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

Abstract

Objective: To determine the level of discrepancy between magnetic resonance imaging (MRI) and ¹⁸F-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 ¹⁸F-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 ¹⁸F-FDG PET-CT or MRI. Forty-one skeletal metastases (39%) detected with MRI did not show increased ¹⁸F-FDG uptake on ¹⁸F-FDG PET-CT (false-negative). Lesion-based sensitivities and specificities were 62% (95%CI 52-71%) and 100% (48-100%) for ¹⁸F-FDG PET-CT; and 99% (97-100%) and 100% (48-100%) for MRI respectively. Bone lesions were more likely to be false-negative on ¹⁸F-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 ¹⁸F-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 ¹⁸F-FDG PET-CT, but not of MRI.

Keywords: Diagnosis; Ewing sarcoma; Magnetic resonance imaging; Osseous metastasis; Positron emission tomography with computerized tomography.

Fig. 1

Flowchart of the inclusion process

Fig. 2

False-positive lesions on ¹⁸F-FDG PET-CT. A 19-year-old woman diagnosed with localized Ewing sarcoma of the seventh rib. Six months after initial treatment consisting of six courses of vincristine, ifosfamide, doxorubicin, and etoposide (VIDE) chemotherapy, 8 courses of vincristine, actinomycin-D, and ifosfamide (VAI) chemotherapy, radiation therapy and surgery, imaging was performed because of chest pain, with local recurrence suspected. a¹⁸F-FDG PET-CT showed two lesions with ¹⁸FDG-uptake in both femora of which the true nature could not be clearly defined; based on the information available they were classified as inconclusive (positive). b and c Low-dose CT images in the transverse and coronal planes of the suspected bone lesions showing sclerosis. d MRI T1- and T2-weighted images show bilateral bone infarctions and no sign of malignancy

Fig. 3

False-negative lesions on ¹⁸F-FDG PET-CT with widespread hematopoietic bone marrow activity. A 23-year-old man diagnosed with localized Ewing sarcoma of the right proximal tibia. Images obtained 6 months after initial treatment (6 × VIDE, surgery, 8 × VAI), at this time undergoing second-line chemotherapy because of recent distant metastasis. a¹⁸F-FDG PET-CT with symmetrical ¹⁸F-FDG uptake in the axial skeleton and proximal extremities. This was classified benign (negative) owing to anemia or recent chemotherapy. b T1-weighted short tau inversion recovery (STIR) MRI images with multifocal metastatic lesions throughout the whole axial skeleton. c STIR images with several skeletal metastases in the left and right ilium and fifth lumbar vertebral body. d T1-weighted turbo spin echo (TSE) images with several skeletal metastases in the left and right ilium and fifth lumbar vertebral body

Fig. 4

False-negative lesions on ¹⁸F-FDG PET-CT. A 17-year-old boy diagnosed with localized Ewing sarcoma of the distal tibia. Images obtained 1 year after finishing treatment (6 × VIDE, amputation, 8 × VAI). a¹⁸F-FDG PET-CT showing no increased ¹⁸F-FDG-uptake at the glenoid of the right shoulder. b T1-weighted (left) and STIR (right) images showing a small nodule (arrow) with a high degree of suspicion for metastasis at the glenoid of the right shoulder. c¹⁸F-FDG PET-CT showing no increased ¹⁸F-FDG-uptake or lytic changes on low-dose CT at the glenoid of the left shoulder. d T1-weighted (left) and STIR (right) images showing a nodule (arrow) with a high degree of suspicion for metastasis at the glenoid of the left shoulder. e¹⁸F-FDG PET-CT showing no increased ¹⁸F-FDG-uptake or lytic changes on low-dose CT at the left proximal tibia and distal femur. f T1-weighted (left) and STIR (right) images showing two nodules with a high degree of suspicion for metastasis at the left proximal tibia and distal femur

Fig. 5

False-negative lesions on ¹⁸F-FDG PET-CT (arrows). A 23-year-old man presenting with metastatic Ewing sarcoma of the right seventh rib. Images obtained at diagnosis, before the start of treatment. a¹⁸F-FDG PET-CT showing increased ¹⁸F-FDG-uptake at the eleventh thoracic vertebrae only. b T1-weighted (left) and STIR (right) images showing nodules with a high degree of suspicion for metastasis at the tenth, eleventh, and twelfth thoracic vertebrae and the third and fifth lumbar vertebrae