A RIDDLE, WRAPPED IN A MYSTERY: MONOCLONAL B LYMPHOCYTOSIS DISCOVERED DURING THE EVALUATION OF A MEDIASTINAL INCIDENTALOMA

TOPIC: Disorders of the Mediastinum TYPE: Medical Student/Resident Case Reports INTRODUCTION: The work-up anterior mediastinal masses (AMM) often warrants consideration malignant etiology. Certain imaging findings may reinforce need for diagnostic biopsy. Moreover, while AMMs disseminate to peripheral blood, incidental hematogenous malignancies also harbor in blood supply non-malignant AMM. CASE PRESENTATION: A 58-year-old female never-smoker was referred our quaternary center multidisciplinary lung mass team an incidentally discovered AMM found on cervical imaging. Physical examination normal, work revealed minimally elevated metamyelocytes and myelocytes, absolute increase immature granulocytes. Thoracic CT elongated, lobulated low-density cystic located within AP window, measured 2.1 x 8.6 1.8 cm without calcification or solid nodularity. Contrast-enhanced MRI thorax be T2 hyperintense, T1 hypointense, with no suspicious enhancing components. There evidence lymphadenopathy intrathoracic metastases.Due abnormal cell differential, EBUS-TBNA performed. Flow cytometry a left hilar lymph node station minute population CD5, CD19, CD20 positive lymphocytes kappa restricted B cells. Additional tissue required definitive diagnosis; this obtained via robotic video-assisted thoracoscopic excisional biopsy Pathological confirmed presence multilocular thymic cyst. persistence low-level monoclonal B-cell population. diagnosis Monoclonal Lymphocytosis low count made. She scheduled long term clinical radiographic follow up. DISCUSSION: While most are benign, more likely malignant. Approximately 25% due lymphoma[1]. For reason, even setting benign appearing findings, hematological abnormalities (eg. metamyelocytes, myelocytes) should prompt further testing evaluate component. In case, initial incidentaloma secondary - Lymphocytosis. (< 500 cells per uL) rarely progress CLL, 1-2% high (> variants can yearly. only 1.8% from high, these clones persist over decades, represent marker impaired immune system[2]. CONCLUSIONS: Imaging hematologic pathological AMMs. Importantly, proliferations confound proper interpretation biopsies. Due percentage lymphoma, lymphocytosis is important etiology consider, attention subtype translate into benefits additional monitoring. REFERENCE #1: Duwe BV, Sterman DH, Musani AI. Tumors mediastinum. Chest. 2005 Oct;128(4):2893-909. doi:10.1378/chest.128.4.2893. #2: Criado I, Rodríguez-Caballero A, Gutiérrez ML, et al. Low-count persists after seven years up associated poorer outcome. Haematologica. 2018;103(7):1198-1208. doi:10.3324/haematol.2017.183954 DISCLOSURES: No relevant relationships by Gustavo Cumbo-Nacheli, source=Web Response Geoffrey Lam, Ranuka Sinniah, Ravi Kanth Velagapudi,