Sen. John McCain's office announced Wednesday he has brain cancer. His daughter, Meghan McCain, posted a somber, but optimistic message on Twitter in light of the news.

"Cancer may afflict him in many ways: but it will not make him surrender," Meghan McCain posted in an image on Twitter. "Nothing ever has."

The tumor is a glioblastoma, a highly malignant cancerous tumor, according to the America Brain Tumor Association.

MORE: McCain's office says he has cancerous brain tumor