(18)F-FDG PET-CT versus MRI for detection of skeletal metastasis in Ewing sarcoma

OBJECTIVE: To determine the level of discrepancy between magnetic resonance imaging (MRI) and (18)F-FDG PET-CT in detecting osseous metastases in patients with Ewing sarcoma. METHODS: Twenty patients with histopathologically confirmed Ewing sarcoma between 2000 and 2017 who underwent (18)F-FDG PET-CT and MRI within a 4-week range were included. Each imaging modality was evaluated by a separate observer. Reference diagnosis of each lesion was based on histopathology or consensus of an expert panel using all available data, including at least 6 months’ follow-up. Sensitivity, specificity, and predictive values were determined. Osseous lesions were analyzed on a patient and a lesion basis. Factors possibly related to false-negative findings were evaluated using Pearson’s Chi-squared or Fisher’s exact test. RESULTS: A total of 112 osseous lesions were diagnosed in 13 patients, 107 malignant and 5 benign. Seven patients showed no metastases on either (18)F-FDG PET-CT or MRI. Forty-one skeletal metastases (39%) detected with MRI did not show increased (18)F-FDG uptake on (18)F-FDG PET-CT (false-negative). Lesion-based sensitivities and specificities were 62% (95%CI 52–71%) and 100% (48–100%) for (18)F-FDG PET-CT; and 99% (97–100%) and 100% (48–100%) for MRI respectively. Bone lesions were more likely to be false-negative on (18)F-FDG PET-CT if hematopoietic bone marrow extension was widespread and active (p = 0.001), during or after (neo)-adjuvant treatment (p = 0.001) or when the lesion was smaller than 10 mm (p < 0.001). CONCLUSION: Although no definite conclusions can be drawn from this small retrospective study, it shows that caution is needed when using (18)F-FDG PET-CT for diagnosing skeletal metastases in Ewing sarcoma. Poor contrast between metastases and active hematopoietic bone marrow, chemotherapeutic treatment, and/or small size significantly decrease the diagnostic yield of (18)F-FDG PET-CT, but not of MRI.