Abstract
A 26 year-old female experienced progressive left sided pulsatile tinnitus and conductive hearing loss for two years, which following an extensive clinical workup, was diagnosed as a left greater petrosal nerve schwannoma located within the petrous apex of the temporal bone. Between neurosurgical management and radiation therapy, multiple therapeutic options were presented to the patient, who ultimately chose stereotactic radiotherapy as an alternative to surgical resection due to the potential morbidity associated with surgery. The patient received three fractions of 600 cGy without subsequent worsening of her symptoms, new onset neurologic symptoms or radiation induced side effects reported at a 3, 6 and 12 month clinic visits. A follow-up MRI at 6 and 12 months post radiation administration demonstrated no further tumor growth.
1
Introduction
Primary schwannomas occurring within the petrous apex of the temporal bone are uncommon and typically arise as an extension of primary tumors from the posterior fossa, Meckel’s cave, or the jugular foramen . They may originate from portions of the facial nerve, trigeminal nerve, or even intraosseously with no apparent cranial nerve origin . Furthermore, only a small fraction of these primary schwannomas arising from the facial nerve originates from the first branch, the greater petrosal nerve (GPN) .
Innervating the lacrimal gland, palate and nasal mucosa, the GPN runs along the anterior surface of the petrous canal apex . Any surgical intervention to remove a GPN schwannoma must be taken with great care given that the GPN lies in close proximity to the carotid artery and trigeminal nerve . Multiple surgical approaches to the petrous apex have been discussed in the literature with varying postoperative successes . The effect of radiotherapy has been studied in various schwannomas and has been shown to be effective in preventing tumor growth, albeit long-term assessments of the benefits in patient quality of life remain uncertain . Given the surgical complications of removing a facial nerve schwannoma, stereotactic radiotherapy provides a more conservative option potentially resulting in less patient morbidity than surgical resection. Because primary tumors within the petrous apex are extremely uncommon, specifically schwannomas arising from the GPN, little is known about the comparative efficacy of stereotactic radiotherapy to surgery. This report highlights a case of a petrous apex schwannoma arising from the GPN that was treated with stereotactic radiation and clinically followed up 12 months post radiation administration.
2
Case presentation
A 26-year-old female with no significant past medical history experienced progressive pulsatile tinnitus and left sided conductive hearing loss for two years prior to medical consultation. The patient denied diplopia, motor paralysis, vertigo, ataxia, dysphagia and hoarseness among other symptoms. She also referenced a familial history of glomus tympanicum. Examination revealed Weber lateralized to the left ear. Otoscopy detected a retracted left tympanic membrane with a pulsatile red mass in the anterior hypotympanum and serous middle ear fluid posteriorly. Audiometry confirmed maximal left sided hearing loss. Further diagnostic studies were ordered.
An initial MRI demonstrated a T2 hyperintense enhancing mass involving the left petrous temporal bone encasing the internal carotid artery (ICA) with extension into the cavernous sinus as well as posteriorly in front of the cochlear promontory ( Fig. 1 ). The mass measured 20 mm transversely × 28 mm anteroposteriorly. An initial CT scan demonstrated opacification of the petrous apex, middle ear and mastoid with erosion of the petrous apex. The patient also underwent an Octreoscan with Indium-111 at 4 and 24 h to assess for a paraganglioma, glomus tympanicum, or other somatostatin receptor positive tumor. SPECT CT images demonstrated mild asymmetrical uptake in the region of the left temporal bone with a diffuse pattern, unlike the focal pattern seen with paraganglioma or glomus tympanicum. Furthermore, a four-vessel cerebral angiogram revealed an avascular lesion.
Subsequently, a transtympanic incisional biopsy of the left middle ear mass was performed, confirming the diagnosis of a schwannoma that was strongly S100 positive, likely originating from the greater superficial petrosal nerve. Definitive management with neurosurgical resection via a combined extended middle cranial fossa transtemporal approach to the petrous apex versus radiation therapy was offered. Given potential complications, such as the risk of facial nerve palsy and injury to the ICA associated with neurosurgery, the patient decided to undergo radiation.
The patient consulted with radiation oncology where therapy with CyberKnife stereotactic radiation was offered as an alternative to single fractioned radiosurgery due to its improved side effect profile. The patient received 600 cGy per fraction for three fractions, a total of 18 Gy at the 79% isodose line. The patient received all three fractions of radiotherapy without complication. Follow-up MRIs taken at the completion of radiation therapy, as well as one month and 6 months after did not demonstrate a change in size of the tumor. Additionally, a follow-up MRI one year following completion of radiotherapy demonstrated no change in the size of the tumor as seen ( Fig. 2, Fig. 3 ). At this time, the patient did not experience worsening of her symptoms or post radiation complications.
