Quality Assurance (QA) for Kilovoltage Cone Beam Computed Tomography (CBCT)

Abstract

The use of an imaging modality for verification of patient and target location immediately prior to application of radiation therapy has spread widely in the last years. It is referred to as Image Guided Radiation Therapy (IGRT). Precise targeting of the tumor is very important and imaging can be immensely helpful in this process. Hence the quality assurance of the imaging systems used to guide the beam is also essential. One could argue that it is as critical as quality assurance for the radiation dose output, since the right amount of radiation dose delivered to the wrong location can be as detrimental as delivering the wrong amount of radiation dose to the right location. Kilovoltage (kV) Cone Beam Computed Tomography (CBCT) is a premiere modality for image guidance, since it produces three-dimensional images of the region of interest of the patient in a relatively short time (1-3 min) in a sufficient quality for the purpose of patient alignment. Popular systems are the Varian OBI (Varian Medical Systems, Inc., Palo Alto, CA, USA) and the Elekta XVI system (Elekta AB, Stockholm, Sweden), shown in figure 1. This chapter introduces the different tests needed for a comprehensive QA of kV CBCT. The tests are related to (1) System safety, (2) Geometric accuracy, that is agreement of the CBCT imaging isocenter with the isocenter of the MV treatment beam, (3) Registration and correction accuracy, that is how accurately can the system position a patient, (4) Image quality, and (5) x-ray tube and generator performance (dose to the patient). It further suggests a frequency with which each of the tests should be performed, organizing them in being part of the daily, monthly and annual QA procedures. In addition, tests which should be performed following system repair or upgrades are described. Responsibility for all tests should be with a Qualified Medical Physicist. Generally the physicist will delegate some of the tests, likely the daily tests to other members of the Radiation Therapy team, for instance the Radiation Therapists. (Klein et al., 2009). For information about in-room kV X-ray imaging beyond CBCT QA see the report of AAPM Task Group 104 (Yin et al., 2009) and (Jaffrey et al., 1999).