Pulmonary endodermal tumor resembling fetal lung. The optically clear nucleus is rich in biotin.

Pulmonary endodermal tumor resembling fetal lung (PET) is a rare pulmonary neoplasm that may represent a distinctive form of pulmonary blastoma lacking the sarcomatous component. A peculiar histologic feature of PET is the presence in the morular area of optically clear nuclei (OCN), which commonly show false-positive immunostaining by the avidin-biotin peroxidase complex (ABC) method. Because recent studies have shown that the nuclei of an identical change seen in endometrioid adenocarcinoma of the ovary and endometrial glandular epithelium during pregnancy are rich in biotin, we conducted an investigation to see if the same was true of a PET occurring in a 45-year-old man. Only the OCN were positively stained when the tissue section was incubated with peroxidase-labeled avidin or streptavidin and developed in a chromogen solution. The reaction was completely blocked by pretreatment with free avidin. An identical nuclear staining pattern was observed when the antibiotin antibody was applied as the primary agent by the indirect immunoperoxidase method. The neoplastic cells were negative for estrogen and progesterone receptors in immunostaining on frozen tissue sections. Ultrastructurally, the OCN were occupied by filamentous substructures of chromatin measuring approximately 10 to 12 nm in diameter. These results suggest that the OCN in PET are part of the unique nuclear change of certain neoplastic and non-neoplastic cells in the body in which abundant biotin accumulates within the nucleus in association with the characteristically arranged chromatin substructure. Further study is necessary to determine whether the OCN generally develop under the influence of female sex hormones. When immunostaining by the ABC method or related techniques is performed in PET and certain other epithelial lesions, the possibility of false positivity in the OCN should be kept in mind to avoid any misinterpretation of the results.