Clinical application of image-guided radiotherapy, IGRT (on the Varian OBI platform)

Image-guided radiation therapy (IGRT) can be used to measure and correct positional errors for target and critical structures immediately prior to or during treatment delivery. Some of the most recent available methods applied for target localization are: transabdominal ultrasound, implanted markers with in room MV or kV X-rays, optical surface tracking systems, implantable electromagnetic markers, in room CT such as kVCT on rail, kilovoltage or megavoltage cone-beam CT (CBCT) and helical megavoltage CT. The verification of the accurate treatment position in conjunction with detailed anatomical information before every fraction can be essential for the outcome of the treatment. In this paper we present the on-board imager (OBI, Varian Medical Systems, Palo Alto, CA) that has been in routine clinical use at the Karolinska University Hospital since June 2004. The OBI has been used for on-line set-up correction of prostate patients using internal gold markers. Displacements of these markers can be monitored radiographically during the treatment course and the registered marker shifts act as a surrogate for prostate motion. For this purpose, on-board kV–kV seems to be an ideal system in terms of image quality. The CBCT function of OBI was installed in March 2005 at our department. It focuses on localizing tumors based on internal anatomy, not just on the conventional external marks or tattoos. The CBCT system provides the capacity for soft tissue imaging in the treatment position and real-time radiographic monitoring during treatment delivery.