Radiation oncologist: A physician who uses radiation as a treatment for cancer.

Radiation therapy: The treatment of cancer with high-energy X-rays. Also called radiotherapy.

Radioactive tracer: A radioactive molecule that can be sent through the body's circulatory or urinary system, with its progress followed by a radiation-sensitive machine.

Recurrence: The reappearance of a cancer after a period of remission.

Remission: Complete or partial disappearance of a cancer, usually after treatment.

Resection: The removal of a malignant tumor in its entirety.

Risk factor: Any agent, substance, condition, or habit that increases a person's chance of developing cancer.