Prospective comparison of 18F-FDG PET/CT and conventional modalities for staging of Ewing sarcoma family of tumours

Introduction: The Ewing sarcoma family of tumours (ESFT) consists of a group of tumours characterized by morphologically similar round-cell neoplasm and by the presence of a common chromosomal translocation. The presence or absence of metastasis at the time of staging predominantly determines the strategy for management, with approximately 25% of patients having detectable metastases at diagnosis. FDG PET/CT provides unique information on the metabolic activity of the tumour and serves as a whole-body screening tool. We aimed to evaluate the role of ¹⁸F-FDG PET/CT in the staging of ESFT as compared to conventional modalities which included ^99mTc-MDPskeletal scintigraphy (SSC), CT chest and bone marrow aspiration/ biopsy (BMAB).

Methods: Patients with a histopathological diagnosis of ESFT, who were to be evaluated for staging were prospectively enrolled. Patients underwent ¹⁸F-FDG PET/CT, ^99mTc-MDPskeletal scintigraphy, CT chest and bone marrow aspiration/ biopsy. PET/CT images were interpreted by two experienced nuclear medicine physicians. Scans were visually inspected for the presence of abnormal radiotracer uptake distinguishable from background activity not located in a site of increased physiological uptake. Metastatic lesions on PET/CT were localised to the bone/bone marrow (BM), lung and/or pleura and lymph nodes. Patients were followed-up clinically and using imaging (PET/CT, CT or SSC), for a minimum period of 6 months, to establish a reference standard.

Results: The final analysis was performed on 33 patients (Median age= 13 years, IQR=11), which included 22 males (66.66%) and 11 females (33.33%). The primary disease was noted to be originating from the skeleton in 17 patients (51.51%), 9 patients (27.27%) had extra-skeletal primary sites, while 7 patients (21.21%) were suffering from Askin tumours of the thoracopulmonary region. After the initial staging, 21 (63.63%) patients were determined to have metastatic disease and 12 (36.36%) were deemed as non-metastatic. Lungs were the most common site of metastasis, followed by bone/ bone marrow and lymph nodes, being detected in 11 (33.33%), 10 (30.3%) and 10 (30.3%) patients respectively. 6 patients had only lung metastasis, while 4 patients had only lymph node metastasis. Only bone/BM involvement was present in 3 patients. FDG PET/CT detected true bone/BM metastases in 9 patients with a sensitivity, specificity and accuracy of 90%, 100% and 96.96% respectively. SSC was performed in 31 patients and it identified true bone/BM metastases in 6 patients with sensitivity, specificity and accuracy of 60%, 95.24% and 83.87% respectively. BMAB was performed in 29 patients and was positive only in 1 patient and was false negative in the 8 patients who had bone/BM metastatic sites detected on PET/CT. FDG PET/CT detected lung metastases in 11 patients with sensitivity, specificity and accuracy of 100%, 90.91% and 93.94% respectively. CT chest was performed in 28 patients and it identified 7 patients with lung metastases with sensitivity, specificity and accuracy of 100%, 75% and 88.46% respectively. FDG PET/CT also detected lymph node metastases in 10 patients with sensitivity, specificity and accuracy of 100%, 91.30% and 93.94% respectively.

Conclusions: As compared to skeletal scintigraphy, FDG PET/CT provided a diagnostic benefit in 4 out of 31 (12.90%, p<0.001) patients. In comparison with bone marrow aspiration and biopsy, FDG PET/CT provided a diagnostic benefit in 8 out of 29 (27.59%, p<0.001) patients. In the detection of pulmonary metastases, FDG PET/CT was more specific and accurate as compared to CT and rightly identified all patients with pulmonary involvement detected on CT. PET/CT also detected unsuspected sites of lymph node metastases.

FDG PET/CT can serve as a comprehensive imaging tool and can obviate the need for further investigations, that may be invasive or incur additional radiation burden to the patient, if used in the staging of Ewing sarcoma family of tumours.