B80-5-19 Beyond Serositis: Hydropneumothorax Unveils a Rare Mesothelioma in Familial Mediterranean Fever

Abstract Introduction Hydropneumothorax is an uncommon manifestation of pleural disease and may obscure underlying malignancy. While malignant pleural mesothelioma (MPM) classically follows asbestos exposure, it can arise without known exposure, particularly in pro-inflammatory milieus. We report an epithelioid MPM initially presenting as chronic pleuritic hydropneumothorax in a patient with Familial Mediterranean Fever (FMF) characterized by recurrent pleuritis, peritonitis, and febrile attacks. Case Description A 62-year-old man with FMF since 1990, complicated by colchicine non-adherence and 3-4 annual attacks characterized by chest-abdominal pain, and fever until 2018, had been adherent to colchicine-therapy since 2018 and remained attack-free. He presented with left-sided pleuritic chest pain and progressive dyspnea for one-month, 10-kg weight loss, and night sweats for six-months. Comorbidities included COPD and ankylosing spondylitis. He was taking colchicine 1 mg/day. He had an 80 pack-year smoking history with 5 years of quitting, but no asbestos or relevant occupational/environmental exposures.Serial chest X-rays and thoracic CTs (2018-2025) showed evolution from a small left pleural effusion to a 6-cm loculated hydropneumothorax with pleural thickening, later progressing to heterogeneous nodular malignant pleural thickening with invasion of intercostal muscles and minimal adjacent bony destruction (Figure-1A-D). By 2020, the patient had developed a spontaneous left hydropneumothorax in the absence of trauma. Although diagnostic thoracentesis and pleural biopsy were recommended, he declined both procedures. PET-CT demonstrated intense FDG uptake along the left pleura (SUVmax:24.8), avid mediastinal/internal mammary/infrahilar nodes, and multiple skeletal metastases (SUVmax:25.1)(Figure-1E). Laboratory tests showed systemic inflammation (CRP:57.8 mg/L, ESR:68 mm/h, serum amyloid-A (SAA):89.7 mg/L) and elevated CA 15-3:105 U/mL.Pleural biopsy revealed atypical mesothelial cells in solid, tubular, and papillary patterns (Immunohistochemistry: CK7+, Keratin 4/18+, Calretinin+, Thrombomodulin+; CK20-, TTF-1-, CDX2-, MOC-31-, mCEA-, Claudin-4-, ERG-) confirming high-grade epithelioid MPM. Combination cisplatin-pemetrexed-pembrolizumab-bevacizumab was initiated. Discussion FMF, repeated or long-standing pleural inflammation due to inadequately controlled disease, can contribute to mesothelial proliferation and oncogenic transformation. Although hydropneumothorax is an unusual finding in FMF, recurrent pleuritic attacks, as seen in our patient, may mask the presence of MPM even in the absence of asbestos exposure.This case highlights that chronic or loculated hydropneumothorax with pleuritic pain should prompt consideration of MPM in FMF patients. Awareness of this rare but preventable complication is crucial, as early recognition of atypical or persistent pleural effusions may allow timely diagnosis of malignant pleural MPM and facilitate multidisciplinary management. This abstract is funded by: None