OBJECTIVE: To determine if a correlation exists between the semi-quantitative bone scintigraphy index (SQBSI) and serum tartrate-resistant acid phosphatase 5b (TRACP5b) activity, a novel osteoclast marker that has been shown to be useful for monitoring bone metastasis in breast cancer (BC) patients. PATIENTS AND METHODS: Among patients enrolled in 2 prospective studies conducted at Tri-Service General Hospital, Taipei, Taiwan, between December 2000 and July 2002, we identified post hoc 52 patients with both BC and bone metastasis who had detailed records of clinical condition, bone scintigraphy, and concordant serum TRACP5b levels. Between January 1, 2003, and December 31, 2005, we performed bone scintigraphy and serum TRACP5b activity assays to monitor these patients, while they were treated according to clinical need. To assess clinical condition, we obtained information from patient records, such as performance status and visual analogue pain score, as well as from selected laboratory tests for tumor markers and serum TRACP5b activity. Those patients with BC and bone metastasis who had undergone whole-body bone scintigraphy and serum TRACP5b activity determination before any therapeutic intervention were designated the pretreated group (n=30). We developed our own formula for calculating SQBSI on the basis of bone scintigraphy findings. RESULTS: A significant correlation was observed between SQBSI and serum TRACP5b activity in pretreated BC patients with bone metastasis, but the strength of the correlation lessened after treatment. No significant correlation was noted between the change in serum TRACP5b activity and the change in SQBSI in treated patients. Compared with the change in SQBSI, the change in TRACP5b activity had higher sensitivity, specificity, and positive predictive value as well as a greater likelihood ratio for reflecting the clinical scenarios of bone morbidity over time. CONCLUSION: As monitors of the response of bone metastasis in BC to treatment, serial determinations of serum TRACP5b activity and SQBSI were both shown to be useful by our preliminary findings. However, serum TRACP5b activity proved the better monitoring tool. If follow-up studies were conducted within 6 months, the combined use of SQBSI and TRACP5b would allow distinction of genuine disease progression from the "flare" phenomenon, in which bone metastasis can appear to progress in bone scintigraphic images although clinical symptoms improve. Larger prospective studies are needed to confirm these findings.
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