Frontiers of Radiation Therapy and Oncology最新文献

Elaborate methods of patient imaging for diagnostics, dose calculation, and radiation delivery are currently used to develop treatment plans with highly conformal patient dose distributions. However, the true delivered dose likely deviates from the planned distribution due to differences in patient position, anatomic changes due to weight loss or tumor shrinkage or variations of linear accelerator output during treatment. All the steps in a radiation treatment from diagnostics to the planning process are based on three-dimensional imaging, with the exception of treatment verification performed with electronic portal imaging devices (EPIDs) and two-dimensional images. Megavoltage cone beam CT (MV CBCT) generates an accurate three-dimensional representation of the patient anatomy, moments before the same X-ray beam is used for treatment. The three-dimensional images will provide additional information on the patient's treatment position and offer a wide range of opportunities to improve the delivery of radiation. The MV CBCT image can be registered with the planning CT for patient setup verification and correction. The periodic acquisition of three-dimensional images will allow the monitoring of anatomical changes over the treatment course due to tumor response or weight loss. The MV CBCT image can also be imported into the planning system to complement the regular CT in the presence of metallic objects or to measure the dosimetric impact of patient misalignment and anatomy modification on dose distribution. By combining exit dosimetry with the EPID and MV CBCT, this technology may play a key role in tracking the dose delivered to the patient, taking us into an era of dose-guided radiation therapy .

Local control and survival of most upper abdominal malignancies are poor. Challenges associated with the safe delivery of tumoricidal doses of radiation therapy to these malignancies include organ motion due to breathing, gastrointestinal filling and peristalsis, and the presence of many normal tissues with a low tolerance to radiation. Intensity-modulated radiation therapy (IMRT) can facilitate normal tissue sparing and dose escalation to these tumors, which has the potential to reduce toxicity and improve local control. Planning studies have demonstrated the potential for dose escalation with IMRT. However, degradation of upper abdominal IMRT plans in the presence of organ motion has also been demonstrated. Thus, organ motion reduction and image guidance strategies should be implemented in conjunction with IMRT. Clinical experience with dose-escalated IMRT is limited, and IMRT should continue to be studied in clinical trials before it is routinely used for upper abdominal malignancies.

In this paper, the current state of intensity-modulated radiation therapy (IMRT) treatment planning systems is reviewed, including some inefficiencies along with useful workarounds and potential advances. Common obstacles in IMRT treatment planning are discussed, including problems due to the lack of scatter tails in optimization dose calculations, unexpected hot spots appearing in uncontoured regions, and uncontrolled tradeoffs inherent in conventional systems. Workarounds that can be applied in current systems are reviewed, including the incorporation of an 'anchor zone' around the target volume (including a margin of separation), which typically induces adequate dose falloff around the target, and the use of pseudostructures to reduce conflicts among objective functions. We propose changing the planning problem statement so that different dosimetric or outcome goals are prioritized as part of the prescription ('prioritized prescription optimization'). Higher-priority goals are turned into constraints for iterations that consider lower-priority goals. This would control tradeoffs between dosimetric objectives. A plan review tool is proposed that specifically summarizes distances from a structure to hot or cold doses ('dose-distance plots'). An algorithm for including scatter in the optimization process is also discussed. Lastly, brief comments are made about the ongoing effort to use outcome models to rank or optimize treatment plans.

Helical tomotherapy is a volumetric image-guided, fully dynamic, intensity-modulated radiation therapy (IMRT) delivery system. The daily use of its pretreatment megavoltage (MV) CT imaging for patient setup verification allows one to correct for interfraction setup error. This is a primary requirement for the accurate delivery of complex IMRT treatment plans, which give differential radiation doses to various target volumes while conformally avoiding normal critical structures. In particular, image guidance using MV CT allows for direct target position verification with the patient in the actual treatment position just prior to therapy delivery. Moreover, since helical MV CT imaging is a slow CT imaging technique, it allows for the encoding of target motion in the resulting MV CT data set, and therefore the pretreatment verification of a motion envelope defined from four-dimensional CT.

