A80-3-15 A Rare Case of Type A Thymoma Presenting as Massive Pleural Effusion With Pleural Seeding

Introduction Thymomas are uncommon anterior mediastinal tumors, typically asymptomatic or incidentally detected. Presentation with massive pleural effusion and pleural seeding is exceedingly rare. Among effusion-associated thymomas, most reported subtypes are WHO Type B2/B3 or mixed histology; pleural dissemination in pure Type A (spindle-cell) thymoma is exceptional. This case emphasizes the diagnostic and pathological nuances of a benign-appearing thymoma with invasive features. Case Description A 58-year-old female presented with cough with scanty sputum since 2 months, progressive dyspnea (Grade II-III) since 1 month, and pleuritic chest pain since 1 month. General examination: Pallor present; no icterus, cyanosis, clubbing, or pedal edema. Respiratory examination: Stony dullness on percussion and absent breath sounds over the left hemithorax. Chest X-ray - massive left-sided pleural effusion. Ultrasound - free fluid in the left pleural cavity. CECT thorax - anterior mediastinal soft-tissue mass with multiple hyperdense pleural nodules, suggesting pleural seeding. Pleural fluid analysis: Lymphocyte-predominant, exudative (ADA 12.4 U/L), cytology negative for malignant cells and AFB. CT-guided core biopsy: Spindle-cell thymoma (WHO Type A) with focal necrosis and mild nuclear atypia. Immunohistochemistry: Positive for p63, CK7, CD20 (in background lymphocytes), and Ki-67, confirming thymic epithelial origin. The patient underwent drainage and was referred for definitive surgical excision with adjuvant therapy Discussion and Novelty Although thymomas with pleural dissemination or effusion have been described, this case is unique for: •WHO Type A (spindle-cell) histology, a subtype rarely associated with pleural involvement. •Massive unilateral effusion with pleural seeding as the initial manifestation. •Negative pleural fluid cytology, illustrating diagnostic pitfalls. •Diagnosis achieved via CT-guided core biopsy, highlighting the usefulness and limitation of minimally invasive sampling. The occurrence of pleural effusion in a benign thymoma is rare, but thymomas with benign histology may still be invasive. A diagnosis of “benign” should not rely solely on histopathology, as more than 30% of thymomas show mixed histologic patterns, and a core needle biopsy may not fully represent the entire lesion due to heterogeneity. Pathology alone cannot determine whether a thymoma is benign or malignant, the extent of invasion on CT, MRI, or during surgery is equally important. Hence, the final diagnosis should combine: •Histologic pattern •Radiologic evidence of invasion •Intraoperative findings This abstract is funded by: None