Ependymoma is an uncommon type of primary brain tumor, accounting for 37% of all brain tumors (1). A rare form of ependymoma known as tanycytic ependymoma (TE) has been classified as Grade II by the World Health Organization (WHO) (2). It was first described and termed by Friede and Pollak in 1978 (3). The majority of TE originates from the ependymal cells of the brain ventricular wall and is characterized by elongated spindle cells. Tanycyte cells are implicated in two types of CNS tumors, namely, tanycytic ependymoma and astroblastoma (4). They are elongated spindly bipolar cells that generally present in the circumventricular organs, particularly in the third ventricle and central canal of spinal cord (5). It is common for TE to be misdiagnosed as a nervous system tumor of the spindle cells (6). The definitive diagnosis of TE requires pathological analysis, including histological characteristics. Electron microscopy may contribute to diagnosis and differential diagnosis. Several cases have been diagnosed as astrocytoma or subependymoma or hemangioblastoma by preoperative imaging examination (7). TE is usually located within the cervical and thoracic (T) spinal cord; however, on rare occasions it can be located within the filum terminale. To the best of our knowledge, only 5 cases of TE associated with the filum terminale have been reported in previous studies (Table I) (6,8–11); however, in the present study, 8 cases of TE in the filum terminale were reported. Combined with data from previous studies, the clinical, radiological and pathological features of TE in the filum terminale were summarized for diagnosis and differential diagnosis.

The present retrospective study investigated clinical data from 8 patients who underwent microsurgery for a TE of the filum terminale between August 2011 and June 2016 at the Department of Neurosurgery of Beijing Tiantan Hospital (Beijing, China). All patients had performed preoperative and postoperative magnetic resonance imaging (MRI) with gadolinium-contrast enhancement. The relevant clinical data (including patient age, sex, symptoms, neuroimaging, preoperative diagnosis, surgical records and follow-up data) were collected via a chart review and telephone interviews. The diagnosis of TE was confirmed by a review of the pathology slides at the Beijing Neurosurgical Institute (Beijing, China). All patients underwent laminectomies to remove the tumor. The following definitions were used: Gross total resection (GTR) was defined as the total macroscopic removal of the tumor; and subtotal resection was defined as the subtotal macroscopic tumor removal. The estimation of the extent of tumor removal was validated by reviewing the postoperative MRI.

Table I depicts a summary of the 5 reported cases of TE located in the filum terminale (6,8–11). Of the 5 patients, 3 were female and 2 were male, 4 patients were adults ageing from 44–69 years old. The remaining patient was a 10-year-old female child, which was reported by Mohindra et al in 2008 (8); the youngest patient with TE that has been reported, to the best of our knowledge. The predominant symptoms experiences by patients included backache, pain and numbness in the lower extremities. All 5 patients received complete surgical resection and postoperative pathology demonstrated that there were notable long spindle cells and pseudorosettes in the tumor, and immunohistochemical staining indicated that the tumor was positive for GFAP, EMA and S-100. With the exception of 1 case in which the prognosis of 1 patient (6) was not described, no recurrence was identified in the remaining 4 patients during the follow-up period.

TE may occur at any age, however the pathogenic sites vary for different ages. Pathogenic sites in children and adolescents are commonly in the cerebral cranium, whilst the majority of pathogenic sites in middle aged and elderly patients are in the spinal cord (11). With TE and spine as keywords, a search of the PUBMED database returned 27 articles (January 10th, 2017; http://www.ncbi.nlm.nih.gov/pmc/), in which 34 cases of spinal TE were reported in total (2,3,6–30). By reviewing the material of these 34 cases of patients with spinal TE, it was revealed that the ages of the patients ranged from 10–76 years, with an average age of 40 years, and that the sex ratio of females to males was 1.25:1. The pathogenic sites were mainly in the cervical spinal cord (15 cases) and T cord (14 cases), whilst the remaining 5 cases occurred in the cauda equine and filum terminale. Combining the data from the present study and the 5 previous cases, it was identified that 10/13 patients with TE of the filum terminale were female. The sex ratio of females to males was 10:3, which was greater than the sex ratio of all patients with spinal TE, indicating that TE of the filum terminale mostly affected females. The clinical manifestations of TE were associated with the pathogenic site of the tumor. For patients exhibiting TE in the filum terminale, the predominant clinical manifestations were backache, pain and numbness in the lower extremities, among others. In terms of the MRI scan, the tumor generally displayed isointense T1 signals and iso- or hyperintense T2 signals whose boundaries were relatively clear and would be strengthened significantly on contrast MRI scans.

