The Cancer Center: Radiation Oncology
External Beam Radiotherapy (EBRT)
External beam radiotherapy is the most common way for delivering a relatively high dose of x-rays to the radiotherapy patient. X-rays are generated inside the linear accelerator and exit through a collimator to a designated region either inside the patient or on the skin. The target volume is determined through a careful evaluation of the patient by the Radiation Oncologist based on a comprehensive consultation including a history, x-rays, pathology reports, CT and/or MRI scans prior to deciding on the appropriate treatment plan for the patient.
Before the start of treatment, a simulation of the plan is performed on a radiotherapy simulator. Simulation employs both a CT based 3D imaging system ("virtual simulation") and a specially designed x-ray machine which can duplicate all of the alignments and configurations of the linear accelerator. Inside the Radiation Oncology Unit the radiation oncologist in consultation with a medical physicist and a senior radiation therapy technologist directs the simulation. After choosing the proper patient setup, the dosimetrist and medical physicist generates an isodose plan, which is then used to provide the energy settings for the one or more ports for delivering the high-energy linac beam to the target volume or area. Starting with the first port on the first day of treatment, the linac alignment is confirmed on a regular basis for each and every port.
Should certain organs need to be protected, the radiation oncologist designs "blocks" to shield them from the high-energy x-rays. Blocks can be made from a material called Cerrobend, which has nearly the density as lead, or can be configured within the head units of some linear accelerators by a device known as a multi-leaf collimator (MLC).