up to 6 months, but not thereafter. Bone-specific alkaline phosphatase dropped by about 30%–35% at 1 month, but remained significant only up to 6 months. Such changes in biochemical markers are not adequate for follow-up, he said.
An audience member noted that PET scans are expensive and not covered for Paget's disease in the United States. Dr. Devogelaer replied, “We hope that the cost will decrease. But we see that with the biological parameters, there is no correlation after treatment. To appreciate the activity of monostotic Paget's disease of bone, we need something else.”