Republican Party chairman Lee Atwater entered a New York hospital Monday for intense radiation treatment of a type of brain tumor that experts say is persistent and dangerous.

Mary Matalin, chief of staff for the Republican National Committee, said in a statement that doctors had determined Atwater has an astrocytoma, a type of tumor that has "the potential to behave aggressively."

Astrocytomas are graded I through IV, although grade IV is usually called by another name. The announcement did not identify the grade of Atwater's tumor.