Histologic distinction between malignant mesothelioma, benign pleural lesion and carcinoma metastasis. Evaluation of the application of morphometry combined with histochemistry and immunostaining.

Thirty men and 7 women with malignant mesothelioma seen at the Free University Hospital from 1st January 1960 until 1st July 1981 were reviewed. The histological, histochemical and morphometrical findings are reported. These findings are compared with 25 cases of pleural metastatic carcinoma and 25 cases of reactive pleural lesions. Fourty-nine percent of malignant mesotheliomas produced hyaluronic acid, however all cases of pleural metastatic carcinomas failed to produce this substance. All cases of malignant mesothelioma were D-PAS negative while 15 cases of pleural metastatic carcinoma showed reactivity to D-PAS. All cases of malignant mesothelioma and 9 cases of metastases were CEA negative. To distinguish malignant mesothelioma from metastases it is advisable to perform the D-PAS staining first. If it is negative mesothelioma can be confirmed by showing hyaluronic acid activity. A positive CEA staining rules out mesothelioma. In our study it was shown that with these methods 18 of 37 mesotheliomas could be identified with certainty, and 22 of the 25 carcinoma metastases. Morphometrically the malignant mesotheliomas could not be distinguished from the metastases, however the reactive pleural lesions had smaller nuclei than the malignant cells with mean values below 30 mu2. In the malignant cases these values had a range from 36 to 101 mu2. In distinguishing between reactive pleural lesions and malignant mesothelioma the production of hyaluronic acid points to the malignant character of the lesion. Thus histochemistry and immunostaining are important in the distinction of malignant mesothelioma from metastases, while the value of morphometry lies mainly in the seperation of reactive lesions from malignant mesothelioma.