Catálogo de publicaciones - libros

Many soft-tissue tumors targeted with extracranial SRS move during respiration. New imaging technologies, motion compensation strategies, and treatment planning algorithms are being developed which enable tracking and treatment of moving tumors in real-time. For this chapter we reviewed the literature to determine known tumor motion amplitudes for lung, liver, and pancreas. Then we analyzed predicted tumor motion for 36 patients and 117 treatment fractions that were previously saved in CyberKnife® (Accuray Incorporated, Sunnyvale, CA) treatment logfiles. These represent 27 tumors in the lung (16 upper lung, 4 middle lung, 7 lower lung) and 9 pancreas patients. For each treatment, the location of the target at end inspiration and end expiration was determined in the patient coordinate system. The origin of the patient coordinate system is at the center of mass of the fiducials as marked on the simulation CT, +x is patient inferior, +y patient left, and +z anterior in a right-handed coordinate system. The mean and variance of respiratory cycle extrema positions were calculated using a program written in MatLab code. Observed motion ranges for all sites except pancreas agree very well with the literature. The largest motion amplitudes of up to 38.7 mm were observed in the lower lung. Twenty-five percent of tumors in the upper lung could have been treated without Synchrony® (Accuray Incorporated, Sunnyvale, CA) with a PTV margin of 2 mm, because the uncertainty is in the range of the technical tracking accuracy of Synchrony of 1.5 mm. Possible causes of large fluctuations around the mean motion could be fiducial tracking errors or irregular breathing. We concluded that a subset of all patients could have been treated using skeletal structure tracking, rather than implanted fiducials, and a PTV margin in the range of the stated tracking accuracy for Synchrony. Defining meaningful parameters to characterize the effects of free breathing is part of ongoing research, since published data from non-dynamic SBRT is limited to short fluoroscopic studies or Cine-CT. The results can be transferred to other treatment modalities to determine PTV margins in standard external beam treatments as well as defining the PTV in the third dimension for 2D motion compensation [ 1 ].

Tumors in the thorax and abdomen move during respiration. One way to man age respiratory motion is to move or shape the radiation beam to dynamically follow the tumor’s changing position, an approach that is often referred to as real-time tracking. The Synchrony® Respiratory Tracking System, which is an integrated subsystem of the CyberKnife® Robotic Radiosurgery System (Accuray, Incorporated, Sunnyvale, CA) is a realization of real-time tracking for tumors that move with respiration. Alignment of each treatment beam with the moving target is maintained in real time by moving the beam dynamically with the target. An advantage of the Synchrony system is that patients can breathe normally during treatment while the robotic manipulator moves the linear accelerator dynamically. The primary concept in the Synchrony system is a correlation model between in ternal tumor position and external marker position. The position of external optical markers, which are attached with Velcro to a snugly fitting vest that the patient wears during treatment, are measured continuously with a stereo camera system. At the start of treatment, the internal tumor position is measured at multiple discrete time points by acquiring orthogonal X-ray images. A linear or quadratic correlation model is generated by fitting the 3D internal tumor positions at different phases of the breathing cycle to the simultane ous external marker positions. An important feature of this method is its ability to fit different models to the inhalation and exhalation breathing phases, which enables accurate tracking even when the tumor or external marker motions exhibit hysteresis. During treatment, the internal tumor position is estimated from the external marker positions using the correlation model, and this information is used to move the linear accelerator dynamically with the target. The model is checked and updated regularly during treatment by acquiring additional X-ray images. This chapter presents the concepts and methods of the Synchrony Respiratory Tracking System. Experimental mea surements and retrospective analysis of clinical data show that the accuracy of the Synchrony System is approximately 1.5 mm.

When using the CyberKnife® (Accuray Incorporated, Sunnyvale, CA) Image-Guided Stereotactic Radiosurgery (SRS) System to treat soft-tissue tumors in anatomic sites other than intracranial or spinal locations — such as in the lung, liver, kidney, prostate, and pancreas — fiducial placement in or close to the tumors is necessary to assist patient alignment and target tracking for precise treatment delivery. Under the assumption of rigid transformation, at least three fiducial markers are required to obtain six-degreesof-freedom transformation parameters, i.e., three translations and three rotations. However, in most cases, soft tissue is highly deformable and non-rigid. This results in three possible scenarios: 1) the rigid body criteria fail and the rotational transformation cannot be obtained, 2) the tumor deformation results in unreliable computed rotational information, and 3) even when the fiducial array meets the rigid body criteria, the orientation of the tumor often has poor correlation with the global body orientation and thus results in dosimetric deviation.

The radiobiological concepts of intrinsic radiation sensitivity, oxygenation, and dose-volume effects have been reasonably delineated in the context of conventional radiotherapy (RT). Yet, for circumstances in which large doses are delivered in single-fraction or hypofractionated regimens, these intrinsic radiobiological concepts are relatively poorly understood. Stereotactic radiosurgery (SRS) is a radical departure from the current RT approach in which large fields, cone downs, and protracted therapies are used for normal tissue preservation and to maximize the therapeutic ratio. SRS is the precise, highly focused delivery of radiation beams to lesions whereby only a fraction of the total dose is received by surrounding normal tissues. The usage of SRS is currently expanding well beyond its roots as an ablative tool for thalamotomies, arteriovenous malformations, and cranial vault tumors. Hence, widely believed dogmas concerning the tolerance of critical structures to conventionally fractionated doses, such as the dose-volume effect, total dose, and time (latency) dependency, have to be reevaluated for hypofractionated radiation therapy.

