Sen. John McCain, R-Ariz., has been diagnosed with brain cancer, and it is unclear when he will return to work in the Senate, his office said in a statement issued Wednesday evening.

McCain, 80, was recently operated on to remove a blood clot from above his left eye. Tissue extracted during the procedure, described as successful, proved to be cancerous, according to a statement from the Mayo Clinic in Phoenix, where the senator is being treated, that was publicized by his office. Treatment options, including chemotherapy, are being reviewed.