LMO2 is a useful marker in distinguishing between indolent t-lymphoblastic proliferations and t-lymphoblastic leukemia/lymphoma.

Abstract

Background: Indolent T-lymphoblastic proliferations (iT-LBP) are reactive expansions of TdT+ T-cells outside of thymic tissue which may be erroneously misdiagnosed as T-lymphoblastic lymphoma (T-LBL). This is in part due to the complex morphologic, immunophenotypic and clinical overlap that can occur between these entities. A singular definitive method for addressing the diagnostic quandary of iT-LBP versus T-LBL is lacking. However, in recent years, LMO2 has been described as a positive marker of T-LBL but consistently negative in reactive immature thymocytes. Design(s): We searched our pathology archives for cases of indolent T-lymphoblastic proliferations and identified 7 with adequate material for further immunohistochemical studies. 3 new cases that have not been described in the literature to date were also identified. We performed immunohistochemical staining for LMO2 on these cases with comparison to LMO2 staining patterns on 12 cases of T-LBL. Result(s): All cases of iT-LBP were negative for LMO2 (0/7) whereas all cases of T-LBL showed expression of LMO2 (12/12) (p-value < 0.001; Fisher exact test). We additionally describe 3 new cases of iTLBP which show typical features of iT-LBP in expression of CD4 and CD8, and TdT, absence of CD34, as well as non-clonal TCR studies and clinical indolence without mediastinal adenopathy. One case was seen in a patient with lymphoid architecture showing Castleman-like features. Conclusion(s): LMO2 is a diagnostically useful marker for differentiating between iT-LBP and T-LBL. As previously described, iTLBP typically express CD4 and CD8 and lack CD34. The absence of mediastinal enlargement is also a characteristic feature of this entity which has not been focused on previously.