18F-FDG PET/CT in solitary plasmacytoma: metabolic behavior and progression to multiple myeloma

Solitary plasmacytoma (SP) is a rare plasma-cell neoplasm, which can develop both in skeletal and/or soft tissue and frequently progresses to multiple myeloma (MM). Our aim was to study the metabolic behavior of SP and the role of 18F-FDG-PET/CT in predicting progression to MM.

Sixty-two patients with SP who underwent 18F-FDG-PET/CT before any treatment were included. PET images were qualitatively and semiquantitatively analyzed by measuring the maximum standardized uptake value body weight (SUVbw), lean body mass (SUVlbm), body surface area (SUVbsa), metabolic tumor volume (MTV), total lesion glycolysis (TLG) and compared with age, sex, site of primary disease, and tumor size.

Fifty-one patients had positive 18F-FDG-PET/CT (average SUVbw was 8.3 ± 4.7; SUVlbm 5.8 ± 2.6; SUVbsa 2 ± 1; MTV 45.4 ± 37; TLG 227 ± 114); the remaining 11 were not 18F-FDG-avid. Tumor size was significantly higher in patients avid lesions compared to FDG not avid; no other features are associated with FDG-avidity. Progression to MM occurred in 29 patients with an average of 18.3 months; MM was more likely to develop in patients with bone plasmacytoma and in patients with 18F-FDG avid lesion. Time to transformation in MM (TTMM) was significantly shorter in patients with osseous SP, in 18F-FDG avid lesion, for SUVlbm > 5.2 and SUVbsa > 1.7.

18F-FDG pathological uptake in SP occurred in most cases, being independently associated with tumor size. PET/CT seemed to be correlated to a higher risk of transformation in MM, in particular for 18F-FDG avid plasmacytoma and SBP. Among semiquantitative features, SUVlbm > 5.2 and SUVbsa > 1.7 were significantly correlated with TTMM.