Tune in to NBC’s “Nightly News with Lester Holt” at 6:30 p.m. ET for more information. As John McCain returns to the U.S. Senate this week, neurosurgeons who specialize in brain cancer say the main concern for his continuing recovery from surgery for aggressive glioblastoma is tiny malignant cells that may have already migrated away from the tumor. Surgery to remove a glioblastoma is challenging for even the most skilled surgeon. Because it's difficult to distinguish tumor tissue from healthy brain tissue, the safe removal of rogue cells can be especially problematic. If too much of the healthy tissue is removed, it could result in brain damage. If too little of the malignant tissue is removed, the cancer cells grow back, often even more aggressive than before. “They have little tentacles that grow into the normal brain that are very hard to see in the operating room, even with a microscope," said Dr. Constantinos Hadjipanayis, chairman of the department of neurosurgery at New York’s Mount Sinai Beth Israel. John McCain's Colleagues Say "Give It Hell, John" Most Common Brain Tumor It is a scenario all too familiar to mother and school teacher, Kim Richardson-Hippler. The 60-year-old from Ann Arbor, Michigan is fighting her third bout with glioblastoma, the same type of deadly brain cancer McCain has. Five years ago, when first diagnosed, Richardson-Hippler's prospects of survival were grim. She recalls how in one quick moment, her life was turned upside down. “Everything changed so suddenly," Richardson-Hippler told NBC News. "The doctor told me, you’re probably going to die soon.” Glioblastoma is the most common malignant brain tumor in the United States, with 12,390 new cases projected to be diagnosed this year. The aggressive cancer spreads into parts of the brain quickly and there is no cure. Radiation and chemotherapy treatments are given primarily with the hope of delaying the tumor’s progress. Most patients survive about 14 months. McCain, R., Ariz., received brain scans after last week's surgery, which confirmed doctors did clear away all the glioblastoma tumor that could be seen, according to a statement issued by the senator's office Wednesday night.