Primary spinal cord tumors comprise 3% to 4% of all primary central nervous system (CNS) tumors in adults.¹ Age-adjusted incidence rates are slightly higher for men than women, 0.68 versus 0.61 per 100,000 person-years, respectively.¹ Most of these tumors present as extramedullary tumors (70%), whereas intramedullary tumors are less common (30%). Approximately 15% to 25% of spinal tumors involve the cervical spine, including the foramen magnum; 50% to 55% involve the thoracic spinal canal; and 25% to 30% involve the lumbosacral spine.

In the pediatric population, spinal cord tumors are slightly more common, 5% to 6% of all primary CNS tumors.¹ Although ependymomas and nerve sheath tumors are the most common histologic types, gliomas make up a larger percentage of tumors in children as compared with adults¹ (Table 28-1).

TABLE 28-1 Distribution of Primary Spinal Cord Tumors by Histologic Type (CBTRUS 2010)

Histologic Type	Adult Distribution	Pediatric Distribution
Nerve sheath tumor	40.5%	15.7%
Glial origin
Ependymoma	33.6%	24.1%
Pilocytic astrocytoma	1.4%	12.6%
Other glioma	4.1%	13.3%
Meningeal tumor	6.1%	10.1%
Hemangioma	2.6%	NA

NA, not available.

The etiology of spinal cord tumors is largely unknown. Depending on the histologic type, they may be associated with genetic syndromes such as neurofibromatosis 1 and 2 as well as von Hippel-Lindau disease.

Prevention and Early Detection

There is no known method for prevention of spinal cord tumors. Early detection relies on careful neurologic examination and consideration of the possibility of a spinal cord tumor in the differential diagnosis. Because of the rarity of these tumors, there are no recommendations for routine screening, even in higher-risk populations.

Biologic Characteristics and Molecular Biology

The diversity of primary spinal axis tumors partly results from the large spectrum of phenotypically distinct cells in the axis that are capable of neoplastic transformation.² The genetic and molecular bases of spinal cord tumors are not yet well understood and vary depending on histologic findings.³ Most tumors are histologically benign but can cause significant morbidity by direct compression of important neural structures.

Histologic typing is an important prognostic factor in spinal cord tumors. Patients with ependymomas have a longer median survival time than those with astrocytomas. Additionally, low-grade (vs. high-grade) and pilocytic (vs. diffuse fibrillary) astrocytomas have a more favorable prognosis.

Anatomy, Pathology, and Pathways of Spread

Anatomy

Enclosed in the bony vertebral column, the spinal cord extends from the medulla oblongata of the brainstem to approximately the level of L1or L2 at the conus medullaris. It measures approximately 43 to 45 cm. The periphery of the cord is composed of white matter, which contains sensory and motor tracts, and the central region contains the butterfly-shaped gray matter, which houses nerve cell bodies. Three layers of meninges, the innermost pia mater, arachnoid mater, and outer dura mater, cover the elliptically shaped spinal cord and are continuous with the brainstem and brain. The delicate pia mater envelops the spinal cord and blood vessels. From this layer extend the denticulate ligaments, which connect with the dense, protective dura mater to stabilize the cord within the spinal canal. The dura mater terminates beyond the conus medullaris at the filum terminale, a strand of fibrous tissue that attaches to the coccyx, providing longitudinal support to the cord. Between these two layers lies the arachnoid, which is filled with cerebrospinal fluid (CSF).

The spinal cord is responsible for transmitting motor and sensory neural signals between the peripheral nervous system and the brain and contains its own independent pathways for coordinating certain reflexes (Fig. 28-1). Functionally, the cord is divided into 31 segments. At each level, a pair of spinal nerves exits: 8 cervical, 12 thoracic, 5 lumbar, and 5 sacral pairs and 1 coccygeal pair. In the cervical region, the spinal nerves exit above the corresponding vertebrae, whereas from the thoracic region and downward, the spinal nerves exit below the equivalent named vertebrae. Although the vertebral column continues to lengthen until adulthood, the canal and the cord are actually much shorter than the column because both stop lengthening at approximately 4 years of age. This growth differential is responsible for the formation of the cauda equina, a collection of the lower lumbar, sacral, and coccygeal nerves that fills the lower spinal column beyond the conus medullaris.

