Carcinomas metastatic to the brain usually grow very well circumscribed, with sharp delineation. Radiosurgery takes advantage of this fact by using the gamma knife for definitive treatment of small metastases. We report a systematic study of the growth pattern of cerebral metastases, focusing on tumor delineation. In 26 cases of 66 metastatic anaplastic small cell carcinomas and in one case of adenocarcinoma, we observed poorly defined borders and a highly diffuse pattern of invasion. Infiltrating carcinoma cells changed to an elongated shape adapting to preexisting tissue structures. This pseudogliomatous growth pattern of some brain metastases--apparently most likely in neuroendocrine carcinomas--is of potential importance for therapeutic strategies in the treatment of brain metastases, especially when considering treatment with radiosurgery and gamma knife.