Stage IV Small Cell Lung Carcinoma – Hilum Overlay Sign

In evaluating densities projecting over the hila on PA chest radiographs, it is important to evaluate whether the opacity blends with the normal hilar shadow of the proximal pulmonary arteries and obscures clear visualization or if the mass and hilar structures are overlapping but distinct from one another. If the hilar vessels are sharply delineated, then it can be assumed that the overlying mass is anterior (Fig. 15) or posterior to the centrally located vascular structures and the intervening aerated lung enables crisp visualization of the pulmonary vessels.^12,13 However, when the mass is inseparable from the proximal pulmonary arteries, it is assumed that the structures are immediately adjacent to one another, with the lack of interposing air obscuring the margins of the normal hilar structures. This sign was originally described by Benjamin Felson.^12,13