Differential diagnosis of fibrosing lung lesions.

Abstract

Pulmonary fibrosis, interstitial infiltrates, and prominent parenchymal vascular shadows are frequently seen in the study of chest roentgenograms. Differentiation of the strand-like shadows observed is often difficult, and the radiologist is dependent upon clinical findings and an understanding of the pathological physiology involved for proper evaluation of the roentgen changes. The conditions which produce shadows representing or simulating pulmonary fibrosis are numerous. In this paper we shall stress the importance of correlation of clinical and roentgen findings. For this purpose the radiologic examination should be comprehensive, including fluoroscopic and multiple film studies. Pulmonary blood vessels are responsible for the majority of normal pulmonary markings; the bronchi and adjacent connective tissue make up the remainder. The markings are largest at the hilar regions and radiate outward in the pulmonary fields, progressively decreasing in size. They are relatively sparse in the apices and lateral portions of the lungs. The shadows are straight and exhibit periodic branching as the larger vessels and bronchi divide. With films of good quality, the blood vessels can be traced from the hilar regions to the periphery, where they become indistinct. The dilatation of pulmonary vessels which occurs in congestion of the lung or in polycythemia vera may present a roentgen appearance that can be confused with interstitial infiltration or fibrosis. With study, the shadows can be identified by their progressively decreasing size as they extend peripherally from the hilus. The dilated vessels extend farther toward the periphery than normal and are tortuous as well as elongated. The shadows are well defined and branching is readily apparent. With congestion, the heart is usually enlarged; in polycythemia it may be of normal size. The presence of inflammation, fibrosis, tumor, and fine foreign material in the interstitial tissues of the lung may be mistaken for vascular prominence; careful study will show that there are differences. Pattern of Fibrosis and Its Distribution It is important to avoid confusing terms in describing fibrosing lesions. In the present discussion, the term “fibrotic appearance” will be used to describe the roentgen shadows being studied, whether due to fibrosis or some other interstitial process. This general designation will be modified by appropriate adjectives, such as “coarse” and “fine.” The term “nodules” will indicate small discrete densities visible in lung fields along the course of or adjacent to the interstitial shadows. Clues to the disease causing the fibrotic appearance may be found by a systematic study of the shadows. Are they uniform in size? Are they modified by any other type of pulmonary disease? Are they localized to any particular region of the lung? Is the major portion in a certain area, such as the hilar region, the bases, or apices? Do they extend to the periphery of the lung?