Basic principles of external beam treatment design for gynecological cancers

The use of radiation therapy for the treatment of gynecological tumors is now well accepted. The treatment often consists of two distinct phases, one in which localized large doses are delivered from internally placed radioactive sources, the other from externally applied radiation. In general, treatment at early stages of the disease relies more on localized doses. The field sizes are generally quite large for external radiation, in order too irradiate small lymph node metastases throughout the entire pelvis: A typical field will extend from the L5 vertebral body to the symphysis pubis and laterally to encompass the bony pelvis. Since a total dose in the range of 5,000 cGy is needed to control microscopic disease in the pelvis, prescriptions for the external beam phase are usually between 4,000 and 5,000 cGy. In more advanced stages, owing to the bulk of disease and the greater role played by external radiation, much larger doses are delivered usually with shrinking fields