Advances in Image-guided Radiotherapy - The Future is in Motion
Advances in Image-guided Radiotherapy - The Future is in Motion
US Oncology Review 2006
Published: October 2008
Published: October 2008
External beam radiotherapy (EBRT) is the most common form of radiation treatment offered to cancer patients. Currently, different types of EB therapy techniques are used. The goal of three-dimensional conformal RT (3-D CRT) is to deliver a full dose of irradiation to the target structure with as little radiation as possible affecting the surrounding normal tissue. Intensity-modulated RT (IMRT) is a further refinement of CRT that allows the dose within a target to be modified, so as to spare specific tissue and organs. In order to take respiratory motion of the target into account another approach is 4-D CRT, which can also be combined using intensity modulated fields. Many of the current advancements in EBRT involve techniques for tracking the motion of tumors—techniques that fall under the collective heading of image-guided radiation therapy (IGRT).
Historical Overview
Different approaches toward IGRT have been followed over the years. In the 1990s, the first electronic portal imaging devices (EPIDs) for linear accelerators (LINACs) were developed, initially with charge-coupled device (CCD) camera optics, later using liquid ion chamber technology, and now mostly based on amorphous silicon flat panels. The next step has been the introduction of room- or gantry-based kilovoltage (kV) radiograph and fluoroscopy devices, also allowing localization by means of bony structures or fiducial markers.
An initial way of obtaining 3-D information was achieved by placing a conventional computed tomography (CT) scanner in the treatment room in a known geometric relationship to the linear accelerator’s isocenter. Now, CT functionality has been integrated in the linear accelerator (LINAC) in order to eliminate the need for a separate scanner. These cone beam CT (CBCT) options are based on either an additional kV system or by using megavolt radiation from the therapy beam source. Most recent publications indicate that on-board imaging devices with a separate kV system offer the largest flexibility with regard to different modes such as radiography, fluoroscopy, and CT.
Gating/4-D Imaging
Most 3-D treatment planning systems utilize CT images based on a diagnostic CT scanner. These scanners limit how the patient can be positioned because of their relatively small opening; in order to overcome these issues, dedicated oncology scanners with a large bore have been developed. The new RT department at the Hirslanden Klinik Aarau was one of the first clinics in Europe to be equipped with a special RT scanner. Such multi-slice systems with up to 16 detector rows allow high-speed scans to be acquired. These scanners additionally offer 4-D functionality, which means the scans are obtained with co-registered respiratory signals.This technique entails the creation of multiple CT slices at each relevant table position for at least the duration of one full respiratory cycle, while simultaneously recording signals from a respiratory motion monitoring system.