Stereotactic radiosurgery is well-established in the treatment of intracranial lesions, but its use in extracranial lesions is relatively new. Stereotactic radiation delivery technologies such as the CyberKnife ® (Accuray Incorporated, Sunnyvale, CA) are being used with increasing frequency to treat lesions outside the cranium, such as in the spine, thorax, and abdomen; lesions that were not previously amenable to radiosurgical treatment due to inherent motion. The critical role of imaging technologies in radiosurgery, both for planning treatments and assessing their effects, makes an understanding of developments in these technologies imperative for radiosurgery professionals. In this chapter, we review how positron emission tomography (PET) is combined with computed tomography (CT) to enhance disease staging, radiosurgery and radiotherapy planning, and follow-up.

The CyberKnife ® (Accuray Incorporated, Sunnyvale, CA) was initially used to treat lesions of the central nervous system (CNS), but in recent years the most rapid increases in utilization have been in the treatment of soft tissue lesions outside the skull and spine. Soft tissue tumors may be broadly classified as lung, abdominal, or pelvic tumors. Like lung tumors, abdominal tumors are sufficiently close to the diaphragm that they normally require correction for motion due to breathing, but the large number of critical organs in the abdomen raises treatment issues that are common to pelvic lesions that move little with respiration, such as prostate adenocarcinoma.

There is a great need for reliable tools for radiosurgery plan evaluation. With the increasing sophistication of radiosurgical treatment planning systems, the radiosurgeon finds a more challenging treatment environment and feels the need to utilize more sophisticated methods of plan evaluation. Ultimately, these methods carry the promise of assisting treatment decisions, or even replacing some of them with consistent, reliable, and verifiable measures of probable treatment success. Among the most common are indices of homogeneity, dose uniformity across the target area, and conformity, the shaping of the radiation dose to the target area. These can often be expressed as simple ratios of treatment target and normal tissue volumes receiving certain radiation doses, although more complicated forms exist. The importance of these tools lies in their rendering of complex concepts into simple values, allowing either more sophisticated additions to multiple clinical treatment parameters, or more simplification of a limited treatment parameter set to a limited metric.

We used the respiratory movement tracking system of the CyberKnife®, called Synchrony® (Accuray Incorporated, Sunnyvale, CA), to develop dose plans delivering 45 Gy (3 times 15 Gy) for the treatment of early stage non-small cell lung cancer (NSCLC). Characteristics of those plans were compared with plans developed for 3-Dimensional conformal radiotherapy (3D-CRT) administering 60 Gy (20 times 3 Gy) based on a slow CT. Ten patients with Stage I NSCLC previously treated with 3DCRT were replanned with the CyberKnife treatment planning system. In the 3D-CRT plan, the planning target volume (PTV) equaled the gross tumor volume (GTV)_slow + 15 mm. In the CyberKnife plan, the PTV equaled the GTV + 8 mm. The physical dose of both treatment plans was converted into the normalized total dose using the linear quadratic model with an α/β_tumor = 10 Gy and α/β_organs at risk(OAR) = 3 Gy. The mean doses administered to the PTV with the CyberKnife and 3D-CRT were 115.8 Gy and 66 Gy, respectively (p < 0.0001). The mean V20 of the CyberKnife and 3D-CRT plan was 8.2% and 6.8%, respectively (p=0.124). Both plans respected the constraints of the other organs at risk (OAR). In this context the CyberKnife can administer a much higher biological dose than 3D-CRT without increasing the dose (V_20) to the lungs.

Image-guided placement of fiducial markers is in some ways an extension of percutaneous procedures such as needle biopsy of lung pathology, which are native to most interventional radiology practices. To that extent, learning the procedure is not difficult for those who are familiar with the basic principles of image-guided lung nodule biopsy. However, there are significant modifications in the procedure that are necessary in order to ensure appropriate placement and distribution of the fiducial markers. Proper positioning of fiducial markers in specific geometric configurations is essential for accurate targeting of the nodule. This chapter focuses on the principles of CT-guided percutaneous placement of fiducial markers. For the most part, this procedure is performed on a consultative basis by interventional radiologists, physicians who specialize in minimally invasive image-guided therapy. Special considerations for patient selection, pre-procedural preparation, techniques, and post-procedural care are explained.

The use of the CyberKnife® (Accuray Incorporated, Sunnyvale, CA) for radiosurgical treatment of lung tumors [ 1 ] typically requires placement of one or more gold fiducials for tracking during treatment. Possible exceptions to this requirement are tumors involving the posterior chest wall, which may be localized with Xsightℳ (Accuray Incorporated, Sunnyvale, CA) based on spinal anatomy. Placement of gold fiducial seeds has been associated with post-placement pneumothorax at rates of 25–40% [ 1 ]. Some of these patients will require treatment by chest tube placement, and all of them will require repeated physician exams and radiological follow-up. In addition, some patients are refused consideration for radiosurgery for fear they will be unable to tolerate a potential pneumothorax. Pneumothorax prevention, therefore, has become a major priority in CyberKnife radiosurgery for lung cancer.