The most likely diagnosis is small cell lung cancer (SCLC). SCLC comprises approximately 15% of all new lung cancer cases diagnosed in the United States and occurs almost exclusively in patients with a history of cigarette smoking. SCLC is the most common pulmonary neuroendocrine tumor in adults. Signs and symptoms of cough, hemoptysis, chest pain, hoarseness, and dyspnea occur frequently. Patients may also present with symptoms related to metastatic disease or to various paraneoplastic syndromes. This patient presents with symptoms of lower extremity weakness with confirmation of upper and lower extremity weakness on neurologic examination. This presentation is consistent with Lambert-Eaton myasthenic syndrome, which has a strong association with SCLC. Because of their rapid growth rate, these tumors are rarely found incidentally. The most common radiographic presentation is a hilar mass with bulky mediastinal lymphadenopathy. It is important to recognize patients with possible SCLC because the cancer is highly aggressive, and most patients start initial chemotherapy during the same hospitalization. SCLC tends to be more aggressive than non–small cell lung cancer (NSCLC); it is usually already disseminated at presentation but is usually more sensitive to chemotherapy and radiation therapy initially.

Most (80%) lung cancers are NSCLC. The most common type is adenocarcinoma of the lung (38% of all lung cancers), followed by squamous cell carcinoma of the lung (20% of all lung cancers). Although NSCLC is still in the differential diagnosis for this patient, the radiographic appearance is classic for SCLC.

Bronchial carcinoid tumors represent only a small percentage of lung cancers and are more common in children and adolescents. Typically, they present with an endobronchial lesion causing proximal airway obstruction.