Ironically, the mustard gas attacks of World War led to the use of chemotherapy for cancer. When autopsies were performed on mustard gas victims, pathologists noted that their lymph glands were destroyed and their bone marrow wiped out. Because bone marrow makes white and red cells for the blood, there were few white cells remaining. Around 1930 James Ewing, a cancer specialist at New York's Memorial Hospital, suggested to colleagues that they try mustard gas on various cancers. It was too toxic for internal use, but it did work on skin cancer.

Source: Edward Shorter, The Health Century (New York: Doubleday, 1987), p. 184.

Sources:

Samuel Hopkins Adams, "What Can We Do About Cancer? The Most Vital and Insistent Question in the Medical World," Ladies' Home Journal, 30 (May 1913): 21-22;

James T. Patterson, The Dread Disease. Cancer and Modern American Culture (Cambridge, Mass.: Harvard...