The management of brain metastases has been important in neurosurgical oncology. Resection of a brain metastasis carries an increased risk of leptomeningeal dissemination than other treatment modalities such as irradiation or pharmacotherapy. We have utilized intraoperative wash cytology of cotton patties covering the brain surface. The cytology information contributes to making a decision of postoperative whole brain radiation. We named the method as "cotton dam", that serves as a check and catch of neoplastic cells on the brain surface during resection surgery.