Head-to-head comparison of [⁶⁸Ga]Ga-PSMA-11 and [¹⁸F]FDG PET/CT in multiple myeloma

The aim of this study was to compare [¹⁸F]FDG and [⁶⁸Ga]Ga-PSMA-11 PET/CT image findings in patients with multiple myeloma (MM).

Twenty consecutive patients with symptomatic biopsy-proven MM were submitted to whole body [¹⁸F]FDG and [⁶⁸Ga]Ga-PSMA-11 PET/CT with a time interval of 1-8 days between procedures. All lesions were counted and had their maximum SUV (SUVmax) measured. Intra-class correlation (ICC) was used to assess the agreement between [¹⁸F]FDG and [⁶⁸Ga]Ga-PSMA-11 PET/CT findings.

A total of 266 lesions were detected in 19/20 patients. [¹⁸F]FDG detected 223/266 (84%) lesions in 17 patients and [⁶⁸Ga]Ga-PSMA-11 190/266 (71%) lesions in 19 patients. Both procedures did not identify any active lesion in 1 patient. Forty-three (16%) lesions were detected only by [⁶⁸Ga]Ga-PSMA-11 and 76 (29%) only by [¹⁸F]FDG. Both tracers identified 147 (55%) lesions. Intralesional mismatch of FDG-PSMA uptake was identified in 25 of these 147 lesions, found in 8 different patients. Different lesions with uptake of only [¹⁸F]FDG or [⁶⁸Ga]Ga-PSMA-11 in the same patient were found in 4 patients. The highest SUVmax of [¹⁸F]FDG and [⁶⁸Ga]Ga-PSMA-11 had a median (min-max) SUVmax of 6.5 (2.0-37.8) and 5.5 (1.7-51.3), respectively. [¹⁸F]FDG and [⁶⁸Ga]Ga-PSMA-11 respectively identified 18 and 19 soft tissue lesions. False-positive [¹⁸F]FDG findings had minimal or no uptake of [⁶⁸Ga]Ga-PSMA-11. Good reliability (ICC ≥ 0.75) was found for number of lesions, number of soft tissue lesions and highest SUVmax in each patient.

[¹⁸F]FDG or [⁶⁸Ga]Ga-PSMA-11 alone can detect most MM lesions. Almost half of the lesions take up only one of the tracers, reflecting increased glycolysis or angiogenesis in specific lesions, and suggesting their possible complementary role in MM. The marked [⁶⁸Ga]Ga-PSMA-11 uptake in some cases raises the possibility of a theranostic approach in selected patients.