Cone beam CT: Transforming radiation treatment guidance, planning, and monitoring

Table 15.1 provides a summary of the milestones in evolution of x-ray imaging technologies to verify treatment setup. Beginning with the introduction of ready pack films for RT in the 1970s, projection imaging with megavoltage (MV) x-rays from the linear accelerator was the mainstay of treatment verification into the 1990s. Portal images were acquired to ensure the correct placement of the beam port or aperture in the body. The lack of conspicuous anatomic information (i.e., visibility of highcontrast bone structures or airways only) in a portal image was compensated with the use of an open (nonshaped) beam to localize the surrounding anatomy. Double exposure imaging, combining both localization and portal image, was thus devised to verify the correct placement of the treatment aperture with respect to the patient anatomy. Weekly double exposure MV imaging remains a standard practice for those treatments that do not require kilovoltage (kV) volumetric image guidance.