Figure 28-1 Spinal cord anatomy (1) Posterior column. (2) Lateral spinothalamic tract. (3) Lateral corticospinal tract. (4) Anterior horn cells. (5) Dorsal root. (6) Ventral root. (7) Dura. (8) Epidural space. (9) Vertebral body.

Adapted from Netter FH, Freidberg SR, Baker RA: Disorders of spinal cord, nerve root and plexus. In Netter FH, Jones HR Jr, Dingle RV [eds]: The Ciba Collection of Medical Illustrations, Vol 1. Nervous System, Part II. West Caldwell, New Jersey, Ciba-Geigy, 1986, pp 181-189. Copyright 1983. Novartis. Reprinted with permission from The Netter Collection of Medical Illustrations, Vol. 1, Part II. Illustrated by Frank H. Netter, M.D. All rights reserved.

The blood supply comes from a single anterior spinal artery (75%) and two smaller posterior arteries (25%). The cervical region relies on the vertebral and inferior cerebellar arteries, while the thoracic and lumbar regions depend on the radicular arteries. Venous drainage occurs through a vast complex network of plexuses.

Tumors are classified based on their location (Fig. 28-2). Intradural tumors include both intramedullary and extramedullary tumors. Intramedullary tumors develop from the cells that make up the spinal cord and cauda equina, and extramedullary tumors arise from the supportive tissues that lie outside the spinal cord and cauda equina, such as the meninges, vascular supply, and connective tissue. Extradural tumors arise from the vertebral canal itself or supportive tissues surrounding the dura.

Figure 28-2 Location within the spinal column. ed, extradural; ie, intradural extramedullary; ii, intradural intramedullary.

From Shiff D: Principles of Neuro-oncology. New York, 2005, McGraw-Hill.

Pathology and Pathways of Spread

The overall and anatomic distributions of histologic types are listed in Tables 28-1 and 28-2. Intradural extramedullary schwannoma (neurilemmoma) and meningioma are the most common intradural tumors, although occasionally they may present as extradural tumors. Other intradural extramedullary tumors include epidermoid and drop metastases of intracranial primary tumors such as medulloblastoma. The most common intramedullary tumors are those derived from glial precursors, most frequently, ependymomas and astrocytomas of low to intermediate grade. Nonneoplastic conditions may mimic spinal cord tumors (Fig. 28-3), including acute transverse myelitis, Angiostrongylus cantonensis infection, infectious meningitis, intradural disc hernia, pseudotumor, sarcoidosis, and tuberculosis.⁴^–⁸

TABLE 28-2 Anatomic Distribution of Spinal Tumors by Histologic Type

Extradural Tumors	Intradural Extramedullary Tumors	Intradural Intramedullary Tumors
Chondroblastoma Chordoma Chondrosarcoma Hemangioma Lipoma Lymphoma Meningioma Metastasis Neuroblastoma Neurofibroma Osteoblastoma Osteochondroma Osteosarcoma Sarcoma Vertebral hemangioma

Ependymoma, myxopapillary type

Figure 28-3 A pseudotumor, mimicking a neoplastic lesion, is shown in a sagittal-view, T1-weighted MR image with contrast (left) and in a T2-weighted image (right).

The predominant mode of spread of primary tumors is by direct extension. Spread along the subarachnoid space with spinal or cranial involvement of the meninges may also occur. One should not always assume that the spinal cord tumor found on imaging is the primary lesion. Although CSF seeding is uncommon, supratentorial tumors that show a tendency toward leptomeningeal metastases include 1.2% of glioblastomas and 1.5% of anaplastic gliomas.⁹ There are no lymphatics in the spinal cord, and hematogenous spread is rare.

Clinical Manifestations, Patient Evaluation, and Staging

The clinical presentation of tumors of the spinal axis is a function of the local anatomy (see Fig. 28-1). Within the spinal canal, there exists a well-defined extradural space containing epidural fat and blood vessels. The extradural space communicates with adjacent extraspinal compartments via the intervertebral foramina.

A spinal cord tumor produces local and distal symptoms and signs; the latter reflect involvement of motor and sensory long tracts within the spinal cord. These allow localization of the level of the lesion based on clinical findings.

Clinical Workup

The diagnostic approach to primary spinal cord tumors is shown in Figure 28-4. In addition to focusing on neurologic symptoms, the history should include an evaluation of the patient’s functional status and symptoms, which may suggest metastatic disease or nonmalignant causes of spinal cord tumors.

