Functional Imaging Using 18-Fluorodeoxyglucose PET in the Management of Primary Mediastinal Large B-Cell Lymphoma: The Contributions of the International Extranodal Lymphoma Study Group

Primary mediastinal large B-cell lymphoma (PMLBCL) is recognized as a distinct disease entity. Treatment outcomes appear better than in other diffuse large B-cell lymphoma (DLBCL) types, partly because of their earlier stage at presentation and the younger age of most patients. If initial treatment fails, however, the results of salvage chemotherapy and myeloablative treatment are poor. The need to avoid relapses after initial therapy has led to controversy over the extent of front-line therapy, particularly whether consolidation radiotherapy to the mediastinum is always required and whether the 18-fluorodeoxyglucose (18F-FDG) uptake detected by PET-CT scan can be used to determine its requirements. Functional imaging using PET-CT generally allows distinguishing of residual mediastinal masses containing active lymphoma from those with only sclerotic material remaining. The International Extranodal Lymphoma Study Group (IELSG) conducted the prospective IELSG-26 study, which showed that a five-point visual scale can be used to define metabolic response after immunochemotherapy and that a cut point based on liver uptake discriminates effectively between high or low risk of failure, with 5-year progression-free survival (PFS) of 99% versus 68% and 5-year overall survival (OS) of 100% versus 83%. This study also showed that a baseline quantitative PET parameter, namely the total lesion glycolysis describing the metabolic tumor burden, can be a powerful predictor of PMLBCL outcomes and warrants further validation as a biomarker. The ongoing IELSG-37 randomized study addresses the need for consolidation mediastinal radiotherapy in patients in whom a complete metabolic response (CMR) can be seen on PET scans after standard immunochemotherapy.