Thymic Carcinoma Presenting as Diffuse Bone Marrow Infiltration Mimicking Lymphoma.

Thymic epithelial tumors (TETs) represent the most common primary neoplasms of the anterior mediastinum, yet thymic carcinoma (TC) stands out as an exceptionally rare and biologically aggressive subtype. Unlike thymomas, TCs exhibit marked cytologic atypia, early invasive potential, and a high predisposition for extrathoracic metastasis. We report the case of a 59-year-old male who presented with a seven-month history of progressive, refractory lumbar pain and functional limitation. Initial clinical and laboratory evaluation revealed normocytic anemia, leukocytosis, elevated inflammatory markers, and increased lactate dehydrogenase and beta-2 microglobulin levels. These findings, combined with systemic bone marrow infiltration observed on spinal magnetic resonance imaging, initially suggested a lymphoproliferative disorder. A subsequent thoracic computed tomography scan identified an anterior mediastinal mass with lymphadenopathy. A bone marrow biopsy evidenced infiltration by a poorly differentiated malignant neoplasm, and an immunohistochemical profile of the mediastinal mass confirmed the diagnosis of stage IVb TC, showing positivity for pan-cytokeratin, tumor protein p63, cluster of differentiation 5 (CD5), epithelial membrane antigen (EMA), and cluster of differentiation 117 (CD117), with a Ki-67 proliferation index of 20%, while remaining negative for hematolymphoid, neuroendocrine, and lung-specific markers. The patient was managed through a multidisciplinary approach with systemic palliative chemotherapy. This case highlights a critical diagnostic challenge where TC mimicked a hematological malignancy. The systemic dissemination of TC, particularly the diffuse bone marrow infiltration, is a rare clinical phenotype that obscures the primary mediastinal origin. Our report emphasizes the necessity of including TC in the differential diagnosis of patients presenting with refractory bone pain and systemic hematological abnormalities, even in the absence of respiratory symptoms. Early diagnostic recognition through strategic immunohistochemical profiling is imperative to avoid clinical bias, prevent the initiation of inappropriate therapies, and facilitate a prompt, evidence-based management strategy for patients with advanced, non-resectable disease.