Evaluation of retention index differences between FDG-avid benign pulmonary nodules and primary lung cancers using dual-time-point FDG-PET imaging

Objectives The aim of this study was to clarify differences in the retention index (RI) between FDG-avid benign pulmonary lesions and malignant lesions, and between tuberculous and non-tuberculous benign lesions by using dual-time-point FDG-PET imaging.

Methods 31 benign lesions (11 tuberculous lesions and 20 non-tuberculous lesions) and 36 primary lung cancers (PLC) with an early SUVmax > 2.5 and maximal diameter > 10 mm were evaluated in this study. We compared the RI values between the each type of lesions, and evaluated their relationship with early SUVmax.

Results The RI values were high in the benign lesion as well as in the PLC (30.7±25.7 vs. 34.2±22.2, mean±SD), and most of them (80.6% of the benign lesions and 88.9% of the PLC) showed RI > 10%, which was generally considered to be an indicator of malignancy. Among the benign lesions, both the tuberculous and non-tuberculous lesions showed high RI values, and there was no significant difference in RI between the two. However, the benign and malignant lesions had a different relationship with early SUVmax. Benign lesions tended to show a lower RI as the early SUVmax increased, and this relation showed a weak negative correlation (R=0.3), whereas PLC showed a persistently high RI with no significant correlation between the two parameters. RI was significantly higher in benign lesions than in PLC (44.3±28.2 vs. 20.4±16.5, p=0.02) when only those lesions with an early SUVmax of 2.5 to 5, but PLC showed higher RI than benign lesions (18.4±16.4 vs. 35.5±22.2, p=0.005) in the lesions with an early SUVmax ≧5.

Conclusions These results suggested that RI was unable to distinguish FDG-avid benign pulmonary lesions from PLC or non-tuberculous from tuberculous lesions.It should be mentioned that benign pulmonary lesions with a moderately high FDG uptake often showed higher RI values than the PLC