This 68 y/o male presented with acute onset of shortness of breath. His medical history was unremarkable with no past history of significant illness. Physical exam and EKG were within normal limits. Blood gases showed hypoxemia. A chest x-ray, posterior Xe133 ventilation scan with rebreathing and washout phases and a Tc99m MAA perfusion scan with multiple projections were performed. Selected images shown above demonstrate findings consistent with a large central lesion. A chest CT was performed confirming the diagnosis. A subsequent biopsy demonstrated a bronchogenic carcinoma.