Figure 28-4 Diagnostic algorithm for primary spinal cord tumors. CSF, cerebrospinal fluid.

Physical examination should involve an extensive neurologic assessment. Flaccid weakness, atrophy, fasciculations, and reduced deep tendon reflexes are seen in corresponding myotomes with involvement of the anterior horn cells and the ventral roots (lower motor neuron lesions). A tumor involving the lateral corticospinal tracts can cause weakness, spasticity, extensor plantar response (Babinski’s sign), and hyperreflexia (upper motor neuron lesion). Motor weakness and spasticity are seen with tumors above the conus medullaris. Weakness and flaccidity are seen with tumors below the conus because the tumors compress the peripheral nerve roots of the cauda equina. A tumor restricted to the conus should not cause weakness, although one also involving the cauda equina or the lumbar enlargement of the spinal cord will do so.

Sensory impairment often helps to localize the level of the tumor based on loss of dermatome function, although the upper level of impaired long-tract function may be several segments below the actual tumor (Fig. 28-5). A lesion involving the lateral spinothalamic tract causes numbness, paresthesias, and decreased temperature sensation over the contralateral limb or trunk below the lesion. This produces the classic finding of a sensory level, which is best demonstrated with a pin or a large metal or cold object to test temperature sensation and with observation for absence of perspiration. A lesion in the posterior column causes an ataxic or tabetic gait. Standing posture is affected with the eyes closed (Romberg’s sign). Paresthesias can occur below the level of the lesion.

Figure 28-5 Anterior (A) and posterior (B) views of the dermatomes. C, cervical; T, thoracic; L, lumbar; S, sacral.

CREDIT LINE: Adapted from Netter FH, Freidberg SR, Baker RA: Disorders of spinal cord, nerve root, and plexus. In Netter FH, Jones HR Jr, Dingle RV (eds): The Ciba Collection of Medical Illustrations, Vol 1. Nervous System, Part II. West Caldwell, New Jersey, Ciba-Geigy, 1986, pp 181-189; Keegan JJ, Garrett FD: The segmental distribution of cutaneous nerves in the limbs of man. Anat Rec 102:409, 1948.

Because pain and temperature pathways cross over in the spinal cord and proprioceptive and motor pathways do not, a unilateral spinal cord lesion can result in ipsilateral paralysis and proprioceptive loss as well as contralateral pain and temperature loss below the level of the lesion. Because light touch travels in two pathways, one that crosses in the spinal cord (spinothalamic tract) and one that does not (posterior columns), it is usually spared in unilateral lesions. This combination of findings is known as the Brown-Séquard syndrome.

Both genitourinary and bowel symptoms can result from a spinal cord tumor. Bladder symptoms include hesitancy, dribbling, incontinence, urgency, or acute retention. Loss of bladder control is seen early in the presentation of tumors in or below the conus and later in tumors above the conus. Cone lesions above L1 can lead to impotence or reflex priapism. Lesions involving S2 to S4 may produce loss of erection and ejaculation ability. Decreased genital sensation can occur from lesions affecting the S2 nerve roots. If bowel control is affected, constipation occurs more frequently than incontinence.

The most common presenting symptom from spinal canal tumors is pain, caused by nerve root compression, bony destruction, or, more subtly, pressure on the dura or deafferentation. The pain may be radicular, midline, or centrally located. Radicular pain is secondary to involvement of the posterior roots and is typically described as shooting pain in the dermatomal distribution. The nerve can be compressed inside of or outside of the dura. Midline spinal pain presents as discomfort localized to the area of the tumor and is thought to arise from pain-sensitive structures in the dura and extradural tissues. Characteristically, pain is more severe with extradural lesions. With spinal cord compression, a dull or burning pain, more widespread than segmental spinal or radicular pain, sometimes occurs. This type of pain, although relatively rare, has numerous designations, including central pain, causalgia, and deafferentation pain. It involves a limb or trunk below the lesion. Pain is the most common presenting symptom in patients with sacrococcygeal tumors. Headache may be caused by elevated intracranial pressure, which is unusual in patients with primary spinal cord tumors, with the exception of large tumors involving the foramen magnum. It can be caused by upward seeding of the tumor or by the development of carcinomatous meningitis from malignant tumors.

