Preliminary Results on the Added Value of Parametric Images Derived from ¹⁸F-fluoroethyl-L-tryptophan PET for...

Increased amino acid transport in gliomas allows imaging of metabolically active tumor volume by PET. Tryptophan analog PET radiotracers can provide additional information by tracking tumoral metabolism via the upregulated immunosuppressive tryptophan-kynurenine pathway. We tested the recently developed tryptophan analog PET tracer [¹⁸F]-fluoro-ethyl-L-tryptophan ([¹⁸F]FETrp) for detecting post-treatment glioblastoma while using a non-invasive approach to generate parametric tryptophan metabolic maps and comparing them with static tracer uptake maps and contrast-enhanced MRI.

Five patients (age: 22-67 years) with previously treated glioblastoma underwent [¹⁸F]FETrp PET/CT imaging. A dynamic acquisition protocol sampled the brain and blood pool non-invasively using the FlowMotion Multiparametric PET software (Siemens Healthineers). Parametric brain images of the unidirectional uptake rate constant (K_i), characterizing irreversible tryptophan trapping and the volume of distribution (V_D) were fused with static uptake (SUV) maps and contrast-enhanced brain MRI. Voxels with elevated K_i, V_D, and SUV were defined, and their spatial associations with contrast-enhanced volumes were characterized by their % volume overlap and the distance between their centroids.

A substantial spatial volume overlap was observed between MRI contrast-enhancing regions and elevated static [¹⁸F]FETrp uptake and V_D. In contrast, the overlap between contrast-enhancing regions and elevated K_i metabolic volumes was low (0-16%), with high K_i areas extending deeper into non-enhancing brain (7-33 mm centroid distance). These non-enhancing high K_i areas showed new contrast-enhancement on follow-up MRI, consistent with tumor progression.

Areas of high tryptophan metabolism detected by [¹⁸F]FETrp PET-derived parametric (K_i) maps extend outside the contrast-enhancing glioblastoma mass in adjacent non-enhancing brain regions that can be missed or underestimated by static uptake images. Consequently, [¹⁸F]FETrp PET metabolic maps have the potential for enhanced detection of non-enhancing glioma infiltration for improved radiation or surgical treatment planning.