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Functional Spine Radiosurgery

The idea of concentrating an extremely high dose of radiation with highly collimated beams to cause changes in brain function, mostly in the treatment of medically refractory pain, gave birth to radiosurgery.1 The evolution of medical therapy for chronic and cancer pain, as well as the relatively short-lived pain relief afforded by destruction of nuclei or pain pathways in the brain, led to the application of radiosurgery to morphological diseases of the brain, such as benign tumors,² vascular lesions,³ and malignances.⁴ The imaging revolution during the last quarter of the 20th century established stereotactic radiosurgery (SRS) as a viable and robust minimally invasive technique to manage extremely difficult benign and malignant pathologies of the brain.^3,5–7 It also established SRS as an important form of treatment for essential trigeminal neuralgia,^8–11 a unique recurrent and frequently medically refractory facial pain syndrome. Treatment of other dermatomal pain syndromes with SRS became a natural evolution of the technique that met substantial barriers due to the technical challenges of targeting precisely outside the skull.

Several factors have delayed the progress of SRS for the treatment of spinal cord-related pain, although the effectiveness of SRS to abate pain related to spinal metastases compressing neural elements is well known. Because the spinal cord is packed with highly eloquent pathways, imprecision in dose delivery cannot be tolerated. Methods of stereotactic localization and exquisite visualization of the spinal cord and spinal nerves were developed during the past decade; however, methods of fixation and tracking of target movement in relation to the isocenter of the SRS device are still in development. The potential of SRS to manage dermatomal pain syndromes is still to be explored.

Authors working on spinal SRS technical development have given the prevalence of metastases to the spinal column and their consequence on a patient’s quality of life as an explanation for the importance of the technique.^12–14 Compromised ambulation, lack of sphincter control, and medically refractory pain are the symptoms that mostly impair the patient’s quality of life. Stabilization and regression of spinal lesions with SRS are already a reality.^15–19 Because the treatment of patients with cancer has improved substantially as a result of developments in systemic chemotherapy, immunotherapy, and radiotherapy, as well as SRS, long-term survivors face the difficult problem of effective pain control during the advanced phase of their disease. Similar difficulties confront patients with chronic pain secondary to benign disease. The application of spinal SRS in the treatment of medically refractory pain requires an understanding of the scope of the problem that one is facing, as well as the limitations of this focal technique.

Refractory Pain Syndromes

There are disagreements among specialists on how to manage persistent pain. These disagreements intensify when opioids and destructive procedures in the nervous system become necessary. Opioids are more acceptable when cancer pain is involved.²⁰ Opioid treatment for persistent pain of benign origin requires careful consideration. Primary care physicians and rheumatologists who prescribe opioids for chronic pain may not consider their long-term use to be a major problem. However, pain specialists, including neurologists, anesthesiologists, neurosurgeons, psychologists, and psychiatrists, are particularly opposed to the extended use of opioids by patients who do not have a limited life span. Complications of long-term use of opioids include intolerance (64%), physical tolerance (34%), withdrawal (17%), and abuse (13%). Psychiatrists are less likely to endorse the long-term use of opioids and are less likely to prescribe them than are anesthesiologists or neurologists.²¹ Frequently, neurosurgical procedures for pain are not even thought of by a large percentage of practitioners.²² Spinal SRS can be introduced in several instances of pain limited to few dermatomes because it is a noninvasive procedure.

Understanding and diagnosing persistent pain are the first steps in indicating SRS in its context. Medically and surgically persistent pain (MSPP) is defined here as persistent pain when all medical and curative surgical measures have been exhausted. Invasive palliative procedures in the nervous system become necessary to control pain when MSPP is present. In this chapter, the term chronic pain stands for MSPP non-cancer pain; the term cancer pain refers to pain from cancer not responding to opioid therapy, or for cases where the side effects of opioid therapy are unbearable to the patient (Table 17. 1). Neurosurgical procedures for these two categories of persistent pain are listed in Table 17.2. Critical evaluation of the neurosurgical procedure for pain leads to the realization of the limitation of spinal SRS in this complex field. However, spinal SRS may be well suited to situations where the current techniques fail or are too invasive to be applied to the frail and medically infirm patient with dermatomal pain.

