X-ray: High-energy radiation used in low doses to diagnose diseases and in high doses to treat cancer.

X-ray therapy: The use of high-energy radiation from x-rays to kill cancer cells and shrink tumors. Radiation may come from a machine outside the body (external-beam radiation therapy) or from materials called radioisotopes. Radioisotopes produce radiation and can be placed in or near the tumor or in the area near cancer cells. This type of radiation treatment is called internal radiation therapy, implant radiation, interstitial radiation, or brachytherapy. Systemic radiation therapy uses a radioactive substance, such as a radiolabeled monoclonal antibody, that circulates throughout the body. X-ray therapy is also called radiation therapy, radiotherapy, and irradiation.

Xenograft: The cells of one species transplanted to another species.

Xeroderma pigmentosum: A rare genetic condition characterized by an eruption of exposed skin occurring in childhood and photosensitivity with severe sunburn; inherited as a recessive autosomal trait in which DNA repair processes are defective so they are more likely to chromosome breaks and cancers when exposed to ultraviolet light.

Xerogram: A type of x-ray in which a picture of the body is recorded on paper rather than on film. In this technique, a plate of selenium, which rests on a thin layer of aluminium oxide, is charged uniformly by passing it in front of a 'scorotron’.

Xeroradiography (ZEE-ro-ray-dee-AH-gra- fee): A type of x-ray in which a picture of the body is recorded on paper rather than on film.

XR9576: A substance that is being studied for its ability to overcome tumor-cell resistance to anticancer drugs. It belongs to the family of drugs called anthranilic acid derivatives.