Pulmonary Metastasis: Given the size (1.6*1.6 cm) and location (posterior segment right upper lobe) of the lesion, and assuming it's a new finding, pulmonary metastasis from a primary malignancy elsewhere in the body is a common cause of such lesions, especially if the patient has a history of cancer.

Granuloma: This could be a result of infectious diseases like tuberculosis or histoplasmosis, especially if the patient has been exposed or has a history suggestive of such infections. The smooth margin is consistent with a granuloma.
Hamartoma: A benign lung tumor that can present as a smooth-edged nodule, often containing fat, which can be confirmed on a CT scan.
Pulmonary Arteriovenous Malformation (AVM): Although less common, an AVM could present as a well-defined nodule, especially if it's small.

Lung Cancer: Although the smooth margin might be less typical for a primary lung cancer (which often has irregular margins), it's crucial not to miss this diagnosis, especially in smokers or those with significant exposure to carcinogens. Early detection significantly improves prognosis.
Infectious Lesions (e.g., Abscess): While less likely given the description, an abscess or other infectious process could present as a well-defined lesion, especially in the early stages or with certain pathogens.

Pulmonary Sclerosing Hemangioma: A rare, benign tumor that can present as a well-defined nodule.
Intrapulmonary Lymph Node: Rarely, an intrapulmonary lymph node can mimic a pulmonary nodule, though this is exceedingly uncommon.
Pulmonary Amyloidosis: Focal amyloid deposits can form nodules in the lung, though this is a rare condition and usually associated with systemic amyloidosis.