Stereotactic radiosurgery for hypoglossal schwannoma

Abstract

Hypoglossal schwannomas (HS) are extremely rare neoplasms. Surgical resection has historically been the treatment of choice but carries a significant risk of postoperative neurological deficits and mortality. Stereotactic radiosurgery (SRS) is a minimally invasive approach that may afford long-term tumour growth. However, literature to determine the safety and effectiveness of SRS in the treatment of HS is scarce. We report on a patient who presented with progressive headache and dysphagia as well as tongue deviation to the left, due to a space-occupying lesion, consistent on brain MRI with a left HS. Primary SRS using a prescription dose of 12 Gy in a single fraction was used to treat the tumour without complications. By last follow-up, the tumour regressed, and the patient’s symptoms improved. Our case shows that radiosurgery can be safe and effective for the management of HS.

Background

Hypoglossal schwannomas (HS) account for 1% of all intracranial schwannomas.^1–3 Symptoms of HS depend on tumour size and location. While HS most commonly present with ipsilateral hypoglossal nerve palsy manifesting as dysphagia and tongue atrophy, larger HS may cause additional symptoms due to brainstem and cerebellar compression as well as invasion of the jugular canal, resulting in lower cranial nerve palsies.

Traditionally, surgical resection has been considered the initial treatment of choice for symptomatic HS, as it provides tissue for definitive diagnosis and can allow for curative gross total resection.^{1 4–6} However, radical tumour resection is often difficult, and the aggressiveness of the resection must be balanced with reducing the risk of damaging adjacent critical neurovascular structures.⁷ Stereotactic radiosurgery (SRS) has been established as the standard of care for small to medium sized (<3 cm) vestibular schwannomas (VS).^8–11 However, the role of SRS in the management of HS is still unclear.

Case presentation

A 68-year-old, right-handed man with no significant medical history presented to neurosurgery clinic with progressive dysphagia and new-onset left-sided headaches of several weeks’ duration. Neurological examination was significant for tongue deviation to the left. There were no other abnormalities on clinical examination.

Investigations

Brain MRI was ordered, as physical examination was positive for left hypoglossal nerve palsy. The MRI revealed a 1.4 cm contrast-enhancing, dumbbell-shaped mass that extended posteriorly and medially, abutted the left lateral and posterior wall of the left vertebral artery, projected into the left cerebello-medullary cistern and expanded the left hypoglossal canal (figure 1A). The mass was consistent on brain MRI with a left-sided HS.

Figure 1.

(A) Preprocedural, axial, T2-weighted, brain MRI significant for a 1.4 cm dumbbell-shaped left hypoglossal canal mass, consistent with hypoglossal schwannomas (HS). (B–F) Axial, contrast-enhanced, T1-weighted, contrast-enhanced brain MRI at the 12, 24, 48, 72, 96-month follow-up time points, respectively, demonstrating tumour stability. (G–H) Axial, T1-weighted, contrast-enhanced brain MRI at 10 and 13 years following SRS, respectively, revealing slight HS regression. The red arrow indicates the tumour on each MRI.

Treatment

The patient initially refused any intervention and decided to proceed with observation of the lesion. However, after several months of conservative management, the patient’s symptoms progressed. At that time, potential treatment options including continued conservative treatment, resection of the lesion and radiosurgery were once more discussed with the patient. The patient weighed the advantages and disadvantages of SRS treatment, which the healthcare team explained. Specifically, the providers informed the patient that radiosurgery held the main advantage of being a minimally invasive, outpatient procedure with a generally lower risk compared with open surgery. However, as the patient was informed, SRS comes with the disadvantage that it cannot remove the entire tumour such that a mass would remain against neural structures after treatment. The providers also considered that, although the patient cannot receive histological confirmation of the aetiology of the mass with radiosurgery, the likelihood that the lesion was a schwannoma was high due to imaging findings. Therefore, management of the patient would not be altered in the absence of such confirmation. After discussing each management option, the patient elected to undergo Gamma Knife stereotactic radiosurgery (GKRS). He was subsequently treated with a margin dose of 12 Gy (maximum dose 26.1 Gy) at the 46% isodose line.

