Mediastinal Gray-Zone Lymphoma: Still an Open Issue

The concept of gray-zone lymphoma (GZL) has been progressively refined since its introduction in the literature in 1998. For several years, it was applied to a rather broad spectrum of conditions, posing the problem of the differential diagnosis between any type of Hodgkin lymphoma (HL) and diffuse large B-cell lymphoma, with special reference to primary mediastinal forms (PMBL). Officially recognised as a provisional entity in the 4th and revised 4th editions of the WHO Classification of Tumour of Haematopoietic and Lymphoid Tissues with the term “B-cell lymphoma unclassifiable with features intermediate between diffuse large B-cell lymphoma and classic Hodgkin lymphoma”, it was limited to tumours showing either morphologic features reminiscent of classic HL (CHL) but carrying a complete B-cell phenotype or conversely provided with a PMBL morphology yet revealing CHL phenotypic characteristics. The definition of GZL has been further revised in the recently published International Lymphoma Classification and 5th edition of the WHO Classification of Haematolymphoid Tumours, which have limited it to mediastinal neoplasms (MGZL) based on emerging molecular evidence. The aim of this review is to critically discuss the issue of MGZL, as well as in light of the suboptimal response to current therapies.