a-extra-pleural lesion=displace both parietal and visceral pleura

resulting in obtuse angle between lesion and chest wall.

+ or - rib erosion.

b-pleural lesion=arise between two layers of pleura producing obtuse

angle with chest wall or become pedunculated and protruded into peripheral

lung parenchyma producing acute angle with chest wall.

c-pulmonary lesion.
sub pleural pulmonary lesion cause acute angle with chest wall.

Reference:BiblioMed Textbook-Computed Body Tomography