Frontiers of Radiation Therapy and Oncology最新文献

Proton beams can provide a substantial dosimetric advantage because of their unique depth-dose characteristics, which can be exploited to achieve significant reductions in normal tissue doses proximal and distal to the target volume. These may allow escalation of tumor doses, potentially improving local control and survival while at the same time reducing toxicity and improving quality of life. While many of the steps in proton and photon treatment planning processes are similar, there are also significant differences. Some of these arise from the unique physical characteristics of protons, while others are the result of their greater vulnerability to uncertainties, especially from inter- and intrafractional variations in anatomy. These factors must be considered in designing margins and field-shaping devices, as well as in designing treatment plans as a whole and in evaluating them. Ongoing research is aimed at better estimation of these uncertainties and their impact on proton therapy, and reducing these uncertainties through image guidance, adaptive radiotherapy and the development of novel imaging devices and dose computation algorithms. For proton therapy delivery, intensity modulation techniques are already in use, and will continue to be developed and utilized increasingly. The advantages include greater flexibility in dose shaping for improved target coverage and reduced normal tissue dose, potential improvement in plan robustness, and improvement in clinical efficiency. A spectrum of imaging techniques can now be used to assist our understanding of proton dosimetry in the patient, and PET imaging is the one that is furthest developed toward the goal of in vivo dose imaging. To decrease the cost of proton therapy and increase its availability, many technical improvements and practical delivery technologies are being developed, including compact proton machines that will soon become clinically available.

Recently developed 4D CT imaging technologies have shown that significant organ motion can occur within radiotherapy fields during treatment. Most often a result of respiration, this motion can cause dose delivery errors that are clinically significant when unmanaged, as demonstrated in many recent investigations. Motion during the regular breathing cycling is important, but day-to-day breathing variations, as may be caused by changes in residual tidal volume, can cause systematic shifts in tumor position. These may cause delivery misalignments because the tumor is not in the same average location at each treatment. Approaches to management of this motion may involve motion-inclusive planning, gating or tracking. 4D CT has been instrumental in most of these approaches. Given the state of treatment planning software, it is not possible to preplan whether a specific patient would benefit from one or another of these methods. Daily imaging (or use of a nonimage-based system such as Calypso) is necessary to locate the tumor, and the location must be correlated with measurements from a system that tracks breathing motion during treatment delivery. This is typically done using an independent metric that characterizes the breathing cycle (e.g. the height of the abdomen). Only then can the treatment plan be accurately implemented. There are many methods to manage tumor motion, though most are challenging to implement and remain poorly supported by vendors. When determining which system to use, an important distinction between competing approaches is whether they are amplitude- or phase-based. Some implementations may use different approaches for different parts of the treatment planning and delivery process, potentially introducing errors in the characterization of breathing motion. While many advances have been achieved and are discussed here, the development of solid, stable and robust processes to effectively manage breathing motion remains a foremost and continuing challenge in radiotherapy.

Helical tomotherapy is a treatment device that is designed to deliver intensity-modulated radiation therapy treatments. Helical tomotherapy systems have been used to treat a wide spectrum of anatomical sites. In addition to its unique delivery technique, the capability to obtain megavoltage-based CT (MVCT) images is highly integrated into the system's image guidance. The introduction of MVCT imaging into clinical practice has prompted a range of technical and clinical investigations. The image quality, image dose and use of MVCT images for dose calculation have been investigated. At the same time, routine clinical use of MVCT imaging has provided a wealth of clinical experience. Both technical and clinical experiences with the MVCT system will be reviewed in this chapter.

The practice of radiation therapy continues to build on rapid advancements in treatment planning and delivery technology, which brings real potential for improving treatment outcomes. Manufacturers have employed advanced computer and imaging technology to produce treatment planning/delivery systems capable of precise shaping of dose distributions, conformal target volume coverage for even the most complex shapes and conformal avoidance of specified sensitive normal structures. However, these new systems have led to a more complex, less intuitive planning and treatment delivery process that presents great challenges for quality assurance/treatment verification. Advances in planning and delivery technologies will continue to occur at record paces, pushing the field toward even higher expectations for radiotherapy accuracy, reliability and applicability and leading the field to new standards of care. However, this optimism must be tempered with the realizations that for this to happen, progress is urgently needed in 3 areas, (1) accuracy in specification of gross tumor volume and clinical target volume, (2) radiation oncology informatics and (3) quality assurance, if we are to keep pace with these rapid planning and delivery developments.

