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Sarcomatoid carcinoma is a rare malignant neoplasm of the lung.1 Its definition was ambiguous until the recent establishment of World Health Organisation (WHO) criteria, according to which it is classified into carcinosarcoma, pleomorphic carcinoma, and spindle cell carcinoma.1 Most of the reported cases of carcinosarcoma or pleomorphic carcinoma of the lung have been large tumours. We present a case of an extremely small carcinosarcoma of the lung with an unfavourable course.
An asymptomatic 69 year old man was admitted to our hospital because of a coin lesion that was detected on a medical examination. He had smoked 20 cigarettes a day for 48 years. Computed tomography showed an irregular shadow of 19 mm maximum diameter in contact with the pleura, situated in the S3 region of the right lung. Neither transbronchial biopsy nor percutaneous needle biopsy yielded positive results. Because thoracoscopic biopsy with frozen section interpretation could not entirely rule out malignancy (fig 1), right upper and middle lobectomy with lymph node dissection was performed. Nine months after surgery he developed a pleural effusion, a cytological preparation of which showed the presence of malignant cells.
The tumour measured 19 × 15 × 7 mm, and was situated just beneath the pleural surface of the middle lobe of the right lung. On cut section it was firm, whitish, and uniform in appearance.
Microscopically, most of the tumour was composed of a pure sarcomatoid region, and the remainder comprised a biphasic region. More than two thirds of the sarcomatoid area was collagenous and showed a deceptively well differentiated appearance. A small focus showing apparent chondroid differentiation was seen within the collagenous area. The remaining sarcomatoid area was composed of atypical spindle cells with pleomorphic nuclei and eosinophilic cytoplasm, in which cross striations were not readily observed. The carcinomatous component showed varying degrees of distinct gland formation, the dimensions of which ranged from large cystic spaces to small tubular structures. The tumour invaded the pleura. There were no positive lymph nodes.
Immunohistochemically, the carcinomatous cells were positive with antibodies to various cytokeratins, including AE1/AE3 (prediluted; Dako, Carpenteria, California, USA). They were negative for vimentin (prediluted; Dako). Sarcomatous cells were negative for cytokeratins and strongly positive for vimentin. S-100 (prediluted; Nichirei, Tokyo, Japan) was positive in the foci with cartilaginous differentiation.
It is important to know just how small a carcinosarcoma of the lung can be, because the existence of small carcinosarcomas suggests that the sarcomatoid transition could take place relatively early. In the literature, small carcinosarcomas of the lung are encountered extremely rarely. Koss et al reviewed the literature and found 34 cases that fit the WHO definition of pulmonary carcinosarcoma, among which only two cases were 2 cm in size.2 Early lesions of carcinosarcoma of the lung may necessitate intraoperative diagnosis. Care should be taken in the interpretation of a frozen section because insufficient sampling could lead to an erroneous diagnosis, such as reactive fibroblastic proliferation.