The clinical presentation of a spinal tumor rarely indicates whether it is extradural or intradural. However, intramedullary tumors can cause a characteristic neurologic syndrome. Early on, decreased temperature and pain sensation occurs in the dermatomes of the spinal segments occupied by the tumor and two or three segments caudal to the lesion. Other sensory modalities are not diminished while the tumor is confined to the central cord. This “dissociated” sensory loss is rarely seen with extramedullary tumors. Additionally, weakness and atrophy are early findings in the myotomes of the corresponding cord segments involved by tumor. As the tumor grows transversely, reflexes produced at the level of the tumor disappear and spastic paralysis with hyperreflexia develops below the lesion. Eventually, all sensory modalities are affected. Cervical tumors can produce Horner’s syndrome by interrupting unilateral autonomic pathways.

Lesions in the conus medullaris can produce several symptoms, including saddle anesthesia, acute urinary retention, incontinence of bowel and bladder, and impotence. If the lesion remains confined to the conus, it will not cause paralysis of the lower limbs and the ankle reflexes are preserved. More commonly, tumors originating in the conus are diagnosed when they have enlarged enough to cause signs characteristic of intramedullary tumors. These findings can also be mixed with those caused by growth of the tumor in the direction of the cauda equina, such as radicular pain.

Lesions of the cauda equina can cause radicular pain in the thigh, weakness, and atrophy of muscles, including the gluteus, hamstring, gastrocnemius, and anterior tibialis muscles. Saddle anesthesia, absent ankle reflexes, impotence, urinary urgency or acute retention, and constipation are also commonly seen. Depending on the location of the lesion, other reflexes may be affected.

Neuroimaging

Certain tumors may display characteristics on plain films such as the calcifications seen in meningiomas. Increased intracranial pressure can cause enlargement of the bony spinal canal, erosion of the pedicles, and scalloping of the posterior wall of the spinal canal. Computed tomography (CT) can be useful for evaluating the vertebral column in extradural tumors. Myelography has been replaced by more advanced, less invasive imaging but can be useful in patients who cannot tolerate MRI,

MRI is the diagnostic study of choice in the evaluation of intramedullary and extramedullary spinal cord lesions. A complete description of the fundamentals of MRI is beyond the scope of this chapter. Most spinal cord tumors enhance with contrast medium, the most common exception being World Health Organization (WHO) grade II gliomas. Edema appears as an area of low-signal intensity on T1-weighted MRI images and high-signal intensity on T2-weighted MRI images. Tumor infiltration may be differentiated from edema by suppression of cerebrospinal fluid (CSF) signal using fluid-attenuated inversion recovery (FLAIR) sequences. MR spectroscopy may help to differentiate tumor from spinal cord necrosis by evaluating the regional distribution of chemicals associated with energy metabolism in the tumor. Imaging of the entire neuraxis is strongly recommended in the initial workup, and brain imaging should be performed in patients with ependymomas.

Cerebrospinal Fluid Examination

Examination of CSF may occasionally be useful for differential diagnosis and for monitoring of therapeutic response. Because of the risk of herniation caused by increased intracranial pressure, neuroimaging studies should precede lumbar puncture. When obstruction of CSF flow occurs because of tumor expansion, patients can develop hydrocephalus; be wary of performing a lumbar puncture in a patient with a spinal cord tumor who presents with a headache.

CSF examination is especially useful in the management of tumors that may seed the subarachnoid space and spread via the CSF, such as medulloblastoma and ependymoma. The appearance of xanthochromic CSF, because of high protein content with an absence of erythrocytes, is characteristic of spinal cord tumors obstructing the subarachnoid space and is examined for malignant cells (cytologic testing), protein, and glucose. A high protein concentration with normal glucose levels and normal cytologic findings is seen in spinal cord tumors, although if the tumor has seeded the meninges, the glucose level may be low and cytologic testing may reveal a malignant tumor.

Histologic Confirmation of Disease

The approach for histologic confirmation depends on the clinical presentation and suspected histologic type. The morbidity of potential biopsy should be weighed against the benefits of confirming the histologic type and whether it would change management. Image guidance can be used to improve safety and efficacy of biopsy. In patients presenting with acute spinal cord compression, surgical decompression should be considered to relieve progression of neurologic symptoms and establish a diagnosis.

