"Thick lung lining" likely refers to a thickening of the layer of tissue (also known as the "pleura") that lines the chest wall and separates it from the lung tissue itself. Thickening of the pleural can be caused by many things including previous infection, damage from injury or trauma, exposure to irritating substances such as asbestos, inflammation such as that caused by lupus or rheumatoid arthritis, or in extremely rare cases, cancer. If you have not already had one, a CT scan of the chest is usually the first step in determining the significance of the thickening of the pleura. You should also expect a detailed history & physical exam to try to determine the underlying cause. Blood work and pulmonary function testing may also be helpful. In most cases, there is no need for invasive testing such as surgical biopsy, but the ultimate diagnosis and treatment plan will depend upon the most likely cause.