George, writing in Catholic New World, a newspaper published by the Chicago archdiocese, said the cancer "will most probably eventually be the cause of my death." He said his doctors plan a new round of chemotherapy, using drugs more aggressive than those in the first round.
"As I prepare for this next round of chemo, I ask for your prayers, which have always sustained me, and for your understanding if I cannot always fulfill the schedule already set for the next several months," he said.
George was first diagnosed with bladder cancer in 2006 which was later found to have spread to his kidneys and liver.
The cardinal turned 75 in 2012 and submitted the letter of resignation required of all archbishops at that age. Pope Francis has not yet accepted his resignation, although George's announcement suggests his retirement could come soon.