Case Report: Flow cytometric differential diagnosis of a peripheral T-cell lymphoma, NOS with complete loss of CD45 and dim expression of CD3.

Peripheral T-cell lymphomas (PTCLs) are a group of non-Hodgkin lymphomas originating from mature T-lymphocytes. Despite encompassing several well-defined entities, about 25% of the PTCLs do not fulfill the requirements of any of the subcategories. These diseases are classified as PTCL, not otherwise specified (PTCL, NOS), and often associated with poor prognosis. Hereby we present a case of a female patient, diagnosed with PTCL, NOS from her skin biopsy specimen. Besides histology and immunohistochemistry, flow cytometry was used for phenotyping and staging (peripheral blood, bone marrow). Pathologic T-cells were found in all the investigated tissues, with a very unusual CD45 negative and surface CD3 dim immunophenotype. For proper differential diagnosis, we determined several markers with immunohistochemistry (CD3, CD4, CD7, CD8, CD30, PD1, Ki-67) and flow cytometry: (CD2, cytoplasmic CD3, surface CD3, CD4, CD5, CD7, CD8, CD9, CD10, CD19, CD20, CD26, CD34, CD38, CD45, CD48, CD56, CD99, CD123, surface TRBC1, cytosplasmic TRBC1, surface TRBC2, cytoplasmic TRBC2, MPO, TdT, Igκ, Igλ). Here we discuss the difficulties of the differential diagnostic process and highlight some potential pitfalls of flow cytometric analysis of the pathologic T-cells with such a rare immunophenotype. Despite several determined markers, the disease characteristics did not meet the criteria of any PTCL subtype, therefore the diagnosis remained PTCL, NOS. Due to the aggressive course of the disease, we lost the patient within 1 year after the diagnosis.