18F-FDG PET/CT for Early Postradiotherapy Assessment in Solitary Bone Plasmacytomas

The purpose of this study was to evaluate the performance and possible prognostic value of early ¹⁸F-FDG PET/CT (FDG PET/CT) assessment after radiotherapy (RT) in patients with solitary bone plasmacytoma (SBP). Methods Twenty-one patients affected by SBP who underwent FDG PET/CT scan for early restaging (≤6 months) postradiotherapy assessment were selected from the PET databases of University College London Hospital of London and San Raffaele Hospital of Milan. Patients with no abnormal uptake were classified as having no pathologic uptake (NPU). A SUV<inf>max</inf> cutoff value of 4 was chosen to discriminate minimal residual uptake (MRU; SUV<inf>max</inf> ≤ 4) from pathologic uptake (PU, SUV<inf>max</inf> >4). Progression-free survival (PFS) rate was estimated using Kaplan-Meier curves and Cox regression analysis. Results In 10 of 21 patients restaged by FDG PET/CT, further previous baseline scan was available also at staging, and results showed positive findings at the level of all biopsy-proven disease areas. Considering MRU as PU, FDG PET/CT showed a sensitivity and specificity of 86% and 29%, respectively. Using SUV<inf>max</inf> >4 as the cutoff, sensitivity and specificity were 86% and 93%, respectively. Kaplan-Meier curves revealed a significant difference in PFS probability between patients classified as positive on FDG PET/CT using a cutoff of SUV<inf>max</inf> >4 (PU) and those classified as negative (NPU + MRU) (log-rank, Mantel-Cox, P = 0.009; χ² = 6.85). Cox regression analysis of PFS using SUV<inf>max</inf> >4 as cutoff revealed an interesting relation in prediction of progression (HR, 9.458). Conclusion ¹⁸F-FDG PET/CT for early restaging after RT in patients with SBP should be considered carefully in view of the lack of specificity of a low SUV<inf>max</inf> value. The good correlation between a high SUV<inf>max</inf> value and follow-up suggests a possible prognostic role for FDG PET/CT in disease progression at early restaging after RT.