Branches of Deception: A Tree-in-Bud Pattern Revealing Advanced Non-small Cell Lung Cancer (NSCLC).

The tree-in-bud (TIB) opacities pattern seen on chest CT scans, characterized by small, branching, nodular opacities, primarily indicates diseases of the small airways, particularly bronchiolitis. We present a rare differential diagnosis for this CT finding.  A 46-year-old female patient with asthma and environmental smoke exposure presented with subacute cough and dyspnea. Chest CT revealed diffuse TIB opacities of the left lung and mediastinal lymphadenopathy. Initial suspicion was for atypical infection. However, bronchoscopy with bronchoalveolar lavage (BAL), brushing, and lymph node FNA revealed atypical and malignant cells. Immunohistochemistry showed tumor cells positive for TTF-1 and negative for p40, confirming the diagnosis of pulmonary adenocarcinoma. Ki-67 highlighted increased proliferative activity. The final diagnosis was stage IIIc non-small cell lung cancer (NSCLC). The patient was treated sequentially with platinum-doublet chemotherapy and multiple systemic therapies, including crizotinib (discontinued due to toxicity) and durvalumab. She is currently on maintenance pembrolizumab for palliative care, with stable disease and improved functional status. This case illustrates how mucinous adenocarcinoma can present with imaging features mimicking infection. Misdiagnosis can delay appropriate treatment. Early cytologic evaluation and immunohistochemistry were key to diagnosis. In high-risk patients with persistent TIB opacities unresponsive to antimicrobials, malignancy should be considered. Prompt tissue sampling and immunophenotyping can prevent diagnostic delay and inform timely management.