Submitting Author: Hasserjian, Robert Paul, MD
Institution: Massachusetts General Hospital
Additional authors:Allison Carey MD PhD, Nancy Lee Harris MD
Session: T Lymphoblastic Leukemia/Lymphoma

45 year old man with no past medical history presented with rapidly progressive shortness of breath, fatigue, and dry cough. CT showed a 12 cm anterior superior complex mediastinal mass with cystic components, as well as a large right pleural effusion with pleural thickening, radiologically consistent with thymoma. Thoracentesis (1500 ml of bloody fluid) revealed atypical keratin-positive epithelial cells in a background of numerous small lymphocytes; with the following flow cytometry immunophenotype: CD2+CD3variable+CD5+CD7+CD4+CD8+CD10dim+TdT+. The effusion rapidly recurred; a core needle biopsy of the mediastinal mass was performed and treatment was begun with doxorubicin, cisplatin, and cyclophosphamide for a presumed diagnosis of Stage IV thymoma. On the limited core biopsy, relatively few keratin positive cells were present in a background of small lymphocytes which were CD3+ and TdT+ with a Ki67 proliferation fraction of 80%, raising the question of a T-lymphoblastic lymphoma/leukemia (T-LBL/ALL). A bone marrow biopsy was normal. To clarify the diagnosis, mediastinoscopic biopsies of the mass were performed one week after chemotherapy administration and pleural fluid was sent for cytology, flow cytometry and cytogenetics.

Biopsies were taken from the mediastinal mass, thickened right pleura, and tissue adjacent to the vena cava. Biopsies were fixed in B+ fixative and formalin. All biopsies were densely fibrotic with variably necrotic sheets of small lymphocytes, admixed with histiocytes. No distinct epithelial cell population was identified. The pleural fluid cytology revealed a mixed population of small lymphocytes and large epithelioid cells.

On immunostains of the mediastinal tissue, only a few scattered clustered keratin+ cells were seen in fibrous areas in the pleural and para-vena caval biopsies; the viable sheet-like areas of lymphocytes were devoid of keratin+ cells. The lymphocytes were CD3+, CD1a+, and CD10+, but were TdT negative.

Flow cytometry of the pleural fluid demonstrated 80% immature T cells: CD3+CD2+CD5+CD7+CD4+CD8+CD1a+TdT+CD10dimCD34-. Immunostains subsequently performed on the original pleural fluid cell block reveal that the epithelioid cells were keratin+, calretinin+, WT1+, CK+, and negative for D240, p63, and BerEP4, consistent with mesothelial cells rather than neoplastic thymic epithelial cells.

Cytogenetics on pleural fluid revealed 47,XY,del(7)(p13),t(7;9)(q34;q34),+20 abnormality in all 10 metaphases analyzed, consistent with a TCRB-NOTCH1 rearrangement.

PCR study of the both the thymus core biopsy tissue and the pleural fluid demonstrated a clonal TCR gamma rearrangement with the Vgamma1-8 primers.

This case illustrates the sometimes difficult differential diagnosis between cortical thymoma and T-LBL presenting as a mediastinal mass. This differential was particularly difficult because of the radiologic appearance simulating a thymoma with pleural spread, the presence of some keratin+ epithelioid cells in the background, and compromise of diagnostic biopsies by small size, fibrosis, and post-therapy changes (including loss of TdT staining by immunohistochemistry). An important clue to the diagnosis of T-LBL/ALL was the presence of immature T cells outside of the confines of epithelial cells, both in pleural fluid and infiltrating fat in the thymic core and mediastinoscopic biopsies. The diagnosis was confirmed by the immunoprofile of the epithelial cells in the pleural fluid (reactive mesothelial cells), confirmation of T-cell clonality, and demonstration of the t(7;9)(q34;q34) TCRB-NOTCH1 rearrangement. Although this activating NOTCH1 translocation is exceedingly rare in T-ALL/LBL (<1% of all cases), most T-LBL/ALL cases have NOTCH1 point mutations (Weng AP et al Science 2004;306:269-271), a finding that ultimately originated from the cloning of the NOTCH1 gene at the t(7;9) site in a case of T-ALL (Ellisen LW et al. Cell 1991;66:649-661).

T-lymphoblastic lymphoma with t(7;9)(q34;q34) TCRB-NOTCH1 rearrangement

T-lymphoblastic lymphoma with t(7;9)(q34;q34); TCRB-NOTCH1

Case 373