Chest Wall Fat Mimicking Tumor

Fig.1: Chest radiograph shows abnormal opacity in the periphery of the right upper lobe, which is broad-based to the pleura and partially well defined on the side opposing the lung. Based on this appearance, pleural location is likely. Differential diagnosis is broad and may include pleural effusion (loculated), thickening, plaque, or mass.
Fig.2: Axial CT shows an extrapleural fat (arrowheads) deep to the costal margin.

Facts

• Extrapleural costal fat can be mistaken for pleural lesions including plaque and tumor
• Thickening of the extrapleural costal fat can be associated with lung scarring
• CT is an easy tool to use differentiate between fat and soft tissue mass
• Fat can be drawn into fissures (as seen on Fig.1). It looks as if it is in the pleural space, but it is covered by parietal pleura - therefore is extrapleural
  

Reference:
Fisher ER, et al. Extrapleural fat collections: pseudotumors and other confusing manifestations. AJR 1993 (July)