The purpose of this work is to provide background and current directions of image guidance for localized prostate cancer treatments. We will describe the external beam hypofractionation protocol for localized prostate cancer currently in progress at Stanford University and the biological bases for large fractions in an abbreviated treatment course for prostate cancer. The need for image guidance in external beam prostate cancer treatments will be discussed. Our experience with two imageguided implementations of hypofractionated radiotherapy for localized prostate cancer will be presented. These are the Cyberknife System (Accuray, Inc.) and the Trilogy System (Varian Medical Systems, Inc.).

Highly conformal radiation therapy tailors treatment to match the target shape and position, minimizing normal tissue damage to a greater extent than previously possible. Technological advances such as intensity-modulated radiation therapy, introduced a decade ago, have yielded significant gains in tumor control and reduced toxicity. Continuing advances have focused on the characterization and control of patient movement, organ motion, and anatomical deformation, which all introduce geometric uncertainty. These sources of uncertainty limit the effectiveness of high-precision treatment. Target localization, performed using appropriate technologies and frequency, is a critical component of treatment quality assurance. Until recently, the target position with respect to the beams has been inferred from surface marks on the patient's skin or through an immobilization device, and verified using megavoltage radiographs of the treatment portal. Advances in imaging technologies have made it possible to image soft tissue volumes in the treatment setting. Real-time tracking is also possible using a variety of technologies, including fluoroscopic imaging and radiopaque markers implanted in or near the tumor. The capacity to acquire volumetric soft tissue images in the treatment setting can also be used to assess anatomical changes over a course of treatment. Enhancing localization practices reduces treatment errors, and gives the capacity to monitor anatomical changes and reduce uncertainties that could influence clinical outcomes. This review presents the technologies available for target localization, and discusses some of the considerations that should be addressed in the implementation of many new clinical processes in radiation oncology.

Primary and metastatic tumors to the lung have been principle targets for the noninvasive high-doseper- fraction treatment programs now officially called stereotactic body radiation therapy (SBRT). Highly focused treatment delivery to moving lung targets requires accurate assessment of tumor position throughout the respiratory cycle. Measures to account for this motion, either by tracking (chasing), gating, or inhibition (breath hold and abdominal compression) must be employed in order to avoid large margins of error that would expose uninvolved normal tissues. The treatments use image guidance and related treatment delivery technology for the purpose of escalating the radiation dose to the tumor itself with as little radiation dose to the surrounding normal tissues as possible. Clinical trials have demonstrated superior local control with SBRT as compared with conventionally fractionated radiotherapy. While late toxicity requires further careful assessment, acute and subacute toxicity are remarkably infrequent. Radiographic and local tissue effects consistent with bronchial damage and downstream collapse with fibrosis are common, especially with adequate doses capable of ablating tumor targets. As such, great care must be taken when employing SBRT near the serially functioning central chest structures including the esophagus and major airways. While mechanisms of this injury remain elusive, ongoing prospective trials offer the hope of finding the ideal application for SBRT in treating pulmonary targets.

Introducing new technologies into radiation oncology clinical practices poses very specific logistical dilemmas. How do we determine that a new technology's dose distribution is better than the 'standard' and what are the methods that can be applied to easily compare the 'new' with the 'old'? We consider how the benchmark dose-volume histogram (DVH) can serve as a conceptual model to approach these issues. Comparing dosimetric differences using benchmark DVHs helps a 'global' comparison of the area under the curve that is intuitive, relatively efficient and easily implemented. These concepts, applied in prostate cancer in this communication, have wider applications in other disease sites and in the introduction of technologies beyond intensity-modulated radiation therapy.

Many recent advances in the technology of radiotherapy have greatly increased the amount of image data that must be rapidly processed. With the increasing use of multimodality imaging for target definition in treatment planning, and daily image guidance in treatment delivery, the importance of image registration emerges as key to improving the radiotherapy planning and delivery process at every step. Both clinicians and nonclinicians are affected in their work efficiency. Image registration can improve the correspondence of information in multimodality imaging, allowing more information to be obtained for tumor and normal tissue definition. Image registration at treatment delivery can improve the accuracy of therapy by taking greater advantage of images available prior to treatment. Technical advances have enhanced the accuracy and efficiency of registration through several approaches to automation, and by beginning to address the tissue deformation that occurs during the planning and therapy period. When using an automated registration technique, the user must understand the components of the registration process and the accuracy and limitations of the algorithm involved. This review presents the fundamental components of image registration, compares the benefits and limitations of different algorithms, demonstrates methods of visualizing registration.