incomplete border sign
The incomplete border sign is useful to depict an extrapulmonary mass on chest radiograph.
An extrapulmonary mass will often have an inner well-defined border and an ill-defined outer margin . This can be attributed to the inner margin being tangential to the x-ray beam and has good inherent contrast with the adjacent lung. On the other hand, the outer margin is en face or partially en face with the x-ray beam and merges with the pleural or chest wall thus the border is obscured.
Differential diagnosis for extrapulmonary mass can be further divided into pleural or extrapleural:
Common pleural masses include:
Extrapleural causes can arise from a component of the chest wall, including bone/cartilage, nerve, vascular, fat, muscle and skin. If there is sign of rib/bone involvement on chest radiograph the lesion is most likely to be extrapleural. In adults, skeletal metastases are the most common chest wall malignancy while chondrosarcoma is the most common primary malignant tumor.