Subcutaneous sacrococcygeal myxopapillary ependymoma misdiagnosed as pilonidal disease

Abstract

Ependymomas are neoplasms which arise from the radial glial cells, which many recent studies have proposed are neural stem cells. Extracranial ependymomas are rare. We present the case report and supporting multimedia of a 37-year-old man who presented with a painless intergluteal swelling which was diagnosed clinically as a pilonidal cyst. However, on excision, he was found to have a subcutaneous sacrococcygeal myxopapillary ependymoma based on histological findings. His management and follow-up are presented and discussed. Given the rare nature of this condition, there is a lack of published guidelines on management and follow-up protocols. Supporting evidence is limited to sporadic case reports. This case highlights the diagnostic challenges and management strategies adopted supported by the best available evidence.

Background

Ependymomas are neoplasms which arise from the radial glial cells, which many recent studies have proposed are neural stem cells.¹ These neoplasms are typically found in the brain and spinal cord (central nervous system), and can rarely present as extraspinal variants, separate from the central nervous system.² Extraspinal ependymomas occurring in the sacrococcygeal region are further classified into presacral and postsacral, depending on their location relative to the sacrum.³ Presacral variants may present with mass effect on the bowel or bladder.³ Postsacral (also described as subcutaneous) ependymomas may present as an intergluteal swelling.^{4 5} This case demonstrates a rare presentation of a subcutaneous sacrococcygeal myxopapillary ependymoma diagnosed clinically as pilonidal disease.

Case presentation

A 37-year-old man presented with an intergluteal swelling which had been enlarging for approximately 3 years. It was non-tender but was associated with discomfort with prolonged sitting. There was no bleeding or discharge from the swelling. He had not required any prior antimicrobial therapy for related infections. He was an otherwise healthy, non-smoker with minimal alcohol intake.

On examination, there was swelling at the superior aspect and slightly to the right of the natal cleft. The swelling was ovoid and cystic in nature with a maximum dimension of 40 mm. No midline pilonidal pits or skin changes were visible.

The patient was scheduled for an elective pilonidal cyst excision based on a clinical diagnosis. Placed in the prone position, this was performed under local anaesthesia. The lesion was identified and excised with a close margin through a 3 cm incision. On excision, a fibrous tract extended towards the tip of the coccyx. No midline communication to the skin was identified. The specimen felt rubbery in consistency with an intact surface layer. The wound was closed in a conventional way with non-absorbable sutures in an off-midline location.

Histology

Received was a 20 mm skin ellipse with an underlying lobulated circumscribed solid tumour measuring 37 mm in diameter, which abutted the peripheral margin.

Histological examination showed a lobulated neoplasm within the deep dermis and subcutis. The neoplasm was composed of solid nests of tumour cells with focal pseudoglandular and papillary areas (figure 1). Prominent well-formed rosettes were present (figure 2), and there were areas of myxoid change (figure 3).

Figure 1.

H&E stain ×4 demonstrating pseudoglandular and papillary areas.

Figure 2.

H&E stain ×20 demonstrating rosettes.

Figure 3.

H&E stain ×10 demonstrating the myxoid area.

The tumour cells were strongly positive for glial fibrillary acidic protein (GFAP) (figure 4), S100, CD99 and CD56, and were negative for epithelial membrane antigen. There was focal cytokeratin positivity.

Figure 4.

Immunohistochemistry ×4 for glial fibrillary acidic protein.

There was focal necrosis present and a mitotic count of 8 per 10 high-power field (HPF), with a Ki-67 stain showing a proliferation rate of 15% (figure 5).

Figure 5.

Ki-67 stain ×10.

There was no lymphovascular invasion or perineural invasion, and the tumour was present at all peripheral margins. The histological diagnosis was of a subcutaneous sacrococcygeal myxopapillary ependymoma.

Investigations

In light of this unexpected diagnosis, the patient was fully staged radiologically. Magnetic resonance imaging (MRI) of the pelvis demonstrated postoperative change with no gross evidence of remaining tumour. CT of the thorax, abdomen and pelvis excluded any associated metastatic disease. Full neuraxial imaging (MRI of the central nervous system) ruled out synchronous intracranial lesions.

Treatment

Institutional multidisciplinary decision making regarding further management was limited by the rare nature of this condition and absence of definite guidelines. Expert opinions were sought both nationally and internationally. The options proposed included conservative management with a watchful waiting approach or excision of the scar with removal of the coccyx. A decision was taken to proceed with wide local excision of the scar to achieve an R₀ excision. Prior to surgery the patient was consented for coccygeal excision if involved. The procedure was performed under general anaesthesia in the prone position with prophylactic antibiotics. There was a nodular area noted at the centre of the scar (figure 6). A wide excision of this area was carried out with an elliptical incision intending to achieve clear radial and deep margins. It was evident that the tumour was not involving the coccyx. The specimen was orientated and sent for frozen section analysis (figure 7). Central persistence of the tumour was identified with clear margins, the closest of which was 7 mm. This obviated the need for more radical tissue excision, and coccygeal excision was not performed. The wound was partly closed with interrupted non-absorbable sutures, and the remainder was left to heal by secondary intention.

