The Great Mimicker: Pulmonary Adenocarcinoma with Interstitial Lung Disease-Like Presentation.

A 52-year-old female with a significant smoking history and newly diagnosed chronic obstructive pulmonary disease (COPD) presented to the emergency department with shortness of breath and blood-tinged sputum. Following a prior hospitalization for a COPD exacerbation, imaging revealed atypical features prompting autoimmune evaluations that were negative. A decline in pulmonary function led to admission for bronchoscopy, which revealed invasive mucinous adenocarcinoma (IMA). This case underscores the diagnostic challenges of IMA, which can mimic interstitial lung disease (ILD), resulting in delayed diagnosis and mismanagement. The high false-negative rate of bronchoalveolar lavage cytology in mucinous adenocarcinoma highlights the need for prompt tissue sampling in similar presentations for timely intervention. LEARNING POINTS: Invasive mucinous adenocarcinoma, a rare subtype of lung adenocarcinoma, often presents with discrete tumours rather than diffuse interstitial involvement, leading to difficulties in diagnosis, which can mimic benign lung conditions.Initial imaging findings such as cystic changes and ground-glass opacities may suggest interstitial lung disease, potentially resulting in an unnecessary autoimmune workup. This emphasizes the importance of considering lung cancer in patients with significant smoking history and worsening respiratory symptoms.Bronchoalveolar lavage cytology may fail to identify malignant cells in cases of mucinous adenocarcinoma due to its lepidic growth pattern. This highlights the need for histopathological confirmation through biopsy to avoid false negatives and ensure appropriate management.