The type of brain tumor that doctors removed from Sen. John McCain on Friday is an aggressive one known as a glioblastoma, a highly malignant form of cancer that spreads quickly due to its association with a large network of blood vessels in the brain. The tumor was associated with a small blood clot above the Arizona Republican's left eye, which doctors removed Friday during a minimally invasive procedure, according to a statement issued by the senator's office Wednesday evening. Scans completed since Friday indicate that doctors were able to remove all the tumor tissue they could see in the 80-year-old senator's brain. "The tissue of concern was completely resected by imaging criteria," the statement read. The procedure and surgery were performed by doctors at the Mayo Clinic Hospital in Phoenix. "The Senator and his family are reviewing further treatment options with his Mayo Clinic care team. Treatment options may include a combination of chemotherapy and radiation," the statement said. Here's what we know about this type of cancer: What is a glioblastoma? A brain tumor is a mass of abnormal cells growing in the brain. They arise from star-shaped cells that make up the supportive tissue of the brain, also known as astrocytes. In rare cases, brain and spinal cord cancers are found to run in families, but in most instances people diagnosed with brain tumors have no family history of the disease, according to the American Cancer Society. The ACS estimates that about 24,000 malignant tumors are diagnosed each year — about three in 10 brain tumors are glioblastomas. Cancerous tumors of the brain and spinal cord differ from other tumors in the body. For one, glioblastomas don't spread to other organs. Although the statement from McCain's office stated that doctors believe they removed all of the tumor tissue, microscopic pieces that look like a sea urchin's tentacles could remain and spread to other parts of the brain, according to NBC News medical correspondent Dr. John Torres. Because gliomas have often spread deep into the brain by the time of diagnosis, it is difficult for the cancerous tissue to be completely removed. McCain has a history of melanoma, a