Extracranial Stereotactic Radiosurgery: Applications for the Spine and Beyond

doi:10.1016/S1042-3680(18)30192-X

Neurosurgery Clinics of North America

Volume 10, Issue 2, April 1999, Pages 257-270

https://doi.org/10.1016/S1042-3680(18)30192-X Get rights and content

The role stereotactic radiosurgery has in the management of malformations of the spine is examined in this article. Specific problems in the application of stereotactic radiosurgery to the spine are discussed and the three techniques for spinal stereotaxis, bone screw fixation, contour mold fixation, and frameless stereotaxis, are reviewed.

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Cord Dose Specification and Validation for Stereotactic Body Radiosurgery of Spine
2009, Medical Dosimetry
Citation Excerpt :
The symptoms of radiation-induced cord necrosis include sensory and motor loss in legs and/or arms as well as sphincter impairment of the bowel and bladder.1 Recently, advanced image-guided stereotactic body radiosurgery (SBRS) has been utilized to deliver a single large tumoricidal dose to the target for improving tumor control rate and reducing the symptoms of the cord compression while minimizing late radiation toxicities to the spinal cord.2-11 Short treatment and quick pain relief also benefit those patients undergoing SBRS.
Effective dose to a portion of the spinal cord in treatment segment, rather than the maximum point dose in the cord surface, was set as the dose limit in stereotactic-body radiosurgery (SBRS) of spine. Such a cord dose specification is sensitive to the volume size and position errors. Thus, we used stereotactic image guidance to minimize phantom positioning errors and compared the results of a 0.6-cm³ Farmer ionization chamber and a 0.01-cm³ compact ionization chamber to determine the detector size effect on 9 SBRS cases. The experimental errors ranging from 2% to 7% were estimated by the deviation of the mean dose in plans to the chamber with spatial displacements of 0.5 mm. The mean and measured doses for the large chamber to individual cases were significantly (∼17%) higher than the doses with the compact chamber placed at the same point. Our experimental results shown that the mean doses to the volume of interest could represent the measured cord doses. For the 9 patients, the mean doses to 10% of the cord were about 10 Gy, while the maximum cord doses varied from 11.6 to 17.6 Gy. The mean dose, possibly correlated with the cord complication, provided us an alternative and reliable cord dose specification in SBRS of spine.
A Study on Target Positioning Error and Its Impact on Dose Variation in Image-Guided Stereotactic Body Radiotherapy for the Spine
2009, International Journal of Radiation Oncology Biology Physics
Citation Excerpt :
This is a noninvasive procedure that uses volumetric imaging information on the treatment day. Some studies on spinal radiosurgery for patients with spinal metastases used surgical fixation as part of the treatment (1, 11). In these invasive techniques general anesthesia was required, and surgical attachment of the stereotactic frame to the spinous process was made using bone screws for immobilization and localization.
To investigate the amount of target positioning error and evaluate its dosimetric impact during image-guided stereotactic body radiotherapy for single-fraction spine treatment.
A prescription dose of 15 Gy and five to nine coplanar intensity-modulated beams were used. The patient was immobilized with a custom-fit vacuum mold, and the target was localized with a volumetric cone-beam CT image. A robotic couch with six degrees of freedom was used for target adjustment. For evaluation a cone-beam CT image was obtained at the end of treatment. Both target positioning error and its dosimetric impact were investigated for the first 9 cases.
For cases studied, translational errors were 0.9 ± 0.5 mm (lateral), 1.2 ± 0.9 mm (longitudinal), 0.7 ± 0.6 mm (vertical), and 1.8 ± 1.0 mm (vector), and rotational errors were 1.6° ± 1.3° (pitch), 0.8° ± 0.9° (roll), and 0.8° ± 0.4° (yaw). For the clinical target volume, D₉₅ (dose to 95% of target volume), D₉₀, D_max, and D_mean were evaluated. Only 1 case showed significant dose variations, reaching up to 18% in D₉₅. The spinal cord dose was evaluated by observing D_0.1 (dose to 0.1 cm³), D_0.5, D_1.0, and D_max. Although 1 case showed a dose change reaching up to 30% in D_max, cord dose was within the planning tolerance limit in all but 2 cases (3% higher in one and 0.4% higher in the other).
