Incremental value of ¹⁸F-FDG PET/CT over conventional imaging in the initial staging of pediatric Ewing sarcoma

Objectives: Ewing sarcoma, a rare tumor of neuroectodermal origin, arising from bone or soft tissue typically affects children and young adults and is one of the tumors bearing extremely poor prognosis. Since the prognosis heavily depends on initial staging, accurate staging of the tumor at diagnosis is of paramount importance. Conventional imaging such as CT and MRI and skeletal scintigraphy are some of the standard diagnostic tests employed for disease staging. ¹⁸F-FDG PET/CT, presently an optional diagnostic modality in Ewing sarcoma has the advantage of whole-body scanning and the combination of functional and anatomic imaging. The present study was conducted to evaluate the incremental value, if any of performing ¹⁸F-FDG PET/CT in pediatric patients with Ewing sarcoma over and above the conventional imaging techniques.

Methods: Records of 89 patients diagnosed with Ewing sarcoma who had undergone ¹⁸F-FDG PET/CT were retrospectively reviewed. Out of these, 23 were included as per the inclusion criteria: age <19 years at diagnosis, histopathologic proven Ewing sarcoma, PET/CT performed prior to any treatment institution, availability of data from other imaging (X-ray, Ultrasound, CT, MRI, Bone scan). The follow-up records of these patients were also reviewed pertaining to further imaging done after PET/CT, sampling of the suspicious lesions for histopathologic examination and clinical findings.

Results: 23 patients (18 boys, 5 girls) with mean age 13.8±4.4 years (range: 4-19) with histopathologic confirmed Ewing sarcoma, were subjected to a total of 39 imaging studies (15 - MRI, 13 - CT, 6 - X-ray, 3 - Ultrasound, 2 - Bone scan) prior to ¹⁸F-FDG PET/CT. 19/23 (82.6%) had Ewing sarcoma with skeletal origin while 4/23 had extra-osseous Ewing sarcoma. ¹⁸F-FDG PET/CT was accurate for localization of the primary site in all the patients (mean SUVmax of primary lesion: 8.2±3.1), while CT was false negative in one and MRI false negative in identification of the primary site in two patients. CT was reported as normal in one patient which was accurately detected by MRI and ¹⁸F-FDG PET/CT. MRI mis-labelled primary lesions in two patients as chronic osteomyelitis and benign cortical sclerosis. ¹⁸F-FDG PET/CT showed good concordance with CT (92.3%) and MRI (86.6%) in primary disease localization. ¹⁸F-FDG PET/CT detected additional lesions in 14/23 patients (60.8%) including lung nodules, soft tissue deposits, nodal lesions and skeletal lesions. Clinical follow-up, further imaging with MRI, HRCT Chest and histopathology of the additional lesions detected on PET/CT confirmed metastatic disease in 11/14 patients (78.6%) while the findings were false positive in 3/14 patients (21.4%). Overall, ¹⁸F-FDG PET/CT could show presence of metastatic disease in 11/23 (47.8%) patients.

Conclusions: ¹⁸F-FDG PET/CT performed better than conventional imaging in disease localization at primary and metastatic sites. Since, accurate initial staging is of paramount importance in terms of deciding further management and in prognostication, ¹⁸F-FDG PET/CT should be considered among the primary investigative modality in the pediatric patients with Ewing sarcoma wherever a high suspicion of metastatic disease exists.