(P 322) DYNAMIC PET/MRI REVEALS METABOLIC-DIFFUSION INTERACTIONS IN SUSPECTED SOFT TISSUE SARCOMAS

Objective: Hybrid imaging with PET/MRI provides a unique opportunity for real-time characterization of tumor tissue composition and biological behavior. Our study explores this field by examining the temporal interactions between functional imaging-derived biomarkers (primary tumor diffusion restriction, amino acid and glucose metabolism) in patients with a suspicion of soft tissue sarcoma using a dynamic PET/MRI data acquisition protocol after administration of both [18F]FET and [18F]FDG.

Methods: A single-site, explorative, prospective study including 19 patients (11f/8m, median age 61.5y [range: 36.6-82.2]) with newly diagnosed tumors and strong suspicion of soft tissue sarcoma (Table 1). At diagnosis, all patients underwent conventional imaging for primary tumor characterization and disease staging, as well as dynamic, regional [18F]FET and [18F]FDG PET/MRI (Siemens Biograph mMR®) of the primary tumor, with a maximum of 7 days between the examinations. Following i.v. administration of 4 MBq/kg radiotracer, continuous PET acquisition was performed 0-60 min post-injection. Within the same timeframe, MRI was performed according to standard tumor protocols, including DWI sequences. The primary tumor was delineated on MRI and the region of interest was transferred to the corresponding PET datasets. Voxel-to-voxel tumor analysis compared semiquantitative measures of diffusion restriction (ADC) with time-dependent amino acid (SUVFET) and glucose metabolism (SUVFDG) across six successive 10 min intervals to characterize primary tumor tissue.

Results: Despite being more time-consuming than standard soft tissue tumor imaging with PET/MRI, the proposed imaging protocol and subsequent analysis proved feasible, and all included patients were able to cooperate. Voxel-to-voxel comparison of semiquantitative parameters for tumor diffusion and glucose metabolism revealed distinct distribution patterns that varied according to histopathological tumor subtype and tumor grade, with correlation coefficients reflecting the heterogenic nature of soft tissue sarcomas/tumors. Furthermore, time-dependent accumulation patterns of both radiotracers demonstrated this biological complexity.

Conclusion: Given the rarity and heterogeneity of soft tissue sarcomas being as major clinical challenges, dynamic functional imaging with PET/MRI demonstrates correlations between tumor diffusion and metabolism, providing a platform for comprehensive imaging characterization of tumor tissue composition and biological behavior.

Clinical Characteristics of Included Patients. Abbreviations: Pt.: patient; M: male; F: female; y: years; gr.: FNCLCC tumor grade; Dediff.: dedifferentiated; Undiff.: undifferentiated; NOS: not otherwise specified; GCT: giant cell tumor; LE: lower extremity; UE: upper extremity. Histopathological extent of necrosis was determined according to the FNCLCC grading system. Tumor size was determined on MRI. The presence of metastatic disease at the time of diagnosis was determined on [18F]FDG PET/CT.