[A case showing a free substance in the thoracic cavity detected during surgery in lung cancer considered to the precursor of the so-called "intrathoracic calculus"].

A 56-year-old male showed a pearl-like free substance in the thoracic cavity during operation for pulmonary cancer. In the thoracic cavity, apart from a small quantity of fluid, no particularly abnormal finding, such as calcification, pleural plaque, etc. was seen in the parietal pleura. Further, the pulmonary pleura showed no abnormal findings, except for pleural indentation at the site of the tumor. On histopathological examination of the free substance, it was spherical, with fat forming the nucleus, and covered by fibrous and hyaline tissue. The fat forming the nucleus of the free substance was assumed to have been derived from a lipoma or fat in the thoracic cavity or on the pulmonary pleura which had become detached. This free substance is believed to be the precursor of an "intrathoracic calculus", as designated by Takiguchi et al. This is very rare and is believed to be only the second such case in Japanese or foreign literature.