The review entitled "Current Status and Optimal Use of Radiosurgery," by Drs. Chang and Adler, presents a summary of recent trends and findings in conventional radiosurgery as well as novel approaches being undertaken at Stanford University with both intracranial and extracranial high-precision treatments. The authors do an excellent job of summarizing what we know about the management of radiosurgical targets in the head, including multiple metastases, acoustic neuromas, and arteriovenous malformations. It is important, however, to emphasize that there is some disagreement about the optimal course of treatment for patients presenting with multiple metastases. The authors’ experience in the application of radiosurgical techniques for nasopharyngeal boosts is unique.
Following a summary of cranial radiosurgery, the article delves into the requirements for fractionated treatments of the brain and discusses extracranial applications. The authors have a close working relationship with Accuray Inc, commercial supplier of the CyberKnife, a robotic treatment system. The group at Stanford has contributed significantly to the evolution of this system into a practical device for precision radiotherapy treatment. Although the authors demonstrate the enormous potential of the system, it is important to expand on their overview of the technology applied to modern high-precision radiotherapy.
Accuracy of Patient Positioning
The CyberKnife system referred to in this article ensures the precision of patient positioning via two components. The primary component is a system that acquires a pair of radiographs, analyzes the radiographs to determine the position and orientation of the patient with respect to the model used in treatment planning, and corrects for deviations in the patient’s or robot’s position. Although this system operates quickly, it does not provide real-time feedback, and thus, precautions should be taken to ensure that sudden movements do not occur. Thus, the need to immobilize the patient is not eliminated with this system, but it does reduce the invasiveness of immobilization and separates the concepts of immobilization and position reproducibility.
As the authors point out, the initial fluoroscopic system is limited in the amount of internal radiographic anatomy presented, and the recent addition of active matrix flat panel imagers (while slower than the fluoroscopic system) has greatly improved their ability to localize internal targets.
The second major component of their localization systemthe tracking of ventilatory movementis achieved by externally monitoring the visible movement of the chest and abdomen. This is necessary, even with the use of internal radiopaque markers, because of the duty cycle (ie, the duration of time) involved in image acquisition, analysis, and feedback. The accuracy with which external movement of the patient correlates with internal movement of the target is still under investigation.
A couple of other methodologies that have been applied or are under investigation for patient positioning and real-time tracking during precision radiotherapy should be pointed out. For cranial applications, a system of tracking infrared light-emitting diodes attached to a bite block was developed by the University of Florida and has reported real-time feedback with submillimeter accuracy. For extracranial targets, ventilatory movement has been addressed via both active breath hold during irradiation and real-time tracking of radiopaque markers. The tradeoffs for tracking vs active breathing control involve patient comfort, reproducibility of position, and the duty cycle of irradiation vs gated-beam hold.
Methods of Dose Delivery
The authors describe the delivery system of the CyberKnife and contrast that with multiple isocenter treatment using circular collimators and arcs. The CyberKnife system presents a different paradigm for the optimization and delivery of treatment, compared to conventional linear accelerators (LINACs), and has realized the potential for highly conformal target irradiation. It should be recognized, however, that LINAC radiosurgery has progressed beyond the concept of circular collimation.
Numerous commercial devices allow treatment with radiosurgery using high-resolution multileaf collimators. These multileaf collimators exist as add-on equipment to standard linear accelerators and integrated components in dedicated LINACs. Using these systems, a highly conformal treatment can be delivered to a target using multiple static-shaped radiation fields. These systems are also capable of dynamic treatment delivery, and have the potential for dynamically conforming to the target as the gantry sweeps through an arc or, for larger targets, of providing intensity modulation to improve avoidance of critical structures. Nedzi et al demonstrated dosimetric improvement for these devices compared to conventional radiosurgical techniques.
In summary, the article by Drs. Chang and Adler does an excellent job of pointing readers toward the dramatic improvements in precision achieved with radiotherapeutic technology. Their overview highlights the potential of using treatment delivery concepts throughout the body that were once thought to be restricted to invasive cranial fixation applications. It is important, however, to realize that such high-precision treatments can potentially be achieved with systems other than the CyberKnife.