Caution should be taken in using CD31 for distinguishing the vasculature of lymph nodes.

CORRESPONDENCE An early lesion of pulmonary carcinosarcoma: possible diagnostic problem in frozen section interpretation 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 carci-noma of the lung have been large tumours. We present a case of an extremely small carci-nosarcoma 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 percutane-ous 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 carcinoma-tous cells were positive with antibodies to various cytokeratins, including AE1/AE3 (pre-diluted; Dako, Carpenteria, California, USA). They were negative for vimentin (prediluted; Dako). Sarcomatous cells were negative for cytokeratins and strongly positive for vimen-tin. S-100 (prediluted; Nichirei, Tokyo, Japan) was positive in the foci with cartilaginous differentiation. It is important to know just how …