The tissue origin of ependymoma has always been controversial (31), however the general opinion is that it originates in the radial glias, namely the tanycytes of ependyma and the precursor cells of neurogliocytes (32,33). Ependymal cells, also termed periventricular cells, have long cell processes extending to the ependymal surface (28). In 1978, Friede and Pollak (3) first proposed TE, reporting6cases according to the characteristics of tumor cells similar to tanycytes. Previous studies on this particular tumor type suggested that it originated in the ependymal cells, and it has been classified as a subtype of ependymoma in the WHO classification of Tumors of the Central Nervous System in 2000 (7,26,34). The genetic characteristics of TE remain unclear; however, it has been reported in the literature that 2 cases were associated with neurofibromatosis type 2 and its tumor suppressor gene neurofibromin 2, which was inactivated; 1 case was potentially caused by family genetic ependymoma (16,17,35).

The majority of TE diagnoses are based on its histological characteristics, notably the tanycyte and immunohistochemical characteristics (32). Electron microscopy is vital in diagnosing TE, due to the difficulty in differentiating it from similar tumors (8). The majority of the cases in this group were initially diagnosed as schwannoma or spinal meningioma by a preoperative imaging examination, and were postoperatively confirmed as misdiagnoses. In pathological diagnosis, improving the understanding of the histological morphology of this tumor is required. Schwannoma is a common spinal canal tumor, which is easy to confuse with TE due to its similar histological morphology of elongated cytoplasmic processes and absent atypia in nuclei. Schwannoma, in the spinal canal, always occurs extra-medullary spaces with well-defined boundaries. Histologically, its thick wall and hyalinized blood vessels are also important in differentiating these two tumors; in addition, schwannoma typically has an Antoni type A and B pattern. In routine pathological examination, a definite diagnosis can be achieved by immunohistochemically labeling for GFAP and S-100 proteins (36). Spinal meningioma is another example of a tumor that TE can be misidentified as. This tumor originates from arachnoid granulations, the majority of which are located subdurally-extra-medullarily, notably in the T vertebra; additionally, dural tail sign is occasionally present in spinal meningioma. This type of tumor is mainly composed of dural cells arranged in patches, or nest-like bulks, of different sizes, with a small amount of mesenchyme between the nest-like bulks. Dural cells are characterized by the following: A large volume; obscure boundaries presenting chimerism; abundant cytoplasm; large nuclei in round or oval shape; and clear nuclear membranes (11,37). The final example of a tumor that TE can be misidentified as is subependymoma. This type of tumor, previously termed subependymal astrocytoma, frequently affects middle-aged and elderly people, with the majority of cases located in the fourth ventricle and lateral ventricles. Histologically, its clustered nuclei are embedded in a dense matrix of glial fibers and, along with the formation of numerous follicular cavities, these can be used as identifying factors in differentiating subependymoma from ependymoma (6,7).

Currently, the primary treatment method for TE is complete surgical resection of the tumor (2,38). For patients undergoing complete surgical resection, postoperative radiotherapy is not necessary (11,26). For patients with postoperative recurrence, the treatments used are surgical resection with postoperative radiotherapy (7,30). Of the patients investigated in the present study group, only 1 patient did not receive complete surgical resection of the tumor as the tumor had adhered firmly to the spinal cord and wrapped around the cauda equina. In order to decrease postoperative complications, a partial resection was opted for instead of a complete surgical resection. Therefore, the treatment principle for TE in the filum terminale is to carefully separate tumor boundaries intraoperatively, spare no effort to protect the cauda equina and strive to achieve complete surgical resection. The prognosis of TE is associated with the patient age, tumor site and the extent of surgical resection. It has been reported that the 5-year and 10-year survival rates for TE are improved compared with those with other subtypes of ependymoma (33,39).

In conclusion, the current case series and literature review suggest that cauda equina TE should be taken into consideration when the tumors exhibit enhancement on MRI. TE is the rarest variant of ependymoma and its diagnosis largely depends on histopathological examination. Neurosurgeons should be aware of this lesion entity. Fortunately, cauda equina TE usually has a favorable prognosis, and GTR or STR are expected whenever possible.