A unique case of thymic indolent T-lymphoblastic proliferation: Redefining the extrathymic boundaries

Per the WHO Classification of Haematolymphoid Tumours (5th edition), indolent T-lymphoblastic proliferation (iT-LBP) is defined as an extrathymic non-clonal proliferation of T lymphoblasts that typically presents as isolated lymphadenopathy or an isolated tumor mass in an otherwise healthy individual. Unlike T-lymphoblastic leukemia/lymphoma (T-ALL/LBL), an aggressive hematologic malignancy characterized by monoclonal T-lymphoblasts and requiring treatment with chemotherapy, iT-LBP is an indolent process that does not require any specific treatment. We report a case of an indolent T-lymphoblastic proliferation with subsequently diagnosed Graves' disease presenting as a mediastinal mass in a young, otherwise healthy woman. Consistent with this diagnosis, she has had a benign clinical course. To the best of our knowledge, this is the first reported case of an iT-LBP-like entity in the thymus, suggesting an expansion of the definition of iT-LBP. Therefore, it is critical for pathologists to be aware that this entity can involve the thymus and not mistake it for other malignancies, especially T-ALL/LBL, given the significant differences in prognosis and management.