Intra-individual comparison of the biodistribution of 18F-FLT versus 18F-FDG in oncologic PET/CT

Objectives To quantitatively evaluate intra-individual differences of 18F-FLT uptake throughout the body compared to 18F-FDG in oncologic patients to better understand the expected uptake in normal and pathological tissues.

Methods Wholebody imaging of 17 patients (75 lesions) was acquired in 18FLT (10±1.0mCi with 65±5min post inj) and 18FDG (14±1.5mCi with 75±10 min post inj) on a Gemini 64 TF PET/CT (Philips) within 1-7 days. 3D ROIs were placed on cerebellum, adipose tissue, periscapular muscle, abdominal aorta, heart, breast, bone marrow (L5), liver, kidney, bladder and metastatic lesions in both exams. SUV values were measured to characterize biodistribution and lesion uptake. Lesions ≥1cm on CT (RECIST 1.1) were defined as “measureable” and evaluated.

Results Comparing the biodistribution, a heterogeneous variance of SUVmax was found: cerebellum (FDG 8.6±2.2 / FLT 0.5±0.1, ΔSUV -95%), heart (5.2±4.2 / 1.8±0.5, -65%), breast tissue (1.0±0.5 / 0.6±0.2, -40%), aorta (2±0.2 / 1.5±0.5, -25%), fat (0.4±0.1 / 0.4±0.1, 0%), muscle (1.0±0.2 / 1.1±0.2, +10%), kidney (10±7.5 / 14.2±10, +42%), bladder (75±67 / 147±158, +95%), liver(2.8±0.5 / 6.8±1.5, +143%) and bone marrow(2.4±0.7 / 9.3±2.5, +288%). For malignant lesions, significant difference in SUV (p<0.0001) were found for 18FDG (9.2±3.9) / 18FLT (4.9±2.7) with an average variance of -41±7% for solid and -46±5% for pulmonary tumors. The tumor-to-background ratio is significantly different comparing the proliferation oriented imaging to metabolic imaging (p<0.001) as expected.

Conclusions The biodistribution between 18FLT and 18FDG PET/CT is heterogeneous and quite different as demonstrated by intra-individual comparison. All “measurable” malignant lesions were visualized by both tracers. FLT revealed a significantly lower uptake/SUVmax, however it did not impact the detectability of any lesion outside the liver and bone-marrow, with intracranial lesion only seen on FLT