Image-Guided Radiation Therapy [ Active ]

Preface

This book presents key image-guided radiation treatment (IGRT) technologies for external beam radiotherapy and caps a multidecade phase of technology development in the realm of conformal, customized radiation treatment. This development phase has been somewhat brief and vigorous, with new IGRT innovations such as increased image fidelity and adaptive radiotherapy continuing through the present day. IGRT had been in development in earnest since the early 1990s as a desired companion to intensity-modulated radiation treatment (IMRT). It was known at the time that beam-intensity modulation would be proven to enable beamlets of radiation dose to be formed and delivered to give highly conformal treatment to target volumes, while at the same time providing avoidance of even nearby normal structures. IMRT was being developed with pathways that were based on particular technological features of each vendor's designs for their multileaf collimators (MLCs) and linear accelerators, e.g., leaf design (width, height, focus, speed, etc.), dose rate control, error checking, and gantry motion control. In a previous decade, the 1980s into the mid-1990s, three-dimensional conformal radiation treatment (3D-CRT) had been developed such that for the first time, using static pretreatment 3D images from computed tomography (CT), anatomical volumes could be identified and segmented for the target, normal structures, and the external contour. As 3D-CRT and IMRT technologies developed, it was recognized that confirming "correct" treatment geometry for every individual fraction might be important, since daily variations in treatment position and the locations of internal structures would lead to blurring (degradation) of the cumulative dose distribution. Thus, IGRT was born from the important requirement for verifying the correct target and normal tissue positions before "beam-ON."

While IGRT technologies are new, the importance of correct patient positioning was recognized very early on and proven with clinical studies. Early important IGRT-related activities of relevance include the following: (1) the development of patient immobilization devices and weekly port field imaging in the 1960s–1970s that paved the way for today's more precise and accurate daily radiation treatments, and (2) in the 1950s–1960s, the addition of a kilovoltage x-ray tube to cobalt teletherapy devices, side-mounted on the gantry head, to provide quality portal images with reasonable contrast and much better sharpness than those obtained with megavoltage gamma rays from a nominal 2-cm diameter cobalt-60 source.

Similar to differing designs and implementations for 3D-CRT and IMRT, IGRT technologies show a richness and creativity that were driven by particular linear accelerator designs, each with existing unique advantages and limitations, as well as the visions of the medical physicists, clinicians, and engineers who conceived of "in-room imaging" immediately before or during treatment. In some cases, image-guidance was an "addon" to existing linear accelerator designs, either free-standing additions or coupled to the gantry, or took advantage of the megavoltage treatment beam to use it for imaging. In other approaches, new basic designs for both treatment and imaging were conceived and implemented that used either kilovoltage or megavoltage imaging and abandoned the conventional C-arm linear accelerator gantry designs. Additional radiological, ultrasound, and optical technologies have provided alternative means to determine or assist with target and/or patient position, including near real-time monitoring for fast feedback of treatment position. In particular, anatomical and biological imaging using CT and positron emission tomography have contributed to the understanding of target volume boundaries and biological behavior, including the effects and accommodation of physiological motion (e.g., respiratory, cardiac, and other) on radiation treatment accuracy, precision, and clinical outcome. Similarly important, computing and imaging science software algorithms and tools serve as the infrastructure to integrate the various IMRT processes into a procedure that can be readily implemented in the clinic. The results of these modalities, software tools, and imaging treatment geometries are a wealth of hybrid IGRT technologies and devices for coupled imaging + treatment inside the radiation treatment room—these IGRT technology classes are reviewed herein.

This book benefits from having nationally and internationally known authors who actively participated in the development of IGRT, imaging, and ancillary technologies. Their expertise is evident in the descriptions of IGRT technologies and their clinical uses and impact. The editor expresses his great appreciation for their important and excellent contributions to make this publication possible such that medical physicists, clinicians, and trainees will profit from their intellect, insight, and scientific curiosity for the benefit of patient care.