Staging

There are no routine staging systems for the classification of spinal tumors.

Primary Therapy

General Principles of Surgery

Surgery has traditionally been the mainstay of therapy for spinal cord tumors.¹⁰ As modern diagnostic and therapeutic techniques such as MRI, the operating microscope, microsurgical tools, and intraoperative neurophysiologic assessment have become available, more aggressive surgical intervention has become a more prevalent treatment strategy.¹¹ This section will discuss general principles of surgery for CNS lesions, depending on anatomic location within the spinal cord. The outcome of surgery and its role in multidisciplinary management are described in the sections for each individual type of tumor. Table 28-3 shows an overview of the outcomes of the most common spinal cord tumors by treatment modality.

TABLE 28-3 Outcomes of the Most Common Spinal Cord Tumors by Treatment Modality

Surgery of Intramedullary Tumors

The most common intramedullary tumors are ependymomas and astrocytomas. Modern series advocate maximum safe resection of intramedullary spinal tumors. The ability to resect these tumors completely varies widely, from less than 10% to as much as 86%, depending on the histologic type.^12,¹³^–¹⁹ Most completely resectable lesions are WHO grade I tumors because higher-grade tumors are defined by their infiltrative potential.^16,²⁰^–²⁴ The gross total resection (GTR) rates for ependymomas range from 19% to 100%, whereas 0% to 43 % of astrocytomas are resectable.²⁵^–²⁸ For infiltrative astrocytomas, the role of surgery is most frequently limited to biopsy for diagnosis. Laminectomy offers limited functional benefit by decompressing the spinal cord in the case of bulky infiltrative tumors. Functional postoperative outcomes may be excellent in selected cases, with the majority of patients (72% to 79%) experiencing either improvement or stabilization of their preoperative presenting symptoms, often after transient postsurgical worsening.^12,^19,^29,³⁰ The incidence of total, permanent postoperative paralysis is reported at 1%.¹¹ Paralysis is often the result of unpredictable, sporadic vascular events.

Intramedullary tumors are almost always approached through a laminectomy exposure with the patient in the prone position.¹¹ After the dura is opened, a longitudinal myelotomy is made over the widened region of the spinal cord, where imaging demonstrates the closest approximation of the tumor to the surface and the incision is deepened several millimeters until the tumor surface is reached. Infiltrative tumors with poorly defined dissection planes cannot be removed completely and, therefore, biopsy for diagnosis is often the safest course of action. When there is an exophytic component to the tumor, maximum safe debulking is typically performed.

Both intramedullary and extramedullary tumors can be associated with a CSF-filled syrinx dilating the central canal. The preferred treatment is tumor removal. If this is not possible, even modest shrinkage of the tumor by means of irradiation can stabilize or even reduce the size of a syrinx. Postoperatively, a syrinx can form from scarring in the central canal. A syrinx drainage procedure may not result in a significant improvement in neurologic function, and the apparent benefit is stabilization or slowing of the decline in function.

Postoperative Management

Corticosteroids are routinely administered before, during, and after surgery to minimize edema of the spinal cord. Intravenous or oral dexamethasone or high-dose intravenous methylprednisolone is most often the agent of choice. Once maximum symptom resolution has occurred, usually approximately 7 days after high-dose glucocorticoids are begun, tapering of steroids as rapidly as possible without resultant symptomatic worsening minimizes the chance for and extent of the numerous serious side effects of these agents.

Complications of Surgery

In patients who are neurologically intact before surgery, the incidence of significant acute morbidity is generally below 5% for extramedullary tumors and as low as 5% for carefully selected patients with intramedullary tumors.¹¹ Spinal deformities, including scoliosis and kyphosis, as well as infection and CSF leakage, can occur.^31,³² The development of postlaminectomy spinal deformities is a serious postoperative complication, occurring in up to 40% of pediatric and adolescent patients, and may be observed as soon as 8 months postoperatively.³³

General Principles of Radiation Therapy

The intent of radiation therapy in the treatment of spinal cord tumors is to improve local control and neurologic function while potentially increasing survival times. Outcomes of radiation therapy and its role in multidisciplinary management are described in the subsequent section for each individual type of tumor.

