Chapter 193 Surgical Resection of Sacral Tumors
Anatomy
Osseous and Ligamentous Structures
Because the lumbosacral and sacroiliac joints transmit the entire weight of the body to the hip bones and lower limbs, these joints and their supporting ligaments must be very strong. The strong dorsal ligamentous complex includes the interosseous ligaments and the dorsal sacroiliac ligaments. The very stout interosseous ligaments connect the sacral tuberosities to the overhanging bone of the iliac tuberosities and represent the single strongest ligaments binding the sacrum to the ilium. The dorsal sacroiliac ligaments are divided into deep (short) and superficial (long) parts. The deeper ligaments connect the sacral and ilial tuberosities and are composed of horizontally oriented fibers; the more superficial ligaments are oriented vertically and stretch from the posterior superior iliac spine to the tubercles of the lateral sacral crest. The caudal portions of the superficial dorsal sacroiliac ligaments blend with the sacrotuberous ligaments.
Neural Anatomy
Both the sympathetic and parasympathetic components of the autonomic nervous system have an intimate relationship with the sacrum. The sacral sympathetic trunk, continuous with the lumbar sympathetic trunk, descends against the ventral surface of the sacrum, converging in front of the coccyx to form the unpaired ganglion impar. Three or four sacral trunk ganglionic enlargements are found on each side of the midline, just medial to the ventral sacral foramina. No white rami communicantes are present in this region; however, the postsynaptic gray rami communicantes from each ganglion join the corresponding sacral or coccygeal nerves for distribution to sweat glands, blood vessels, and erector pilori muscles. In addition, the sacral sympathetic trunks provide fine branches to the superior hypogastric plexus. The superior hypogastric plexus is the caudal continuation of the periaortic sympathetic plexus; it lies on the anterior surface of the fifth lumbar vertebra and upper sacrum in the retroperitoneal tissue. Fibers of the superior hypogastric plexus diverge into right and left hypogastric nerves opposite the first sacral vertebra. The term hypogastric nerve may be a misnomer, because the structure is really a narrow plexus of fibers. The hypogastric nerves represent the principal sympathetic inputs to the inferior hypogastric plexus.
Diagnosis
Clinical Presentation
Sacral tumors are rare and difficult to diagnose at an early stage.1 The major reasons for this delay include the unique capacity of the osseous sacrum and sacral canal to allow neoplastic expansion without causing significant symptoms and the often nonspecific nature of complaints when they do arise. Tumors that originate within the sacral canal can erode or invade the walls of the sacrum and/or expand cephalad within the spinal canal. Tumor can also enter the pelvis via the ventral sacral foramina. A slow-growing, regionally expansive neoplasm can attain a large size without causing symptoms early in the course of the illness. Aggressive, rapidly growing tumors are more likely to cause mechanical instability and/or neurologic compromise earlier during the disease course.
The earliest presenting symptom in patients with sacral tumors is pain located in the lower back or sacrococcygeal region.1–3 Referred pain to the leg or buttock can occur secondary to irritation of the first sacral root or iliolumbar trunk.3 The early presentation of sacral lesions is therefore very similar to that of lumbar spondylosis. By the time a sacral lesion is diagnosed, some patients have been treated and occasionally even operated on for suspected lumbar intervertebral disc pathology.
Radiologic Evaluation
A sacral tumor can be easily overlooked on standard radiographs. The curved shape of the sacrum, its position within the pelvic girdle, and overlying bowel gas are common sources of obscuration. Destructive changes must be advanced before they become evident on plain radiographs.4 Adequate imaging should display the entire sacrum and coccyx on lateral views, and the sacrum should be visualized en face on anteroposterior views. A malignant process is suggested when lytic lesions without sharply defined borders are seen. Well-defined sclerotic margins, reflecting reactive changes in the surrounding bone, imply the presence of a benign or chronic process.
CT and MRI more readily allow the detection, characterization, and staging of sacral tumors.4,5 CT has the advantage of providing excellent bony detail and showing tumor matrix calcification. CT is also useful to image the abdomen for evidence of visceral involvement. Major advantages of MRI studies include the detailed depiction of associated soft tissue masses and the ability to assess the anatomy in multiple planes. The rostral extent of sacral involvement, which is particularly critical to surgical planning, is best appreciated on a midsagittal view.
