The PA radiograph shows a large left mid zone mass which can be localised to the posterior of the chest by using the silhouette sign. The left heart border remains visible indicating that the mass cannot be anterior. Some of the left hilar vessels remain visible (hilum overlay sign) indicating that the mass is not centered at the hilum. The mass therefore must be located posteriorly within the apical portion of the left lower lobe. This is confirmed on the lateral radiograph.