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[A case of enlarging rounded atelectasis difficult to distinguish from a pulmonary neoplasm].

A 55-year-old man with occupational exposure to asbestos presented with a chest roentgenographic abnormality that was found during a physical check-up. Computed tomography disclosed pleural thickening and a pulmonary mass in the left lower lobe. A fine-needle biopsy was done, but the presence of a malignant lesion was not confirmed. The patient was asymptomatic. He was observed closely, and 10 months later computed tomography showed that the mass had enlarged. A fine-needle biopsy was done again, but no evidence of malignancy was found. We suspected that it was rounded atelectasis, but could not rule out a neoplasm. An exploratory thoracotomy was done, and the histopathological findings showed that the mass was lung tissue with alveolar wall fibrosis that had resulted in thickening of the wall. The mass was diagnosed as rounded atelectasis. Rounded atelectasis requires no special treatment, but if the mass increases in size as in the case of this patient, an exploratory thoracotomy is required.