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.

The same finding is also described as the incomplete rim sign: the rim of the lesion can be followed only where aerated lung completes it and is lost where it merges with soft tissue. Read it together with the other features of extrapulmonary origin — obtuse rather than acute angles with the chest wall, a broad pleural base, a smooth convex margin facing the lung, displacement rather than encasement of adjacent structures, and rib or vertebral erosion or periosteal reaction when the chest wall is involved. A neurogenic tumour in the paravertebral gutter is the classic posterior mediastinal example, and empyema, extrapleural haematoma and extrapleural lipoma are the common benign causes. Because radiography localises but cannot characterise, CT is used for definitive assessment, with ultrasound for pleural and chest wall lesions and MRI for neurogenic tumours and vertebral involvement.

Alternative names: Incomplete rim sign; extrapleural sign

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.