Outcome and follow-up

There were no complications during or after radiosurgery, and the patient was discharged home on the same day. During both 6-month and 12-month follow-ups, the patient experienced resolution of headaches and dysphagia. On physical examination, he still retained mild left-sided tongue deviation. The MRI showed stable schwannoma with no growth. During 3-year follow-up, the patient continued to have deviation of tongue, with no other symptoms, and the MRI revealed a decrease in the size of the portion of the tumour extending into the left lateral medullary cistern. During 13-year follow-up, the patient had no new neurological signs and symptoms, and the MRI revealed a persistent decrease in tumour size (1.3 cm) relative to pre-SRS treatment (figure 1B–H).

Discussion

SRS is a non-invasive technique used to treat a variety of benign and malignant brain pathologies.¹² In SRS, a sophisticated 3D computerised image is used to focus highly concentrated dose of radiation to precise locations in the brain, where the targeted tumour cells are distorted and destroyed. Generally, the radiation dosage varies depending on the type and location of the tumour.

SRS of central nervous system (CNS) tumours has been shown to have a number of benefits.^{12 13} SRS is non-invasive, painless and can be performed in an outpatient setting, which can reduce the risk of infection, minimise recovery time and reduce complications relative to surgical resection. SRS is also very targeted and can be ideal for difficult-to-reach tumours or those that are closer to critical structures, which cannot be safely resected surgically.

SRS has been established as the standard of treatment for small and medium sized VS.^{9 10} Moreover, it has been used successfully as an adjuvant therapy following subtotal resection of VS and at VS recurrence.⁸ SRS has also been demonstrated to be safe and effective treatment option for some non-VS, such as facial and trigeminal nerve schwannomas.¹³

However, despite the proven benefits of SRS in the treatment of other schwannomas, there is little evidence regarding the safety and effectiveness of SRS for HS limited to case reports and retrospective series, most of which are part of a larger series reporting on SRS treatment of non-VS. In one retrospective study, Suri et al reported on seven patients with HS that were managed with primary (n=2) or adjuvant GKSRS (n=5), using a tumour margin dose of 12–13 Gy. At last follow-up, tumour stability and tumour regression was noted in two and five patients, respectively. No complications attributable to SRS were noted.¹⁴ Two additional studies involving the treatment of small-to-medium HS with primary (n=1) or adjuvant (n=1) SRS at similar doses have also reported stability or regression of the tumour size and symptom resolution in follow-up times greater than 11 months.^{15 16} Similarly, single-session SRS at a prescription dose of 12 Gy in our patient resulted in durable tumour regression and sustained improvement of his symptoms.

In SRS of HS, three complications have been reported in the literature. In one case report, after the patient was treated with upfront SRS using a 13.6 Gy margin dose, their symptoms improved, and a slight regression of the tumour was noted initially on follow-up. However, 8.5 years later, the patient’s symptoms returned, and a subsequent brain MRI revealed enlargement of the tumour. Following gross total resection of the tumour, subsequent histopathology revealed malignant peripheral nerve sheath tumour (MPNST). Although neurofibromatosis-2 (NF-2) was suspected, the authors hypothesised that SRS treatment may have contributed to the development of MPNST.¹⁷ In this case, the patient may not have been an ideal candidate for SRS. Specifically, SRS of CNS tumours arising as a result of NF-2 can pose a challenge, and outcomes following SRS have known to be variable.¹⁸ In contrast, our patient presented as an ideal candidate for primary SRS and experienced a decrease in the size of his tumour and stabilisation of his pre-SRS symptoms, barring complications from SRS.

The second complication was reported in another case, where a patient, presenting with a relatively large HS, had worsening symptoms, including the loss of speech and ambulation, following primary SRS treatment that used a median margin dose of 13.5 Gy. Tumour progression was also noted on brain MRI.¹³ In the third case, one patient developed eustachian tube dysfunction following primary SRS. The median margin dose used in that study was 18 Gy.¹⁹

In summary, we report the case of a 68-year-old man who presented with progressive symptoms due to a left-sided HS. Single-session GKRS using a prescription dose of 12 Gy at the 46% isodose line resulted in tumour regression and long-term sustained improvement of his symptoms. There still remains a dearth of evidence regarding the indications, optimal treatment parameters, safety and effectiveness of SRS for the management of HS. Well-designed studies with large numbers of patients are necessary to establish the safety and effectiveness of SRS in the treatment of HS.

Learning points.

• Stereotactic radiosurgery (SRS) may be a safe treatment option for small-to-medium hypoglossal schwannoma (HS).

• SRS may result in long-term control of HS and of symptoms related to the HS.

• More well-designed studies are necessary to establish the safety and effectiveness of SRS in the treatment of HS.

Ethics statements

Patient consent for publication

Consent obtained directly from patient(s).