Simulation in the Determination and Definition of Treatment Volume and Treatment Planning

Abstract

One of the cornerstones of modern radiation therapy practice are volumetric patient image datasets from computed tomography (CT), magnetic resonance imaging (MRI), positron emission tomography (PET), and ultrasound (US). Current radiation therapy imaging devices (simulators) include (1) Conventional Simulators (based on conventional x-ray radiography planar imaging); (2) CT-simulators (based on CT scanners); (3) PET/CT-simulators (based on PET/CT scanners and); (4) MR-simulators (based on MR scanners). The radiation therapy simulator has been an integral component of the treatment planning process since the 1960s. Conventional simulators are designed to mimic the linear accelerator geometry while providing a diagnostic-quality x-ray beam for anatomic imaging. Due to the increased use of 3D imaging for treatment planning, conventional simulators are less popular than in the past. Still, this technology continues to be developed and retains its presence in most radiation oncology departments. Shortly after the introduction of clinical CT scanners in early 1970s it was realized that this imaging modality had much to offer in a radiation oncology setting because they provided a volumetric view of the patient’s normal and tumor anatomy with excellent spatial accuracy. In response, CT simulators were developed during the 1980s and 1990s and today CT simulator is the main imaging device in radiation therapy. CT simulators have developed to a point where CT scanner manufacturers are designing scanners specifically for CT simulation purposes. PET imaging can also provide valuable information about tumors and PET/CT scanners have been implemented as radiation therapy simulators and can be found in many radiotherapy centers. Finally, MRI scanners have also been implemented and are used for treatment simulation in radiation therapy. MRI scanners are less commonly found in radiotherapy departments, but due to the imaging advantages that MRI has to offer it is expected that the use of MRI scanner for radiotherapy simulation will experience significant growth. These devices have enabled better delineation of treatment volumes and critical structures while improving our ability to image patients in better treatment positions and with improved immobilization devices. Successful radiation therapy imaging (simulation) program must consider capabilities of individual imaging equipment, immobilization equipment, and the needs of individual techniques and treatment sites. This chapter describes the radiation therapy simulation process, design and features of conventional, CT, PET/CT, and MRI simulators, and their use for treatment planning.