QS21: A plant extract that may improve the ability of the immune system to respond to disease. It is being studied in combination with vaccine therapy.

Quadrantectomy: Surgical removal of the region of the breast (approximately one quarter) containing cancer.

Quality of life: The overall enjoyment of life. Many clinical trials measure aspects of an individual's sense of well-being and ability to perform various tasks to assess the effects of cancer and its treatment on the quality of life.