Morphology and classification of 96 primary cardiac neoplasms in Fischer 344 rats.

Ninety-six primary cardiac neoplasms were identified from 79,971 Fischer 344 (F344) rats used in chronic toxicity and carcinogenicity studies by the National Toxicology Program (NTP) and National Cancer Institute (NCI), for an overall incidence of 0.1%. Neoplasms were classified as: 60 endocardial schwannomas, 23 intramural schwannomas, eight atriocaval mesotheliomas, three paragangliomas, one pericardial mesothelioma, and one hemangioma. Metastases occurred in four rats with endocardial schwannoma. Histological appearance of the endocardial and intramural schwannomas was consistent with origin from nerve sheath. Two of six endocardial schwannomas available for immunohistochemical staining were weakly positive for S-100 antigen. The atriocaval mesotheliomas, while morphologically resembling adenocarcinoma, were positive for vimentin and keratin, indicating mesothelial origin. Seventy of the 96 cardiac neoplasms occurred in rats 2 years of age or older at time of death. There were no sex or treatment-related differences in the incidence of these neoplasms, with the exception of atriocaval mesothelioma, which was more common in males.