In addition to the bone scan, CT scans of the chest, abdomen, and pelvis are warranted to rule out metastatic pathology. Intravenous pyelography and/or barium enema may be indicated in evaluating sacral tumors with significant pelvic invasion as well as for colorectal tumors that invade the sacrum.6 Angiography is useful in defining the vascularity of sacral tumors and for preoperative tumor embolization, especially in the case of highly vascular lesions such as giant cell tumor or aneurysmal bone cyst.7,8
Biopsy
The percutaneous CT-guided biopsy involves minimal risk and is currently the method of choice. The site of entry and the trajectory of the needle should be carefully selected so that they can easily be included within the margins of any subsequent resection. If possible, the puncture site should be located in the midline posteriorly. It may be worthwhile to introduce a small droplet of sterile permanent ink into the needle tract to tattoo the skin for later identification.
Pathology
Sacral tumors are categorized as those that originate from the neural elements or their supporting tissues, those arising from bone, and those that metastasize from distal sites or are the result of direct invasion from adjacent pelvic structures. The most common sacral tumors are metastatic, and the most common primary sacral tumor is chordoma.2 Some non-neoplastic entities, including developmental cysts and inflammatory conditions, mimic sacral tumors on imaging studies. A broad differential diagnosis of the various lesions encountered in this area is provided in Table 193-1.
Congenital Lesions |
Posterior sacral meningocele, meningomyelocele, and lipomyelomeningocele |
Developmental cysts |
Dermoid and epidermoid cysts |
Retrorectal tailgut cysts |
Enteric duplication cysts |
Anterior sacral meningocele |
Lateral meningocele |
Occult intrasacral meningocele |
Perineural (Tarlov’s) cysts |
Tumors |
Primitive Neuroectodermal Tumors |
Teratoma |
Hamartoma |
Chordoma |
Inflammatory Lesions |
Osteomyelitis |
Abscess (pelvic abscess, perirectal abscess) |
Neurogenic Lesions |
Schwannoma and neurofibroma |
Ependymoma |
Ganglioneuroma |
Neuroblastoma |
Osseous Lesions |
Bone island |
Osteoid osteoma and osteoblastoma |
Osteochondroma |
Hemangioma |
Aneurysmal bone cyst |
Giant cell tumor |
Chondrosarcoma |
Osteosarcoma |
Ewing’s sarcoma |
Paget’s disease (monostotic) |
Metastatic Lesions |
Hematogenous spread (lung, breast, prostate, kidney, lymphoma) |
Locally invasive lesions (colorectal and gynecologic malignancies, sarcoma) |
Miscellaneous Lesions |
Carcinoid tumor |
Lymphoma |
Solitary plasmacytoma, multiple myeloma |
Meningioma |
Hemangiopericytoma |
Congenital Tumors
Sacrococcygeal Teratoma
A teratoma is a lesion containing tissue from all three germ layers, represented by either well-differentiated or immature elements. Skin, teeth, central nervous system tissue, and respiratory and alimentary mucosa may be found within these tumors. Sacrococcygeal teratomas are the most common sacral tumor in neonates. In contrast, these tumors are rare in adults.9 They develop during intrauterine growth and can grow large enough to cause dystocia.10 The diagnosis is often made on prenatal ultrasonography.11 After birth they manifest as an exophytic mass located between the anus and coccyx, covered by normal skin. Presacral and sometimes combined pre- and postsacral (dumbbell-shaped) lesions also occur.
Chordoma
Chordoma is the most common primary bone tumor of the sacrum.12 Sacral chordomas occur almost twice as often in men compared with women and are uncommon in persons younger than 40 years. The most common presenting symptom is pain in the lower back or sciatic region.3 Chordomas can reach a very large size before constipation (from rectal compression) or lower extremity paresis (due to sacral plexus involvement) occurs.
Chordomas are considered congenital because they are thought to arise from notochordal remnants. Chordoma is characterized microscopically by the appearance of vacuolated “physaliferous” cells. The midline location of these tumors also relates to this proposed etiology.13
Chordomas are typically slow growing but locally aggressive. Significant extracompartmental growth is often seen by the time of diagnosis. Most sacral chordomas manifest as surgical stage IB with anterior extension into the pelvis.14 The tumor often displaces but does not invade the rectum, because the tough periosteum and presacral fascia resist the transgression of disease. Metastasis is usually a late event.
The usual CT appearance consists of lytic bone destruction in addition to a disproportionately large soft tissue mass. Calcification is present in 30% to 70% of cases.5 Unlike most bone tumors, chordomas can show reduced uptake or normal distribution of isotope on bone scan.
