18F‑FDG PET/CT in metastatic chordoma: a retrospective analysis of imaging features and clinical impact

Background: Systematic data on ¹⁸ F-fluorodeoxyglucose positron emission tomography/computed tomography ( ¹⁸ F-FDG PET/CT) in metastatic chordoma are scarce. This study aimed to evaluate its imaging characteristics, diagnostic consistency, and potential impact on clinical assessment and management. Methods: In this single-center retrospective analysis, 21 patients with pathologically confirmed chordoma and prior treatment underwent ¹⁸ F-FDG PET/CT for suspected recurrence or metastasis. Metastatic disease was diagnosed per a composite standard (biopsy, imaging progression, or characteristic multimodal findings). Relevant clinical and histopathological data were collected. Images were independently reviewed by two experienced nuclear medicine physicians for metabolic activity and whole-body disease assessment. They assessed metabolic activity at the primary site and performed whole-body evaluation. For each metastatic lesion, maximum standardized uptake value (SUVmax) and size were measured; CT features were also documented. Interobserver agreement for key assessments was formally evaluated. Results: Metastatic disease was identified in 11 of 21 patients (52.4%). Metastases were found in bone (7 patients), soft tissue (8 patients), and lung (5 patients). Site-specific metabolic patterns emerged: pulmonary metastases had lower FDG avidity (median SUVmax 2.3) correlated with size, whereas bone and soft-tissue avidity (SUVmax 3.8-3.9) was size-independent. A higher SUVmax was associated with poorly-differentiated histology (6.4 vs. 3.6, P =0.023). Notably, in 4 out of 21 patients (19.0%), PET/CT detected metastases outside the field of view of conventional imaging. Interobserver agreement was perfect for metastatic status and excellent for total lesion counts. A case illustrated differential treatment response linked to baseline metabolic avidity. Conclusions: ¹⁸ F-FDG PET/CT offers a reproducible whole-body assessment for chordoma, enabling the detection of occult metastases and revealing clinically relevant metabolic heterogeneity. These findings highlight its potential in surveillance and personalized management, warranting further prospective validation.