The field of stereotactic radiosurgery is rapidly advancing as a result of both improvements in radiosurgical equipment and better physician understanding of the clinical applications of stereotactic radiosurgery. This
The review entitled "Current Status and Optimal Use ofRadiosurgery," by Drs. Chang and Adler, presents a summary of recent trends andfindings in conventional radiosurgery as well as novel approaches beingundertaken at Stanford University with both intracranial and extracranialhigh-precision treatments. The authors do an excellent job of summarizing whatwe know about the management of radiosurgical targets in the head, includingmultiple metastases, acoustic neuromas, and arteriovenous malformations. It isimportant, however, to emphasize that there is some disagreement about theoptimal course of treatment for patients presenting with multiple metastases.The authors’ experience in the application of radiosurgical techniques fornasopharyngeal boosts is unique.
Following a summary of cranial radiosurgery, the article delvesinto the requirements for fractionated treatments of the brain and discussesextracranial applications. The authors have a close working relationship withAccuray Inc, commercial supplier of the CyberKnife, a robotic treatment system.The group at Stanford has contributed significantly to the evolution of thissystem into a practical device for precision radiotherapy treatment. Althoughthe authors demonstrate the enormous potential of the system, it is important toexpand on their overview of the technology applied to modern high-precisionradiotherapy.
Accuracy of Patient Positioning
The CyberKnife system referred to in this article ensures theprecision of patient positioning via two components. The primary component is asystem that acquires a pair of radiographs, analyzes the radiographs todetermine the position and orientation of the patient with respect to the modelused in treatment planning, and corrects for deviations in the patient’s orrobot’s position. Although this system operates quickly, it does not providereal-time feedback, and thus, precautions should be taken to ensure that suddenmovements do not occur. Thus, the need to immobilize the patient is noteliminated with this system, but it does reduce the invasiveness ofimmobilization and separates the concepts of immobilization and positionreproducibility.
As the authors point out, the initial fluoroscopic system islimited in the amount of internal radiographic anatomy presented, and the recentaddition of active matrix flat panel imagers (while slower than the fluoroscopicsystem) has greatly improved their ability to localize internal targets.
The second major component of their localization systemthetracking of ventilatory movementis achieved by externally monitoring thevisible movement of the chest and abdomen. This is necessary, even with the useof internal radiopaque markers, because of the duty cycle (ie, the duration oftime) involved in image acquisition, analysis, and feedback. The accuracy withwhich external movement of the patient correlates with internal movement of thetarget is still under investigation.
A couple of other methodologies that have been applied or areunder investigation for patient positioning and real-time tracking duringprecision radiotherapy should be pointed out. For cranial applications, a systemof tracking infrared light-emitting diodes attached to a bite block wasdeveloped by the University of Florida and has reported real-time feedback withsubmillimeter accuracy. For extracranial targets, ventilatory movement hasbeen addressed via both active breath hold during irradiation and real-timetracking of radiopaque markers. The tradeoffs for tracking vs activebreathing control involve patient comfort, reproducibility of position, and theduty cycle of irradiation vs gated-beam hold.
Methods of Dose Delivery
The authors describe the delivery system of the CyberKnife andcontrast that with multiple isocenter treatment using circular collimators andarcs. The CyberKnife system presents a different paradigm for the optimizationand delivery of treatment, compared to conventional linear accelerators(LINACs), and has realized the potential for highly conformal targetirradiation. It should be recognized, however, that LINAC radiosurgery hasprogressed beyond the concept of circular collimation.
Numerous commercial devices allow treatment with radiosurgeryusing high-resolution multileaf collimators. These multileaf collimators existas add-on equipment to standard linear accelerators and integrated components indedicated LINACs. Using these systems, a highly conformal treatment can bedelivered to a target using multiple static-shaped radiation fields. Thesesystems are also capable of dynamic treatment delivery, and have the potentialfor dynamically conforming to the target as the gantry sweeps through an arc or,for larger targets, of providing intensity modulation to improve avoidance ofcritical structures. Nedzi et al demonstrated dosimetric improvement forthese devices compared to conventional radiosurgical techniques.
In summary, the article by Drs. Chang and Adler does anexcellent job of pointing readers toward the dramatic improvements in precisionachieved with radiotherapeutic technology. Their overview highlights thepotential of using treatment delivery concepts throughout the body that wereonce thought to be restricted to invasive cranial fixation applications. It isimportant, however, to realize that such high-precision treatments canpotentially be achieved with systems other than the CyberKnife.
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