Because of the potential for growth and developmental side effects, the use of irradiation in children and infants with spinal cord tumors is controversial. Some authors do not recommend postoperative radiation therapy, and suggest that observation with close imaging follow-up and re-resection at the time of second occurrence is a better approach.¹² With this strategy, radiation therapy may be avoided or used only for children with multiply recurrent tumors.

General Principles of Chemotherapy

The efficacy of chemotherapy may be limited by the intact blood–nervous system barrier. Regional drug delivery may be in the form of intracerebrospinal fluid therapy, intra-arterial infusion, and intratumoral therapy. Extramedullary spinal tumors gain their blood supply from meningeal blood vessels that are significantly more permeable than those of the brain, and chemotherapy may be of more use for these tumors. Drugs that have some efficacy in treating primary CNS malignant tumors and that are thought to cross the blood-brain barrier have been used in the treatment of malignant glial tumors of the spinal cord; however, no clear benefit from the use of chemotherapy has been demonstrated thus far.*

Locally Advanced Disease and Palliation

The principles of treatment are the same for patients with more locally advanced disease and palliation as for definitive treatment. Because of the severity of symptoms related to lack of control of spinal cord tumors, in most cases the dose should not be substantially decreased, although this may be a consideration with very large fields. End-stage patients requiring palliation of pain and mass effect are typically treated based on the general principles of palliative management, and the reader is referred to the chapter on the corresponding topic. For radiotherapeutic palliation, fractionation regimens are similar to those used for spinal column metastases.

Irradiation Techniques and Tolerance

Definitions of Treatment Volumes

Three-dimensional treatment planning based on International Commission on Radiation Units (ICRU) volume definitions is the norm.^39,⁴⁰ The gross target volume (GTV) represents the grossly visible disease burden. Typically, this is the T1-enhancing abnormality on MRI or the nonenhancing tumor seen on T2 or FLAIR images. If there is no residual abnormality after a surgical resection, the tumor resection cavity is defined to be the GTV. Surrounding edema is not considered part of the GTV. The clinical target volume (CTV) is the T2 or FLAIR abnormality (which does include edema) on MRI as well as any areas potentially containing microscopic disease. Suggested definitions and doses to be delivered to the GTV and CTV are listed by histologic diagnosis in Table 28-4. The planning target volume (PTV) adds a dosimetric margin that takes into account uncertainties in daily treatment setup and physiologic variations that are difficult or impossible to control, such as (potential) fluctuations in the mass effect from cord edema that may occur over the course of treatment. Organs at risk typically include the thyroid and salivary glands, esophagus, lungs, heart, stomach, small bowel, liver, kidneys, bladders, ovaries, testicles, and uninvolved portions of the spinal cord itself.

Treatment Techniques

The most common approaches include a single posterior field (PA), opposed lateral fields, a PA field with opposed laterals, and oblique wedge-pair fields.⁴¹ They are typically designed to treat the CTV and GTV. For tumors in the cervicothoracic region, a split-beam approach is sometimes used. The central axis is placed just above the shoulders. Opposed lateral fields are used to treat the upper spine, whereas a PA field is used for the area of the spine below. Tumors in the thoracic region are often treated with a three-field approach using a PA field and opposed lateral beams. In the lumbar region, care must be taken to minimize the dose to the kidneys; a four-field approach using AP/PA and opposed lateral beams with the AP/PA beams preferentially weighted may be useful.

The advent of intensity-modulated radiation therapy (IMRT) techniques now allows for the development of sophisticated treatment plans, using several beam directions and treatment fields. It is therefore crucial to compare different treatment and field setups using dose-volume histograms in order to select the most appropriate option.

Certain neoplasms require treatment of the entire craniospinal axis. Several modifications of this approach are used in clinical practice (Fig. 28-6). Patients may be treated either in the supine or prone position, often in an immobilization cast to ensure daily positional reproducibility.^42,⁴³ The intracranial contents, including the upper one or two segments of the cervical cord, are treated through opposed lateral fields. Customized blocks protect the normal head and neck tissues from the primary radiation beam. The spine is treated through one or two posterior fields, depending on the size of the patient. In one method, the collimator for the lateral cranial fields is angled to match the divergence of the upper border of the adjacent spinal field, and the treatment couch is angled so that the inferior border of the cranial field is perpendicular to the superior edge of the spinal field. Alternatively, one may dispense with collimator and couch angles by calculating appropriate gaps. The gap is calculated so that the 50% isodose lines meet at the level of the anterior spinal cord. All junction lines are moved 0.5 to 1 cm every 8 to 10 Gy to avoid overdosing or underdosing segments of the cord. This is accomplished by shortening the inferior margin of the lateral cranial fields, symmetrically lengthening the superior and inferior margins of the posterior spinal field, and shortening the cranial margin of the caudal spinal field; a fixed block is placed at the inferior margin of the caudal spinal field to keep the lower margin of the irradiated volume at the same location.