The implemented image-guided stereotactic body radiotherapy provides precise target localization. However, despite reasonably precise spatial precision, dosimetric perturbation can be significant because of both extremely steep dose gradients and close distances between the target and the spinal cord.
Image-Guided Stereotactic Radiosurgery Using a Specially Designed High-Dose-Rate Linac
2007, Medical Dosimetry
Citation Excerpt :
Increasingly complex treatment plans and paradigms with multi-isocenter delivery require more treatment time in radiosurgery, which is further compounded in IGRT. Several methods have been proposed to extend stereotactic localization to extracranial sites.1–7 High-precision radiation delivery to extracranial sites is more challenging; target position can shift relative to bony anatomy between image acquisition and treatment.
Stereotactic radiosurgery and image-guided radiotherapy (IGRT) place enhanced demands on treatment delivery machines. In this study, we describe a high-dose-rate output accelerator as a part of our stereotactic IGRT delivery system. The linac is a Siemens Oncor without a flattening filter, and enables dose rates to reach 1000 monitor units (MUs) per minute. Even at this high-dose-rate, the linac dosimetry system remains robust; constancy, linearity, and beam energy remain within 1% for 3 to 1000 MU. Dose profiles for larger field sizes are not flat, but they are radially symmetric and, as such, able to be modeled by a treatment planning system. Target localization is performed via optical guidance utilizing a 3-dimensional (3D) ultrasound probe coupled to an array of 4 infrared light-emitting diodes. These diodes are identified by a fixed infrared camera system that determines diode position and, by extension, all objects imaged in the room coordinate system. This system provides sub-millimeter localization accuracy for cranial applications and better than 1.5 mm for extracranial applications. Because stereotactic IGRT can require significantly longer times for treatment delivery, the advantages of the high-dose-rate design and its direct impact on IGRT are discussed.
Optimal number of beams for stereotactic body radiotherapy of lung and liver lesions
2006, International Journal of Radiation Oncology Biology Physics
Citation Excerpt :
Because of the success of stereotactic radiosurgery for many intracranial applications, there has been an interest in extending the technology to extracranial sites. Initial promising results of radiosurgery treatments for lung and liver tumors (1, 2) further increased interest in development of the technology with several methods used to extend stereotactic localization to liver, lung, and paraspinal targets, with excellent clinical results (3–8). Compared with intracranial radiosurgery, extracranial tumors are more technically challenging to treat accurately because of internal target motion relative to bony anatomy.
Purpose: The aim of this study was to determine the optimal number of coplanar and noncoplanar external beams in the setting of stereotactic body radiotherapy (SBRT).
Methods and Materials: Spherical targets were delineated within 2 separate extracranial sites, the lung and liver, with diameters varying from 2 cm to 7 cm to cover the range of volumes used in SBRT. Treatment plans were created for all target volumes using 5 to 15 geometrically optimized coplanar and noncoplanar conformal beams. Dose gradient and normal tissue complication probability (NTCP) were evaluated for each set of beam configurations and for each target size.
Results: For all lung and liver target volumes, the dose gradient improved with an increase in beam number from 5 to 15 for both coplanar and noncoplanar beam configurations. NTCP decreased as the beam number increased from 5 to 9 beams for all target sizes for both coplanar and noncoplanar beams. There is no significant improvement in NTCP when more than 9 beams were used for treatment planning regardless of target size.
Conclusion: Based on dosimetric criteria, the optimal number of external beams is 13 to 15 for SBRT using either coplanar or noncoplanar beam bouquets. Simple biologic models indicate that the optimal number of beams is 9 for SBRT of lung and liver lesions >2 cm, whereas smaller lesions may benefit from plans using up to 13 beams.
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