Determination of skeletal tumor burden on ¹⁸F-fluoride PET/CT

The purpose of this study was to define a method to assess skeletal tumor burden with ¹⁸F-labeled sodium fluoride PET/CT (¹⁸F-fluoride PET/CT) and evaluate the reproducibility of these measurements. Methods: Ninety-eight consecutive patients (90 men; mean age ± SD, 65.7 ± 14.2 y) underwent 158 ¹⁸F-fluoride PET/CT scans for evaluation of skeletal metastatic disease. In order to determine the mean normal bone SUV, initially a 1-cm spheric volume of interest (VOI) was placed over 5 bone sites: T12, L5, sacrum, right iliac bone, and right femur. For each patient, the mean SUV_max for all sites was generated. Afterward, a threshold value of normal bone uptake was established. Subsequently, skeletal tumor burden was determined by generating volumetric data using a whole-body segmentation method. Any SUV_max below the normal threshold was excluded from analysis, as were VOIs not related to metastatic disease. Statistics for the remaining VOIs were then generated and defined as the skeletal metastatic tumor burden by 2 parameters: total lesion fluoride uptake above an SUV_max of 10 (TLF₁₀) and fluoride tumor volume above an SUV_max of 10 (FTV₁₀). TLF10 and FTV₁₀ reproducibility was determined using 2 independent and experienced PET/CT interpreters analyzing a subset of 13 ¹⁸F-fluoride PET/CT scans. Results: Mean (±SD) normal bone SUV_max was 6.62 ± 1.55 for T12, 6.11 ± 1.73 for L5, 4.59 ± 1.74 for sacrum, 5.39 ± 1.72 for right iliac bone, and 3.90 ± 1.57 for right femur. The mean normal SUV_max for all 543 sites was 5.32 ± 0.99. On the basis of these values, an SUV_max threshold of 10 was chosen to exclude normal bone from the volumetric calculations. Semiautomated measurements of TLF₁₀ and FTV₁₀ exhibited high interobserver reproducibility, within ±0.77% and ±3.62% of the interinterpreter average for TLF10 and FTV10, respectively. Conclusion: Determination of skeletal tumor burden with ¹⁸F-fluoride PET/CT is feasible and highly reproducible. Using an SUV_max threshold of 10 excludes nearly all normal bone activity from volumetric calculations.