Stereotactic body radiation therapy (SBRT) is a potent noninvasive means of administering high radiation doses to demarcated tumor deposits in extracranial locations. The treatments use image guidance and related advanced treatment delivery technologies for the purpose of escalating the radiation dose to the tumor, while sharply minimizing the radiation doses to surrounding normal tissues. The local tumor control outcomes for SBRT have been higher than any previously published for the radiotherapy of frequently occurring carcinomas. In addition, the pattern, timing and severity of the toxicities have been very different than from those seen with conventional radiotherapy. These issues pose challenges to our understanding of the radiobiological mechanisms and the optimal uses of SBRT. In this review, the clinical characteristics and outcomes of SBRT are presented in the context of their possible underlying mechanisms. While some of these considerations remain theoretical, they may outline at least qualitative understandings of the observed clinical effects, and motivate continuing research into the effects of SBRT that guide its most effective use in the clinic.

Healthcare economists generally agree that the development and rapid introduction of new technologies and the expanding utilization of existing ones in national healthcare systems have been significant factors in the dramatic and potentially unsustainable growth in healthcare spending. Creating a rational system for evaluation of emerging technologies in this country has been complicated by 3 broad issues: the often conflicting needs and expectations of the variety of stakeholders; an arcane and often illogical system of service valuation and payment; and the lack of a standardized, transparent and validated approach to the measurement of 'value.' Recent discussions on reforming the elements of healthcare delivery have increased focus on these systemic shortcomings and conflicts. As a specialty that is clinically wedded to modern and increasingly expensive technology, radiation oncology has often been singled out for scrutiny. A thorough examination and understanding of the various factors and controversies involved in technology development, implementation and valuation analysis is essential to rational growth and development of the specialty.

3D knowledge of the tumor position during abdominal and thoracic radiotherapy is an important component of motion management in radiation therapy. A wide variety of real-time position monitoring systems are available or under development. These are based on a diversity of modalities including radiofrequency, radioisotopes, ultrasound and MRI in addition to the optical, kilovoltage and megavoltage imaging systems available on conventional accelerators. These systems are also providing new insights into the magnitude and complexity of target and normal tissue motion during a course of therapy, and are driving the development of real-time targeting systems. Real-time targeting devices to align the tumor and the radiation beam have built upon technologies of robots, multileaf collimators, and couch-based and gimbaled positioning systems. The integration and widespread dissemination of systems that locate and target moving tumors are ongoing developments in the early 21st century, and future systems are likely to include the functionality of targeting temporally changing tumors and normal tissue physiology as well as anatomy.

Over the past decade, fundamental advances in image-guided radiation therapy (IGRT) have been made that are now being implemented in clinical practice. Imaging technologies to direct and confirm beam accuracy at the time of radiotherapy delivery have been intensively researched and developed. More recently, these imaging data have been used to evaluate and even modify the daily dose delivery of intended treatment plans. The rationale for the use of IGRT, to improve tumor control while limiting normal tissue toxicity, is a universal goal in radiotherapy. Avoidance of unexpected under- or overdosing during treatment is the most important benefit of IGRT, and has led to its integration into the use of advanced radiotherapy planning/delivery technologies for many clinical applications. Evidence-based strategies to effectively use IGRT in the clinic are still emerging. The evolving role of IGRT and some proposed strategies to exploit its potential benefits in the clinic will be presented, emphasizing the perspective of the radiation clinician. Practical strategies will be proposed to exploit the potential benefits of IGRT technologies in the clinic.

Intensity-modulated arc therapy (IMAT) is a rotational approach to radiation therapy delivered on a conventional linear accelerator using a conventional multileaf collimator. There are 2 key advantages of IMAT. First, the rotational nature of the delivery provides great flexibility in shaping each dose distribution. As a result, IMAT can provide dosimetric advantages relative to fixed-field intensity-modulated radiation therapy (IMRT). The second advantage is the highly efficient nature of the delivery. For centers with an active IMRT program, the clinical implementation of IMAT should be relatively straightforward. For clinical implementation of IMAT, it is important to fully characterize the accuracy of the dose model used, and the performance of the quality assurance equipment.

Experience with intensity-modulated radiation therapy (IMRT) for head and neck cancer is building greater understanding of the requirements for therapy planning. Delineation of the lymphatic targets for IMRT of the head and neck is a crucial step in this planning, and often determines the risks of marginal or out-of-field local/regional tumor recurrence. Definition of the gross tumor volumes needs to take into account both radiological (CT, MRI, PET) and clinical findings. Understanding of the appropriate CTVs is developing based on: (a) established knowledge of the natural history and spread patterns of head and neck cancer, (b) the accruing experience of clinicians using IMRT, and (c) evaluations of patient outcomes following consistent treatment approaches as determined by institution practice patterns and prospective clinical studies. This chapter will outline the important steps in lymphatic target definition for head and neck cancer, and will discuss several special clinical concerns for these patients and their management.