Occurrence of Pineal Gland Tumors in Combined Chronic Toxicity/Carcinogenicity Studies in Wistar Rats.

Pineal gland tumors are very rare brain lesions in rats as well as in other species including humans. A total of 8 (out of 1,360 examined) Wistar rats from 3 different combined chronic toxicity/carcinogenicity or mere carcinogenicity studies revealed pineal gland tumors. The tumors were regarded to be spontaneous and unrelated to treatment. The morphology and immunohistochemical evaluation led to the diagnosis malignant pinealoma. The main characteristics that were variably developed within the tumors were the following: cellular atypia, high mitotic index, giant cells, necrosis, Homer Wright rosettes, Flexner-Wintersteiner rosettes and pseudorosettes, positive immunohistochemical reaction for synaptophysin, and neuron-specific enolase. The pineal gland is not a protocol organ for histopathological examination in carcinogenicity studies. Nevertheless, the pineal gland can occasionally be encountered on the routine brain section or if it is the origin of a tumor protruding into the brain, the finding will be recorded. Therefore, although known to be a rare tumor in rats, pineal neoplasms should be included in the list of possible differential diagnoses for brain tumors, especially when the tumor is located in the region of the pineal body.