Difficult diagnostic and therapeutic cases: case 1 .true thymic hyperplasia in patient treated for T-cell lymphoma.

A 24-year-old woman presented with rapidly enlarging supraclavicular lymphadenopathy. Biopsy showed complete effacement of the lymph node by a neoplastic population morphologically consistent with lymphoma/leukemia of precursor T-lymphocytes (so-called T-cell lymphoblastic lymphoma/leukemia). The phenotypic profile was as follows: CD34+, TdT+, CD3+, CD1a−, CD4−, CD8−, and Ki-67/MIB-1 95%. No involvement of the bone marrow and peripheral blood was found. She underwent chemotherapy according to the L2 protocol, followed by autologous bone marrow transplantation, which produced complete remission. Six months later, a follow-up computed tomography scan displayed a newly developed mass in the anterior-superior mediastinum that was regarded as suspicious for disease relapse (Fig 1). Given that no other signs and symptoms of malignant lymphoma were present, the mediastinal tumor was surgically removed. On gross examination, it was an encapsulated bilobate mass that measured 11 × 8 × 3.5 cm and weighed 180 g. At light microscopy, the mass consisted of normal thymic tissue with typical lobular architecture, Hassall's corpuscles, and well-defined cortical and medullary compartments (Fig 2, low power with cortex on left and medulla with Hassall's corpuscles on right; Fig 3, high power, with cortical thymocytes and admixed macrophages and epithelial cells). Immunohistochemistry on paraffin sections were carried out by the alkaline phosphatase-antialkaline phosphatase complexes technique [1] with application of specific antibodies against cytokeratins, CD1a, CD2, CD3, CD4, CD8, CD20, CD30, Bcl-2 oncogene product, Bcl-6 protein, TdT, Oct-1, Oct-2, BOB.1, and Ki-67, which showed the expected positivities both in the cortex and medulla [2,3] for true thymic hyperplasia (TTH). Most, if not all, cortical thymocytes did express the proliferation-associated nuclear antigen Ki-67. Interestingly, no lymphoid follicles were observed, in contrast to the thymic lymphoid hyperplasia occurring in patients with myasthenia gravis and other autoimmune disorders [4]. Twenty-eight months after the onset of disease, the patient died of acute respiratory failure as a result of Cytomegalovirus pneumonitis. Unfortunately, the permission for autopsy was denied.