Role of 18F-FDG-PET/CT in the staging of metastatic rhabdomyosarcoma: A report from the European paediatric Soft tissue sarcoma Study Group

27 September 2021

The largest comparison study between PET/CT and the standard radiology workup in Rhabdomyosarcoma

The aim of this study was to evaluate the ability of 18F-FDG-PET/CT in the staging of metastatic rhabdomyosarcoma patients, compared to the standard radiology workup (SRW) and to bone marrow aspirates/bone marrow biopsies.
Thanks to the great collaboration of the EpSSG centers, the reports of 118 of 121 eligible patients for this study, enrolled in the EpSSG RMS 2008 protocol, were obtained.
The imaging (18F-FDG-PET/CT and MRI/CT-scan/bone scan) reports were systematically reviewed by two authors supported by a nuclear medicine doctor; the number and sites of metastatic lesions detected by SRW and 18F-FDG-PET/CT were noted. The sensitivity of the different investigation methods was calculated using as reference the final clinical interpretation by the treating physician, i.e. if the lesion demonstrated by the radiological investigations were considered, and treated, as a metastasis or not.
In 4 patients (3.4%), 18F-FDG-PET/CT changed the staging from localized to metastatic disease and revealed a greater number of sites of metastasis (mean 1.94/patients, compared to 1.72/patients of SRW).
18F-FDG-PET/CT showed a higher sensitivity than SRW in recognizing locoregional (96.2% vs. 78.5%, p-value = 0.0013) and distant (94.8% vs. 79.3%, p-value = 0.0242) nodal involvement.
Conversely, the sensitivity was lower for intrathoracic sites (lung 79.6% vs. 100%, p-value=0.0025).
For bone metastasis, 18F-FDG-PET/CT was more sensitive than bone scintigraphy (96.4% vs. 67.9%, p-value=0.0116). The 18F-FDG-PET/CT sensitivity and specificity to detect marrow involvement were 91.8% and 93.8%, respectively.
These findings lead to important conclusions: first, the greater sensitivity of 18F-FDG-PET/CT compared to other radiological techniques, in particular for lymph node involvement (frequent site of involvement in rhabdomyosarcoma), reinforces the recommendations for its use in initial diagnostics; second, for the study of bone involvement, if 18F-FDG-PET/CT is available can replace the bone scan; third, chest-CT remains essential to detect lesions in intrathoracic sites, which can be performed in a one stop-shot routine examination or on a dedicated chest-CT scan.
18F-FDG-PET/CT therefore plays an important role in rhabdomyosarcoma and will be the subject of further studies in the new EpSSG FaR-RMS protocol.
Author: Federico Mercolini and EpSSG colleagues