On January 29, 1896, the E. H. Grubb company, a vacuum tube manufacturer, used X-rays for the first time to treat breast cancer. The suggestion appears to have come from a doctor named Reuben Ludlam. He was the doctor the patient, Rose Lee, was seeing at the time.
Rose Lee underwent a one-hour treatment session. She was suffering from a breast cancer recurrence after having a mastectomy. A metastatic tumor had developed in her chest, and her case was terminal.
At ten in the morning on January 28, lead sheets were placed around the breast to shield the rest of her chest. Emil Grubbe, a homeopathic physician and chemist, suspended a Crooks tube three inches above the tumor. The treatment session went on for an hour. Rose Lee repeated the treatment several times over the next few weeks.
Despite being a leap forward in medicine, Rose Lee is believed to have passed away within a month of the treatment.
 “How Playing with Dangerous X-Rays Led to the Discovery of Radiation Treatment for Cancer | PBS NewsHour,” https://www.pbs.org/newshour/health/emil-grubbe-first-use-radiation-treat-breast-cancer.