Pulmonary meningothelial-like nodules: new insights into a common but poorly understood entity.

Although pulmonary meningothelial-like nodules (MLNs) have been recognized for decades, their nature and significance remain uncertain. This study was undertaken to evaluate MLNs in a wide range of specimens to clarify their incidence, distribution, relation to age and underlying disease, and histogenesis. Five hundred surgical lung biopsies, 25 extensively sampled lobectomies, 20 resections for pneumothoraces in persons younger than 30 years, and 92 pediatric autopsies were examined. Immunohistochemistry was performed in selected cases. One hundred eighty-six MLNs were identified in 81 cases, including 69 of 500 (13.8%) surgical biopsies and 12 of 25 (48%) lobectomies. No MLNs were found in pneumothorax resections or pediatric autopsies. Patients ranged from 22 to 84 years (mean, 62), with only 4 younger than 40 years. There were 56 women and 25 men (female:male=2.2:1). The highest incidence of MLNs was in thromboembolic disease/infarcts (5/12; 42%) and respiratory bronchiolitis-associated interstitial lung disease/desquamative interstitial pneumonia (9/35; 26%). MLNs were randomly distributed in alveolar septa with no consistent relation to small blood vessels. Immunohistochemistry demonstrated positivity for CD56 (18/18) in addition to progesterone receptor (18/18), epithelial membrane antigen (11/11), and vimentin (2/2). The high incidence of MLNs in our study may be related to underlying chronic lung disease. The finding of MLNs in almost half of extensively sampled lobectomies suggests that they may be present in all abnormal lungs if sufficiently sampled. Their absence in infants and children indicates that they are not congenital rests. The positive staining for CD56 is novel, and as CD56 has been reported in meningiomas, this finding supports meningothelial differentiation.