Figure 28-6 Two- and three-dimensional treatment planning images of craniospinal axis radiation therapy. Patients are treated in the supine or prone position, and an immobilization cast often is used to ensure positional reproducibility. Intracranial contents and the upper one or two segments of the cervical cord are treated through opposed lateral fields. Customized blocks protect the normal head and neck tissues from the primary radiation beam. The spine is treated through one or two posterior fields.

IMRT is a treatment delivery method that may be used to further optimize the dose distribution. Because multiple critical sensitive organs are located near the spinal cord, improved dose distribution should allow the dose to these structures to be minimized. Because most spinal cord tumors typically recur locally, IMRT should allow the exploration of anatomic/biologic treatment planning and delivery in order to optimize different doses to different cell populations within heterogeneous volumes. The use of image guidance may also allow for reduced margins for sparing of critical structures. The use of IMRT is currently under investigation at multiple institutions.^44,⁴⁵

Stereotactic radiosurgery (SRS) and body radiotherapy (SBRT) deliver a highly conformal high dose of radiotherapy with stereotactic guidance. Doses may be delivered as a single dose (SRS) or as a short fractionated course (SBRT). Potential advantages include patient convenience with a shorter treatment course and the steep dose gradient at the edge of the field, which may allow for greater tissue sparing. SRS for primary spinal cord tumors remains experimental at this time but is an active area of research.⁴⁶^–⁴⁸

Tolerance of the Spinal Cord and Lumbosacral Nerve Roots

A dose of 45 to 50.4 Gy in 25 to 28 fractions over 5 or weeks is usually considered to be safe. The risk of myelopathy is less than 1%, well below the steep portion of the dose-response curve.^46,^49,⁵⁰ It is estimated that with conventionally fractionated irradiation (1.8 to 2 Gy per fraction in five fractions per week), at 5 years the incidence of myelopathy is 5% for doses in the range of 57 to 61 Gy (tolerance dose TD_5/5) and 50% for doses of 68 to 73 Gy (TD_50/5).⁴⁹ There is no convincing evidence that the cervical and thoracic cord differ in their radiosensitivity and there appears to be little change in tolerance with variations in the length of cord irradiated.⁴⁹ Table 28-5 shows a range of isomorbid fractionation schemes, all of which carry a 5% risk of radiation myelopathy.

TABLE 28-5 Fractionation Schemes with a 5% Risk of Radiation-Induced Spinal Cord Myelopathy

Dose per Fraction (Gy)	No. Fractions	Total Dose (Gy)
2	29	58
3	13	39
3.3	11	33
4	7	28
5	5	25
10	1	10

Data from references 50, 98, 116, 206–209, 210, 211–214.

The tolerance of the lumbosacral nerve roots appears to be somewhat higher than that of the spinal cord. Most series report a 0% complication rate if patients are treated to doses of 70 Gy (or equivalent) as long as fraction sizes are kept at or below 2 Gy.⁵¹^–⁵³

The Quantitative Analysis of Normal Tissue Effects in the Clinic (QUANTEC) reviewed data from preclinical, conventional, reirradiation, and SBRT studies in order to refine normal tissue dose/volume tolerance guidelines.⁵⁴ Based on this, a total dose of 54, 60, or 69 Gy with conventional 2-Gy once-daily fractionation was associated with a myelopathy rate of less than 1%, 6%, and 50%, with a calculated strong dependence on dose/fraction (α/β = 0.87 Gy). With reirradiation of the full cord cross section at conventional fractionation, cord tolerance appears to increase at least 2% at 6 months after an initial course of radiation therapy. For partial cord irradiation as part of spinal SRS, a maximum cord dose of 13 Gy in a single fraction or 20 Gy in three fractions appeared to have a minimal (<1%) risk of injury.⁵⁵