The use of ionizing radiation as a therapeutic agent has been recognized for almost a century, and continues to be widely used for the treatment and palliation of many human cancers. Ionizing radiation can also be mutagenic or lethal to individual cells, thus a critical balance must be achieved when using radiation as a form of anticancer treatment to ensure tumor cell death with minimal side effects to normal tissue and organ function. Morphological damage following nonlethal exposure to whole-body ionizing radiation is detectable only in a few select cell types. Histological studies on tissue derived from irradiated mammals reveal immediate and extensive death to specific cells in the spleen, thymus, bone marrow, and intestinal epithelium. This phenomenon of cellular and nuclear disintegration, now defined as apoptosis (1 ) and long known to be an outcome of whole-body irradiation in these mammalian cell types, has been most carefully studied in spleen, thymus, and intestinal epithelia (2 ).
