Causes of failure of radiotherapy in head and neck cancer.

Causes of failure of radiation therapy are reviewed and illustrated with clinical examples from cancers of the head and neck region. Radiobiological factors relating to volume of cancer, hypoxia, tumor cell kinetics, intrinsic cellular radiosensitivity and repair capability are considered, along with physical factors relating to fraction size and inadvertent underdosage. In addition, reference is made to failures attributable to a sigmoid dose response curve for tumor control and to the development of second primary cancers. The distinction is made between those causes of failure that can be minimized by optimal application of concepts and techniques readily available to all radiotherapists, those that are not amenable to any modification of radiotherapeutic technique, and those that are potentially remediable by new treatment strategies based on the radiobiological attributes of individual tumors.