Today marked the announcement by Science, the venerable magazine published by the American Association for the Advancement of Science, of their breakthrough for the year. There were nine runners up: genetic microsurgery; brain imaging; cloned human embryos; growing mini-organs; the birth of cosmic rays; creation of new solar cells; repair of brain cells; bacteria living in the human body; and design of vaccines to combat childhood diseases. After much debate, the editors of Science chose cancer immunotherapy as the winner for 2013.

Jennifer Couzin-Frankel, the editor who described the breakthrough asked, “Did we risk hyping an approach whose ultimate impact remains unknown? Were we irresponsible to label as a breakthrough a strategy that has touched a tiny fraction of cancer patients and helped only some of them?” She and her colleagues concluded that clinical trials have demonstrated the potential of immunotherapy and convinced even the skeptics that the treatment is now living up to its promise.

The “war on cancer” began in America just over 40 years ago and the main weapons have been surgery, radiation and chemotherapy, treatments that are often associated with adverse side effects. The latest weapon, cancer immunotherapy, is based on a simple premise — harnessing the body’s own immune system to do battle with the tumour. The technique has been around for over 20 years, where one of the early pioneers was James Allison who showed that a new protein on the surface of T cells, called CTLA-4, prevented the T cells from launching immune attacks. By blocking the CTLA-4 molecule, the immune system could be set free to destroy the cancer cells.

While the anecdotes supporting the efficacy of cancer immunotherapy are heart-warming — a 6-year-old with leukemia whose disease is now in remission, and a man with metastatic kidney cancer whose tumour continued to shrink even after treatment stopped — it is the hard data that’s transforming the field. Scientists from Johns Hopkins and Yale have reported results for 300 patients showing that tumours shrunk by more than half in 31% of melanoma patients, 29% with kidney cancer and 17% with lung cancer.

Although breast cancer has historically not been considered an ideal target for immunotherapy, the situation has changed in the past few years. Herceptin has been successful while cancer vaccines have also shown early promise. As Couzin-Frankel concludes, “One book has closed, and a new one has been opened. How it will end is anyone’s guess.”