En bloc excision is the treatment of choice for sacral chordomas.2,12,13 The extent of surgical resection has been found to play a major role in determining the length of disease-free survival.13 Although a distinct capsule is often seen within the soft tissues, a radical wide posterior margin of the gluteal muscles should be employed to reduce the risk for local recurrence.15 The margins of chordoma within bone are often indistinct. Surgical resection should extend at least one sacral segment beyond the area of gross disease.2
The value of radiotherapy as primary or adjuvant treatment for chordoma has been debated.12 Supplementary radiotherapy may be a useful adjunct to surgical care, but it is not sufficient as stand-alone therapy. In a clinical series spanning 40 years, York and colleagues2 reported that the addition of radiation therapy significantly prolonged the disease-free interval for patients undergoing subtotal resection (2.12 years versus 8 months). Others have suggested that radiotherapy is of limited value in most cases.13
Currently, chemotherapy does not play a role for the treatment of chordomas, although the use of imatinib mesylate is being investigated.16
Neurogenic Tumors
Schwannoma and Neurofibroma
Sacral schwannomas are much more common than neurofibromas. They grow within the sacral canal and only rarely expand through the anterior sacral foramina into the presacral space.17,18 A complete resection of these benign tumors is potentially curative because, with the exception of plexiform neurofibromas, they do not infiltrate beyond their capsular envelope. The surgical approach depends on the size and location of the tumor, especially the degree of intraspinal and presacral extension. The majority of lesions coming to neurosurgical attention are largely confined to the sacral canal and can be resected completely using a posterior approach. This approach allows direct visualization of the relationship between the sacral nerve roots and the tumor. If there is a small presacral component, limited access to this region may be obtained by transforaminal resection. Tumors with a large presacral component should often be removed through an anterior transabdominal approach. Giant intrasacral schwannomas have been described for which radical sacral resection was performed. 17,18
Ependymoma
Sacral ependymomas are most commonly the benign myxopapillary type. They arise from ependymal cell clusters within the terminal filum and expand the sacral canal.19 Rare extradural sacrococcygeal ependymomas have been reported, including subcutaneous, presacral, and intrasacral varieties.20
Patients with sacral region ependymomas typically present with pain in either a lower back or sciatic distribution. By the time of diagnosis, which averages 2 to 3 years after the onset of symptoms, many patients exhibit some form of cauda equina syndrome.3 As with other slow-growing sacral tumors that tend to be diagnosed at an advanced stage, ependymomas may be associated with extensive bony destruction and a large soft tissue mass.19
A complete en bloc resection is advisable in order to prevent local recurrence or cerebrospinal fluid dissemination. Intradural lesions can be resected completely via a posterior approach.20 Intraoperative somatosensory-evoked potentials are useful, because dissection of tumor from the roots of the cauda equina can be difficult. The approach to extradural lesions depends on tumor location. Radiotherapy may be useful in cases of subtotal removal or recurrence.
Ganglioneuroma
Ganglioneuromas are rare slow-growing tumors composed of sympathetic ganglion cells.21 They are thought to represent the benign counterpart of malignant neuroblastomas. Ganglioneuromas can arise anywhere from the base of the skull to the pelvis. Like neuroblastomas, most ganglioneuromas arise in the abdomen, predominantly from the adrenal gland. The small percentages of pelvic tumors likely arise from sacral extensions of the sympathetic chain. As tumor expands within the pelvis, it can extend through a sacral foramen into the epidural space, causing sacral nerve root compression. The treatment is complete surgical removal.
Primary Osseous Tumors
Less than 10% of all primary bone tumors occur in the spine, with the exception of osteoblastoma, which has a 40% incidence of vertebral involvement.22 The incidence of sacral involvement among osseous tumors varies considerably. Bone tumors are a histologically diverse group of neoplasms. Some lesions are of low biological activity, such as osteoid osteoma, osteoblastoma,22,23 and aneurysmal bone cyst.24 High-grade lesions include chondrosarcoma and osteosarcoma. Although giant cell tumors are histologically benign, they are locally invasive and have a high risk for recurrence.25
Aneurysmal Bone Cysts
There is much debate over the best treatment for spinal aneurysmal bone cysts. Among the treatment modalities, complete surgical excision of the lesion remains the most effective, with the lowest rate of recurrence.26,27 Alternatively, these lesions are often treated by intralesional curettage and bone grafting. The disadvantage of this technique is that it is often associated with greater blood loss as well as high recurrence rates ranging from 20% to 60%.28,29 Selective embolization has been shown to avoid excessive bleeding when used with surgery and has also proved to be an effective sole treatment in cases that are difficult to treat surgically. Several less-invasive methods have been studied for the treatment of aneurysmal bone cysts including particulate embolization, intralesional injection of alcoholic zein, radiotherapy alone,30 and radiotherapy combined with surgery.31 Radiotherapy can be used as an alternative or adjunct to surgery when aneurysmal bone cysts are located in sites where an adequate resection cannot be done without resulting in significant morbidity or poor cosmetic results.
