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TOPIC: Lung Pathology TYPE: Fellow Case Reports INTRODUCTION: We present a diagnostic challenging case of spindle cell thymoma mimicking a left lung upper lobe mass along the mediastinal pleura. CASE PRESENTATION: A 54-year-old female is hospitalized with hypoxic respiratory failure due to COVID 19 pneumonia. Patient developed ARDS, pneumomediastinum, and septic shock with secondary bacterial infection requiring mechanical ventilator support and antibiotic therapy. Patient had a prolonged complicated hospital course with multiorgan failure including kidney injury, lacunar stroke and developed severe critical illness myopathy. Patient underwent tracheostomy and was discharge to long term acute care facility to wean from the ventilator. Due to persistent need of supplemental oxygen and intermittent tachycardia, chest X ray was performed which showed alveolar and interstitial infiltrates. Subsequently, CT angiogram chest was performed which showed no pulmonary embolism but a 7 cm left lung upper lobe mass extending from hilum along the mediastinum. Positron emission tomography (PET) performed showed minimal uptake in the mass without enhanced uptake in any other location. Percutaneous biopsy was pursued which showed numerous oval spindle cells clusters with rare lymphocyte suggestive of spindle cell thymoma. During rehabilitation, patient was later diagnosed with myasthenia gravis and planned to undergo thymectomy after functional status of the patient improves. DISCUSSION: Thymomas are the most common epithelial cell tumor of the anterior mediastinum composed of neoplastic epithelial cells with a variable number of lymphocytes which determines various histologic subtypes of thymic neoplasms based on Worl Health Organization (WHO) classification. Histologic subtyping of thymoma need complete resection of tumor but core biopsy can differentiate thymoma from carcinoma. Our patient had WHO type A spindle cell thymoma which is composed of fusiform or oval spindle cells with rare lymphocytes. It accounts for 22% of all thymomas with no gender predilection and mean age of diagnosis is sixth decade. Thymomas are commonly located in prevascular anterior mediastinum but, rarely, as described in our patient it can present as hilar mass extending along the mediastinal pleural into the left upper lobe which can mimic lung tumor. Prognosis is based on resectability of tumor, staging and histologic features. CONCLUSIONS: Thymomas can mimic lung malignancy on imaging studies and need histologic exam to confirm the diagnosis. REFERENCE #1: The histomorphologic spectrum of spindle cell thymoma. Annikka Weissferdt MD, FRCPath⁎, Cesar A. Moran MD, PMID: 23260329 DISCLOSURES: No relevant relationships by Yusra Ansari, source=Web Response No relevant relationships by Priyanka Bhat, source=Web Response No relevant relationships by Tahir Muhammad Abdullah Khan, source=Web Response No relevant relationships by Karan Singh, source=Web Response