PET/CT studies of multiple myeloma using ¹⁸ F-FDG and ¹⁸ F-NaF: comparison of distribution patterns and tracers’ pharmacokinetics

The aim of this prospective study is to evaluate the combined use of fluorine-18 fluorodeoxyglucose (¹⁸ F-FDG) and fluorine-18 sodium fluoride (¹⁸ F-NaF) PET/CT in the skeletal assessment of patients with multiple myeloma (MM) and to compare the efficacy of these two PET tracers regarding detection of myeloma-indicative osseous lesions.

The study includes 60 patients with multiple myeloma (MM) diagnosed according to standard criteria. All patients underwent dynamic (dPET/CT) scanning of the pelvis as well as whole body PET/CT studies with both tracers. The interval between the two exams was one day. Sites of focal increased ¹⁸ F-FDG uptake were considered as highly suspicious of myelomatous involvement. The lesions detected on the ¹⁸ F-NaF PET/CT scans were then correlated with those detected on ¹⁸ F-FDG PET/CT, which served as a reference. Moreover, the ¹⁸ F-FDG PET/CT results were also correlated with the low-dose CT findings. The evaluation of dPET/CT studies was based on qualitative evaluation, SUV calculation, and quantitative analysis based on a 2-tissue compartment model and a non-compartmental approach.

Whole body ¹⁸ F-FDG PET/CT revealed approximately 343 focal lesions while ¹⁸ F-NaF PET/CT revealed 135 MM-indicative lesions (39 % correlation). CT demonstrated 150 lesions that correlated with those in ¹⁸ F-FDG PET/CT (44 % correlation). Six patients demonstrated a diffuse pattern of disease with ¹⁸ F-FDG, while 15 of them had a mixed (diffuse and focal) pattern of skeletal ¹⁸ F-FDG uptake. A high number of degenerative, traumatic and arthritic disease lesions were detected with ¹⁸ F-NaF PET/CT. In three patients with multiple focal ¹⁸ F-FDG-uptake, ¹⁸ F-NaF PET/CT failed to demonstrate any bone lesion. The dPET/CT scanning of the pelvic area with ¹⁸ F-FDG and ¹⁸ F-NaF revealed 77 and 24 MM-indicative lesions, respectively. Kinetic analysis of ¹⁸ F-FDG revealed the following mean values: SUV_aver = 5.1, k₁ = 0.37 (1/min), k₃ = 0.10 (1/min), V_B = 0.06, influx = 0.04 (1/min), FD = 1.28; the respective values for ¹⁸ F-NaF were SUV_average = 10.7, k₁ = 0.25 (1/min), k₃ = 0.34 (1/min), V_B = 0.02, influx = 0.10 (1/min), FD = 1.37. Apart from the correlation between V_B of ¹⁸ F-FDG and k₁ of ¹⁸ F-NaF (r = 0.54), no other significant correlation was observed between the two tracers’ kinetic parameters. We found a significant correlation between FD and SUV_average (r = 0.93), FD and SUV_max (r = 0.80), FD and influx ( r = 0.85), as well as between influx and SUV_average (r = 0.74) for ¹⁸ F-FDG. In ¹⁸ F-NaF we observed the most significant correlations between FD and SUV_average (r = 0.97), FD and SUV_max (r = 0.87), and between influx and k₁ (r = 0.72).

The combined use of ¹⁸ F-FDG PET/CT and ¹⁸ F-NaF PET/CT provides different molecular information regarding the biological processes that take place in a MM osseous lesion. ¹⁸ F-FDG PET/CT proved to be a more specific biomarker than ¹⁸ F-NaF PET/CT in multiple myeloma skeletal assessment.