Osteoid Osteoma and Osteoblastoma
Osteoid osteomas and osteoblastomas are bone-producing lesions that often occur in long bones but may also be found in the spine. Although these tumors have a predilection for posterior elements of the mobile spine, they tend to involve the body of the sacrum.22 These lesions are similar histologically to each other, containing osteoblasts that produce osteoid and woven bone. Osteoid osteomas are small, self-limited, and benign, whereas osteoblastomas are often larger, are more aggressive, and can become malignant.23
Osteoid osteomas and osteoblastomas are common causes of painful scoliosis in children and adolescents. The most common presenting symptom is neck or back pain that often responds to aspirin or nonsteroidal anti-inflammatory drugs (NSAIDs). Chemotherapy has played a minimal role in the treatment of these lesions and has only been used in selected patients with recurrent aggressive lesions or in patients with surgically inaccessible lesions. Surgery becomes the primary treatment for osteoid osteoma and osteoblastoma as the pain becomes more severe and less responsive to medication. Complete resection is often curative for these lesions. Incomplete resections are more common in osteoblastomas than in osteoid osteomas because osteoblastomas are larger and more commonly involve extraosseous tissue. Osteoblastomas recur in 10% to 15% of cases,32 and the recurrence rate of osteoid osteomas is 4.5%.33
Giant Cell Tumor
Giant cell tumors of the spinal column have a predilection for the thoracolumbar and sacral regions. It is the second most common primary sacral tumor after chordoma. Patients with spinal involvement usually present in the third and forth decades of life. Giant cell tumors are believed to arise from mononuclear cells of macrophage origin. Histologically, they are multinucleated giant cells and macrophages that can contain areas resembling aneurismal bone cysts, and care should be taken when interpreting bone biopsy samples to avoid an erroneous diagnosis. Although they are histologically benign, approximately 5% to 10% of giant cell tumors undergo malignant degeneration and assume a more aggressive course.34
Clinical management of giant cell tumors is difficult given their high propensity for recurrence. En bloc resection with wide margins is the gold standard for treating these lesions.35 Unfortunately, these tumors often grow to a large size before initial diagnosis, and extension of the tumor into the spinal canal and adjacent soft tissues is not uncommon. Therefore, subtotal resection in concert with adjuvant chemotherapy, arterial embolization, and/or radiosurgery is the typical treatment when en bloc resection is not feasible.
Arterial embolization is a critical adjuvant therapy that can increase the safety of surgery as well as improve progression-free survival. Preoperative embolization is a useful tool that minimizes intraoperative blood loss during the resection of these highly vascularized tumors. Embolization may be especially useful to limit bleeding during intralesional resections. Interestingly, arterial embolization may also be useful as a primary treatment modality.8
Chondroma and Chondrosarcoma
Complete surgical resection with negative margins is the primary treatment for chondromas. Less than 10% of patients experience tumor recurrence after complete resection.36 If incomplete resection is performed, residual tumor is at risk for sarcomatous degeneration. Because chondromas are radioresistant lesions, radiosurgery currently does not have a significant role in the treatment of these tumors.
Chondrosarcomas are slow-growing tumors that commonly manifest with focal pain or neurologic deficits. Although chondrosarcomas can be divided into multiple pathologic subgroups, the most important characteristic with respect to clinical outcome is the World Health Organization (WHO) grade. Grade I chondrosarcomas have a 90% 10-year survival rate compared to 30% to 40% for high-grade lesions.37
Regardless of WHO tumor grade, the gold standard for treating chondrosarcomas is gross total resection with negative margins. The literature suggests that en bloc resection is associated with long-term recurrence-free survival. En bloc resection can result in recurrence rates of 20% or less.38 In cases of tumor recurrence, repeat resection can lead to improved survival. There is no well-defined role for radiation or chemotherapy for these lesions.