A well defined opacity in lung can be due to
pneumonia,
cancer, tuberculosis, a benign
lung nodule,
or due to an infarct. Less well defined
or diffuse opacities are usually multiple in nature and often due to inflammation, tuberculosis (military type), asbestosis, silicosis, auto-immune diseases etc.
If the lung nodule is not calcified nor has cavity, it is benign if it is less than 10 mm in size.