Mass

Definition

A lung mass is any pulmonary lesion seen as an opacity greater than 30 mm in diameter irrespective of its contour, border, or attenuation characteristics. The mass usually implies a solid or partly solid opacity. CT allows more exact evaluation of size, location, and attenuation [1] (Figs. 2.1 and 2.2).

Diseases Causing the Pattern

The most common cause is malignant tumor including lung cancer (Figs. 2.1 and 2.2), bronchus-associated lymphoid tissue (BALT) lymphoma, and pulmonary sarcoma [2] (Fig. 2.3). Benign condition such as benign tumor, rounded atelectasis [3, 4], and progressive massive fibrosis [5] (Fig. 2.4) may present with a lung mass. Chronic pneumonia including actinomycosis [6] (Fig. 2.5) and semi-invasive aspergillosis (chronic necrotizing pulmonary aspergillosis) [7] may also manifest a mass lesion (Table 2.1).

Distribution

Likelihood ratio for malignancy in upper and middle lobe nodule or mass is 1.22 as compared with 0.66 in lower lobe nodule [8]. Otherwise, there is no central-subpleural or bronchovascular predominance.

Clinical Considerations

Rounded atelectasis is seen in patients with asbestos exposure, while progressive massive Þbrosis (PMF) is seen in patients with silicosis or coal workersÕ pneumoconiosis [5]. Pulmonary actinomycosis is usually seen in immunocompetent patients

with respiratory disorders (emphysema and chronic bronchitis), poor oral hygiene, alcoholism, and chronic debilitating diseases [9]. Semi-invasive aspergillosis usually occur in patients with underlying illness such as diabetes, pulmonary tuberculosis, or chronic alcoholism [7].

Key Points for Differential Diagnosis

1.Pulmonary sarcomas are rare and account for 0.5 % of all primary malignant lung disease [10]. Histologic types include malignant Þbrous histiocytoma, pleuropulmonary synovial sarcoma, malignant nerve sheath tumor, leiomyosarcoma, Þbrosarcoma, and hemangiopericytoma [2, 11].

2.The signal intensity manifested on T2-weighted images provides a clue for the differentiation of lung cancer from progressive massive Þbrosis. When a mass lesion is depicted with high signal intensity on T2-weighted MR images, the Þnding is highly suggestive of lung cancer. Progressive massive Þbrosis appears as a lowor black-signal-intensity abnormality on T2-wighted MR images [12].

3.Rounded atelectasis appears on CT as a rounded mass abutting a thickened pleural surface in the lung periphery. The margin closest to the hilum is blurred by the entering vessels. Typically, bronchi and vessels curve into the mass at its hilar pole, giving the so-called comet tail sign [3, 4].

4.Dense consolidation or a mass is the main pattern of pulmonary actinomycosis and semi-invasive aspergillosis.

5.The presence of geographic central low-attenuation area (microabscesses with sulfur granule) within the lesion and peripheral enhancement are suggestive of pulmonary actinomycosis [6].