A tram-line shadow located immediately anterior to the tumor shadow in the right lower lung field was observed in a patient with adenocarcinoma of the lung. The extripated lung specimen revealed that the right lower bronchus was compressed anteriorly by tumor formation and there was no thickening of the bronchial wall. Superimposition effect of water dense materials, on the one hand by tumor shadow, and the other hand by bronchial wall, was considered to explain the mechanism of the appearance of the tram-line shadow.
This reasoning was tested by a model experiment using ham and straws. Frequency of tram-line shadows was tested in cases with non-diffuse lung diseases in which the right lower lung field was free of pathological changes. In 100 such cases, tram-line shadows in the right lower lung field were observed in 31 cases and most of the shadow was made by superimposition of the bronchus upon pulmonary vascular shadows.
These findings clearly show that tram-line shadows observed in the lung field doe not necessarily indicate abnormal conditions, such as bronchial wall thickening mentioned heretofore, but can reveal radiographic findings observed in normal condition due to superimposition of water dense materials, namely the bronchial wall and pulmonary vasculature.
