B-Acute Lymphoblastic Leukemia/Lymphoblastic Lymphoma Mimicking Fibrosing Mediastinitis: A Case Report and Diagnostic Insight

BACKGROUND Fibrosing mediastinitis (FM) is a rare, fibroproliferative disorder within the mediastinum. It is extremely rare for hematologic malignancies to develop as FM. CASE REPORT A 32-year-old Japanese man with a 1-month history of headache and 2-week history of facial swelling underwent chest computed tomography (CT); a diffuse mass-like lesion was revealed in the anterior mediastinum with severe stenosis of vital mediastinal organs. After a surgical biopsy, an initial diagnosis of idiopathic FM was made. The FM lesions responded mildly to corticosteroids but recurred repeatedly. Sixteen months after the treatment initiation, blasts appeared in the peripheral blood (PB), and the patient was diagnosed with B-acute lymphoblastic leukemia/lymphoblastic lymphoma (B-ALL/LBL). Chemotherapy led to complete remission of the B-ALL/LBL and almost complete disappearance of FM-like lesions. Immunohistochemistry of the mediastinal biopsy specimen taken before the blasts' appearance in PB demonstrated a CD34/CD7/terminal deoxynucleotidyl transferase-positive population, an identical pattern of expression common to the blasts in the patient's PB and bone marrow. CONCLUSIONS This is the first case report of B-ALL/LBL presenting as FM. This case underscores the importance of considering the possibility of latent hematologic malignancy even in the absence of new symptoms other than those caused by FM lesions for a long period of time. This is the first demonstration that leukemia cells may be present in the FM lesions from the initial stage of disease onset. Even if a diagnosis of idiopathic FM is confirmed, continued suspicion of the presence of hematologic malignancy is vital for improving patient outcomes.