The trial participants were among the 2 to 3 percent of cancer patients with tumors that should respond to immunotherapy, a drug that overcomes barriers that prevent the immune system from attacking cancers. The result was stunning, and could bring hope to the limited cohort of patients contending with these cancers. In 49 of the patients, who had rectal cancer, the tumors disappeared and, after five years, have not recurred. Out of all 103 patients, cancers recurred in only five. Three got additional doses of immunotherapy and one, whose tumor recurred in a lymph node, had the lymph node removed.