Figure 6.

Surgical site prepped, draped and marked in prone position.

Figure 7.

Specimen marked prior to frozen section analysis. Long: lateral; short: superior; double: deep.

Outcome and follow-up

In view of the finding of residual disease in the re-excised specimen, a restaging CT of the thorax, abdomen and pelvis was performed. Following multidisciplinary team (MDT) discussion, a follow-up strategy consisting of 4-monthly clinical review, 6-monthly MRI of the pelvis and yearly chest X-ray was instigated. The patient is currently 16 months postsurgery and is doing well. The wound has healed satisfactorily, and clinical and radiological follow-up to date is negative for any recurrence.

Discussion

Subcutaneous sacrococcygeal myxopapillary ependymomas are extremely rare. The first reported case was by Mallory in 1902.⁶ They can occur near the gluteal cleft^{3 4} and can be initially diagnosed as pilonidal disease based on clinical findings.^{4 5} These tumours tend to be slow-growing and are often quite large before causing symptoms.^{3 7} The age of presentation varies between 2 months and 67 years old.³ Histopathological examination reveals a rosette pattern and immunohistochemical staining is positive for GFAP and S100.^{7 8} Typical histological features of myxopapillary ependymomas are seen, such as cuboidal to elongated tumour cells radially arranged in a papillary fashion around hyalinised fibrovascular cores with intervening myxoid material between the tumour cells and blood vessels.⁹ Our specimen also shows anaplastic features such as focal necrosis, a mitotic count of 8 per 10 HPF and a Ki-67 proliferation rate of 15%. Anaplasia in myxopapillary ependymomas is extremely rare, with less than 10 reported cases in the literature. If only subcutaneous myxopapillary ependymomas are considered, there are only two reported cases.^{10 11} The presence of both anaplastic features and the rosette pattern (which corresponds to ependymoblastic differentiation) put this tumour into the WHO grade IV category.

In a retrospective case series by Chai et al,¹² 33 patients with 25 WHO grade II ependymomas and 8 anaplastic ependymomas (WHO grade III) were analysed in terms of progression-free survival (PFS). There was no statistically significant relationship with sex, tumour location, pathology or postoperative adjuvant treatment. The extent of removal had a significant effect on PFS, with a 1-year PFS of 100% for gross total resection vs 75% for subtotal resection.¹² It appears that pathology has no impact on overall survival when gross total resection is performed; however, it can only be assumed that if there was tumour recurrence, or if subtotal resection was performed, the likelihood of metastases developing would be higher in anaplastic tumours than in lower grade tumours.¹³

There are no formal guidelines on the management of this condition. However, the National Comprehensive Cancer Network guidelines for the management of intracranial and spinal ependymomas may be extrapolated to this variant. The treatment of choice is complete resection where possible and/or subtotal resection, with or without radiotherapy. Repeat imaging and observation are recommended to ensure no local or distant recurrence.¹³ Resection within the central nervous system to achieve clear margins is challenging due to associated morbidity and functional disturbance. The oncological principle of performing an R₀ resection, as in our case, has proven effective for the treatment of subcutaneous ependymomas.¹⁴ There is some evidence suggesting radiotherapy may be useful in cases of margin-positive disease^{2 15}; however, it appears that there is no role for chemotherapy.^{8 14}

These tumours are slow-growing and tend to metastasise to the lymph nodes, spine and lungs.^{8 14 15} The prognosis varies from curative treatment to aggressive metastatic disease, leading to death. Following development of metastases, disease progression tends to be slow but relentless.¹⁶ Locoregional and distant recurrences can occur several decades after primary tumour excision; thus, long-term surveillance is crucial.^{2 8 17}

This case reinforces the importance of local and international collaboration to provide optimum care to patients with unexpected or rare conditions. It also increases the body of evidence available on the topic of ependymomas and presents an evidence-based pragmatic treatment and follow-up strategy.

Learning points.

• Ependymomas are neoplasms which arise from the radial glial cells, which many recent studies have proposed are neural stem cells.

• This case serves as a reminder that routine presentations and procedures occasionally lead to unexpected or obscure diagnoses and an open mind is required.

• The management of rare conditions is sometimes limited by the absence of defined clinical guidelines.

• Collaboration with international experts and treatment tailored to the individual presentation and patient provides a reasonable approach.