Interfraction and Intrafraction Motion–IGRT and Dynamic Targeting
Tumors are not stationary. They move between and during daily treatments.They are subject to ‘interfraction’ motion due to unavoidable day-to-day variations in anatomy or in how patients are positioned for treatment. And they are subject to ‘intrafraction’ motion, due to respiration and other physiological processes.
To deal with ‘interfraction’ motion, radiation therapy is embracing a strategy called image-guided radiation therapy (IGRT). IGRT tools enable clinicians to image a tumor just prior to treatment and to adjust for set-up errors or changes in tumor position. They include technologies like the On-Board Imager™ device from Varian Medical Systems, a kilovoltage X-ray imaging system that is mounted on the radiotherapy treatment machine or linear accelerator. The On-Board Imager provides high-resolution radiographic, fluoroscopic, and cone-beam CT images of the tumor just before and during treatment. Varian’s SonArray™ system is another IGRT technology that uses ultrasound imaging for daily prostate localization. And the company’s RPM™ Respiratory Gating system can be used to synchronize imaging and dose delivery with a patient’s breathing cycle.
These and other Dynamic Targeting™ IGRT technologies enable doctors to plan and compensate for tumor motion in realtime.
Stereotactic Strategies in Radiation Oncology
Because Dynamic Targeting IGRT improves precision, it also raises the possibility of reducing the 30 to 40 daily treatment sessions or ‘fractions’ normally needed for delivering a total dose of radiation.With improved imaging and delivery technology, some small lesions could be treated in a single session with ‘stereotactic radiosurgery’. Others could be treated in as few as three to five sessions with ‘hypofractionated radiotherapy.’