Application of spinal SRS to chronic pain syndromes has to be in the context of a multidisciplinary approach to pain management. Collaboration of pain specialists, as happens in modern pain clinics, is indispensable for the proper application of palliative invasive procedures in chronic pain patients. These patients frequently have unrealistic expectations of neurosurgical pain procedures. A succession of failed procedures increases frustration and complicates their management. There is a paucity of methods designed to measure success of surgery for pain.21 Results that are considered excellent by some surgeons are not accepted by others. Several variables contribute to the investigators’ disagreement; these include follow-up length, differences in technique applied, patient selection, and the subjectivity of pain. The evaluation becomes more difficult when drug dependence and secondary gains are at play.

Technical Aspects of Stereotactic Radiosurgery for the Spine

The progress of computed image fusion has allowed the development of stereotactic techniques that no longer depend on rigid fixation,²³ as did the initial surgically invasive attempts of spinal SRS.²⁴ The Stanford group pioneered the frameless approach that uses image matching of pretreatment computed tomography (CT) and oblique x-rays obtaine d at the time of treatment.¹⁸ Several groups have adopted this technique for spine SRS. Further development of patient positioning has relied on reflective markers attached to a patient’s surface and registration by infraredemitting cameras, similar to the prevalent image-guided systems in neurosurgery operating rooms (ExacTrac xray 6D, BrainLab, AG, Feldkirchen, Germany). The marriage of stereotactic triangulation technique with powerful fusion of images obtained in the radiosurgical suite supported the treatment of spinallesions.15 Now this same technique can be developed for the treatment of carefully selected refractory pain syndromes.

Instrumentation

The Novalis Body (BrainLab) is one commercial system used for SRS. It consists of several stereotactic components. Infrared passive reflectors attached noninvasively to the patient’s surface are used for positioning of the target close to the isocenter of a linear accelerator (linac). Radiographic image guidance is used for fine positioning adjustments based on internal anatomy (the spine). The infrared guidance is also used to monitor external patient motion during treatment. The infrared positioning system consists of a pair of cameras in the radiosurgery room that emit and detect infrared radiation reflected from markers placed on a patient’s skin. Data are disseminated to give real-time positional information about the patient; translation and rotation in all three major axes are displayed on multiple monitors. The treatment couch is driven automatically to position the patient near the isocenter based on information from the infrared positioning system. The infrared cameras have a very high effective resolution and can determine the position of individual reflectors with a standard deviation of 0.1 to 0.2 mm.

The radiographic image guidance system consists of a pair of kilovoltage x-ray tubes suspended from the ceiling and an amorphous silicon detector mounted to the linac couch. A pair of radiographs is exposed following infrared positioning to determine the position of the spine relative to the isocenter. The radiographs are digitally transferred to a computer, where they are compared with digitally reconstructed radiographs (DRRs) generated from a pretreatment CT. DRRs represent the position of a perfectly aligned patient. Radiographs and DRRs are fused automatically to determine if any patient positioning adjustments are necessary, and the treatment couch is moved to reposition the patient appropriately. After shifts have been applied, anterior and lateral port films are exposed to confirm patient positioning. The infrared positioning system is used to monitor external patient motion throughout the treatment process. A vacuum immobilization bag and plastic wrap restraining system minimize patient movement during treatment.

Related posts:

Multidisciplinary Combined Modality Approach for Cancer of the Spinal Cord Radiosurgery of Spinal Metastases Spinal Cord Arteriovenous Malformation Radiosurgery Imaging and Target Localization Treatment Delivery Biomechanical Assessment of Spinal Instability and Stabilization

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