PET evaluation of bone metastases: Degree of hypermetabolic activity of FDG-PET depends on the degree of osteolytic or osteoblastic component in the bone metastases

Abstract

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Objectives: To investigate natures of discordant or concordant scan finding between PET and other diagnostic imaging studies for bone metastases and its clinical relevance. Methods: Retrospective reviews of 75 patients whose PET or PET/CT scan finding, suspicious of bone metastasis were analyzed for follow-up clinical outcome. The PET scan findings were correlated with other imaging studies including bone scan, CT, MRI, with respect to a type of primary tumor, nature of osteolytic, sclerotic, or mixed, and concordancy or discordancy with other imaging studies. In the subset of the patients who had PET/CT scan, SUV and Hounsfields Unit (HU) values were calculated in the metastatic sites and normal bone sites. Results: Thirty-two patients had bone metastases; In 13 of these, PET finding showed more hypermetabolic metastatic lesions than that of other imaging studies: the types of primary tumor are breast, lung, lymphoma and thyroid; In 12 patients, PET and other imaging studies showed concordant appearance in the breast, lung, lymphoma, colon and esophageal tumor; In 6 patients, PET scan showed less lesions than other imaging studies in colon and lung. SUV values of metastatic sites ranged from 3.6 to 14.4 (mean of 8.1) whereas SUV values of normal bone ranged 0.6 to 3.1 (mean of 2.2). 95% CI values of maximum and minimum HU values of metastatic sites ranged from 548 to 903 (mean 724) and 26 to 146 (118) whereas normal bone sites ranged 324-to 439 (mean 391) and 11.5-38 (mean value 25). No statistically significant positive or negative correlation between the SUV and HU values is noted. However, qualitatively PET scan is more positive and hypermetabolic in the osteolytic tumor and there was no statistical difference in the proportion of such occurrence between the tumor types. Conclusions: One should be cautious about concluding the presence of bony metastases by PET scan finding alone. PET scan finding of bone metastases can be variable, either concordant or discordant with other imaging studies. Hypermetabolic metabolic foci are more prevalent in the osteolytic tumor. False negative FDG PET finding could be noted in the 6/35 patients (17%) with osteoblastic bone metastases.