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Objective:
Dual time point imaging (DTPI) has been found to enhance the specificity of FDG PET/CT for diagnostic and prognostic purposes. The purpose of this study is to use a CT-based approach of PET/CT quantification to compare early and delayed scans with dual time-point imaging (DTPI) of the bone marrow in patients with non-Hodgkin`s lymphoma (NHL).

Materials and Methods:
Data from FDG-PET/CT scans performed at the University of California, Davis from 20 patients referred for initial staging of NHL were analyzed (age 12–74.9 years, mean = 44.12 ± 19.70 years; 9 male patients, 11 female patients). It included 40 scans conducted 60 and 120 minutes following IV injection of 8 mCi of FDG. Out of 20 patients, one patient was excluded from the study as its baseline global SUVmean of bone marrow was an outlier. Interim scans after two cycles of chemotherapy (7/20 patients) were assessed at both 60 and 120 minutes after injection of 8 mCi of FDG. Segmentation of the entire skeleton on a fused PET/CT image was accomplished using an iterative thresholding technique that defines a continuous region based on Hounsfield units (150 to 1500) using OsiriX MD (version 12.5.2 software; Pixmeo SARL; Bernex, Switzerland). This enabled the quantification of the global FDG uptake in the bone and bone marrow, providing a global SUVmean. Global SUVmean of bone marrow at 60 and 120 minutes were compared.

Results:
For baseline scans, the bone marrow at 60 minutes had an average SUVmean of 0.83 ± 0.16 (range: 0.42–1.18), whereas the average SUVmean at 120 minutes was 0.83 ± 0.24 (range: 0.32–1.43). The average 60–120 min percent change was -0.37% ± 17.86% (range: -34.26%–48.09%) (p = 0.96). For posttreatment scans, there was an overall decrease in FDG uptake in the bone marrow from 60–120 minutes. At 60 minutes, the average of SUV mean was 0.71 ± 0.22 (range: 0.24–0.89), whereas the average of SUVmean at 120 minutes was 0.66 ± 0.21 (range: 0.23–0.89). The difference in the average SUVmean at 120 minutes and 60 minutes was found to be -0.06 ± 0.05 (range: -0.16–0.09) (p = 0.04), and the average 60–120 min percent change was -7.73% ± 6.07% (range: -17.79 % to 0.95%).

Conclusion:
This study indicates that FDG uptake in the bone marrow does not change significantly from 60–120 minutes during DTPI of the bone marrow with total body FDG PET/CT in this small initial staging lymphoma population. However, after treatment, the bone marrow demonstrated a decrease in FDG uptake when a 60-minute scan was compared to 120 minutes scans; this may be related to the chemotherapy effect. Thus, in the posttreatment FDG PET/CT images of NHL, focal lesions are more appreciable in bone marrow at 120 minutes images compared to 60 minutes. Given the small cohort of subjects, further analysis is warranted.