B80-5-31 Invasive Mucinous Adenocarcinoma Presenting as Refractory Bilateral Pneumonia With Rapid Clinical Decline in a Non-smoking Elderly Patient

Abstract Introduction Invasive mucinous adenocarcinoma (IMA) of the lung is a rare subtype characterized by TTF-1 negativity and atypical immune-profiles that can mimic infectious pneumonia radiologically. Pneumonic-type presentations with bilateral consolidations and air bronchograms pose significant diagnostic challenges, often resulting in delayed diagnosis and treatment. We present a case highlighting the clinical, pathologic, and prognostic features of IMA presenting as acute hypoxemic respiratory failure thought to be secondary to community-acquired pneumonia. Case Presentation An 81-year-old male, lifelong non-smoker with a past medical history significant for coronary artery disease, hypertension, and chronic kidney disease, initially presented with progressive shortness of breath and productive cough of three months duration. He was admitted for acute hypoxemic respiratory failure with bilateral pulmonary infiltrates shown on chest imaging, initially treated as pneumonia with antibiotics and planned corticosteroids for possible cryptogenic organizing pneumonia.During that admission, bronchoscopy with bronchoalveolar lavage (BAL) and transbronchial biopsy of the right lower lobe was performed. BAL cytology was negative for malignancy, but biopsy tissue was sent for outside consultation. The patient was discharged on home oxygen at 4 L/min but did not initiate the recommended prednisone taper.Over the following weeks, the patient’s respiratory symptoms progressively worsened, leading to readmission with severe hypoxemia and hypotension requiring vasopressor support. Laboratory studies revealed leukocytosis, acute kidney injury with worsening creatinine and elevated inflammatory markers. Repeat chest CT demonstrated interval worsening of bilateral airspace opacities with a cavitary lesion in the left upper lobe. Blood and sputum cultures remained negative.Around this time, transbronchial biopsy results confirmed invasive mucinous adenocarcinoma of the lung characterized histologically by a lepidic growth pattern and abundant extracellular mucin. Immunohistochemical staining was notable for positivity for CK7, patchy CK20 and CDX2, and negativity for TTF-1, consistent with mucinous adenocarcinoma. The radiologic pattern with multifocal consolidations and air bronchograms mimicked pneumonia, contributing to diagnostic delay.Despite aggressive supportive care including high-flow oxygen and broad-spectrum antibiotics, the patient’s condition deteriorated with development of pleural effusions, hypoxic respiratory failure, and gastrointestinal bleeding. Given his clinical decline and wishes, he was transitioned to comfort care and passed away 4 days later. Discussion IMA presenting as pneumonic-like consolidation represents a diagnostic challenge requiring high clinical suspicion in nonsmokers with progressive respiratory symptoms unresponsive to antibiotics. TTF-1 negativity with focal CDX2/CK20 positivity is characteristic but necessitates careful exclusion of gastrointestinal metastases. Early tissue diagnosis through bronchoscopy is essential for appropriate therapeutic decision-making and prognostication in this aggressive presentation pattern. This abstract is funded by: None