Lung Cancer: Given the size (8 cm) and location (lower lobe), lung cancer is a strong consideration, especially in patients with a history of smoking or other risk factors. The size of the lesion suggests a more advanced stage if it is indeed cancer.

Pneumonia: Although typically smaller, a large opacity could represent a severe or necrotizing pneumonia, especially if the patient has symptoms like fever, cough, and shortness of breath.
Pulmonary Abscess: A large, cavitated lesion could be an abscess, particularly if there's a history of aspiration or a severe infection.
Hamartoma: While usually smaller and more commonly found in the upper lobes, a large hamartoma could present as an 8 cm opacity, though this would be less likely.

Pulmonary Embolism with Infarction: Although typically presenting with multiple, smaller wedge-shaped opacities, a large infarct could appear as a single large opacity. This diagnosis is critical due to its high mortality if untreated.
Tuberculosis: Especially in endemic areas or in immunocompromised patients, TB can present with large opacities and must be considered to prevent missing a treatable disease.

Pulmonary Lymphoma: Primary or secondary lymphoma of the lung can present as large opacities, though this is less common.
Metastatic Disease: If the patient has a known primary malignancy, metastatic disease to the lung could present as one or more large opacities.
Hydatid Cyst: In endemic areas, a large opacity could represent a hydatid cyst of the lung, which is caused by Echinococcus granulosus infection.
Diaphragmatic Hernia: Although more typically presenting with bowel or liver in the thorax, a large diaphragmatic hernia could mimic an 8 cm opacity, especially if it contains omentum or other tissue.