An Unusual First Presentation of Colon Cancer: Pulmonary Lymphangitic Carcinomatosis.

Pulmonary lymphangitic carcinomatosis (PLC) is a rare manifestation of metastatic disease that presents with nonspecific respiratory symptoms. We describe a young male who presented with progressive dyspnea and cough and was initially managed for pneumonia. Chest imaging demonstrated diffuse interstitial abnormalities with mediastinal lymphadenopathy and pleural effusions, prompting evaluation for interstitial lung disease (ILD). Extensive infectious and rheumatologic testing was unrevealing. Cytologic analysis of pleural fluid revealed adenocarcinoma, and subsequent imaging identified a primary sigmoid colon malignancy. Despite the initiation of chemotherapy, the patient rapidly deteriorated. This case underscores the diagnostic challenges of PLC and emphasizes the importance of maintaining a broad differential diagnosis in patients with persistent respiratory symptoms and imaging findings similar to ILD. It also highlights the importance of considering occult colorectal malignancy in cases of PLC.