Focal irradiation and image fusion techniques (IGRT)--Part IV.

s e t s n mage-guided radiation therapy (IGRT) is currently an merging technology and is undergoing rapid transormation. There have been different interpretations of his technology. Medical Dosimetry journal has deoted 4 special volumes on this topic. With the recent evelopment in positron emission tomography (PET), n particular, the commercial availability of PET-CT ybrid scanner, the advancement of IGRT has been erceived as the integration of images from various edical imaging modalities to support the delineation f targets for three-dimensional (3D) treatment planing. Another interpretation has been toward the aplication of medical imaging modalities to set up atients prior to dose delivery to reduce patient setup ncertainties. Various technologies have been develped to support this concept including (a) ultrasound maging, (b) in-room CT (also known as CT-on-rail), c) mobile kilovoltage x-ray unit, and (d) cone-beam T technology using kilovoltage and megavoltage raiation beams. The integration of these IGRT technolgies to the dose delivery systems has been described n the first IGRT special volume of the Medical Doimetry journal. Of particular interest in the integraion of IGRT technologies to the dose delivery system s the implementation of cone-beam CT techniques. his cone-beam CT technique allows patient data to be cquired volumetrically by rotating the gantry of the ose delivery system around the patient. The patient ata are then reconstructed to obtain transaxial and/or ultiplanar CT images. Along with the advancement f medical imaging modalities for diagnostic radiolgy and radiation oncology, there have been increasng concerns about the impact of voluntary and involntary patient movements. Static in silico treatment lans may have little resemblance to the dose distriutions achieved in living, breathing, fidgeting paients who may gain or lose 10% of their body weight ver a 7-week course of fractionated treatments. Inoluntary patient movement such as breathing has een addressed using (a) forced breathing technique, b) shallow breathing technique, and (c) motion-gating echniques. Motion-gating techniques and the technolgy of cone-beam CT has been reviewed in the second GRT special volume of Medical Dosimetry. The third and fourth special volumes in the IGRT eries are devoted to focal irradiation. In the past, focal rradiation has been limited to the treatment of intracraial lesions. Stereotactic frame systems fixed to the skull ave been used to guide the dose delivery. Because the xation of the frame systems is invasive and difficult to e