Incomplete border sign | Radiology Signs

Answer:

The incomplete border sign identifies a mass as extrapleural or otherwise extrapulmonary rather than arising within the lung. The lesion shows a crisp, well-defined border on the side facing the lung, but that border fades and becomes indistinct peripherally where the mass merges with the chest wall or pleura, so its outline is never completely traceable. A true intrapulmonary mass, being surrounded by aerated lung on all sides, shows a sharp margin throughout its circumference. Supporting features of an extrapleural origin are obtuse angles with the chest wall, tapered pleural margins, displacement rather than encasement of adjacent lung, and rib or soft-tissue involvement. Typical causes include pleural fibroma, lipoma, metastasis, myeloma or other rib lesions, extrapleural haematoma, and mesothelioma. CT confirms the origin directly and demonstrates the chest wall component.

Why is it called so?

The border of the mass can only be followed part of the way round before it is lost, so the outline is incomplete.

Pathophysiology

The sign is a projection effect. The inner surface of an extrapleural mass bulges into aerated lung and lies tangential to the x-ray beam, producing a sharply defined, high-contrast edge. Its outer surface lies against the chest wall, en face or oblique to the beam, and is bounded by soft tissue of identical attenuation, so no edge is generated and the margin dissolves. A mass entirely surrounded by lung has air along its whole circumference and therefore shows a complete border.

Alternative names: None

Other associated named signs: Silhouette sign, extrapleural sign, cervicothoracic sign, and holly leaf sign of pleural plaques

References

1. Hsu CC, Henry TS, Chung JH, Little BP. The incomplete border sign. J Thorac Imaging. 2014;29(4):W48. PMID: 24858533.