Benign CNS tumors
Vestibular schwannoma (acoustic neuroma)
A benign inner-ear nerve tumor where focused radiation stops growth while sparing hearing and facial function.
Definition
A vestibular schwannoma is a benign tumor of Schwann cells on the vestibular branch of the eighth cranial nerve, arising in the internal auditory canal and growing into the cerebellopontine angle. Patients present with progressive unilateral sensorineural hearing loss, tinnitus, imbalance, and — with larger tumors — trigeminal numbness, facial weakness, or brainstem compression. Bilateral lesions define neurofibromatosis type 2.
How radiation treats it
Radiation induces growth arrest by damaging the proliferating Schwann cells while the vestibular nerve fibers, which are relatively radioresistant, continue conducting. The tumor typically stops growing and slowly shrinks over years. Treatment is chosen when the tumor is small-to-medium, growing radiographically, or when surgery would put hearing or facial function at high risk — the goal is indefinite growth control, not excision.
Side effects
- Temporary imbalance or mild dizziness during the weeks after treatment.
- Hearing in the treated ear often remains stable, but gradual decline occurs in a minority and is tracked with serial audiograms.
- Facial and trigeminal neuropathy are rare (under 5%) with modern dose constraints and conformal plans.
Simulation process
A high-resolution thin-slice contrast MRI fused to a stereotactic CT localizes the tumor and the adjacent facial and trigeminal nerves, brainstem, and cochlea. The cochlea is contoured to keep the mean cochlear dose low and maximize hearing preservation. For fractionated stereotactic radiotherapy, a frameless thermoplastic mask is made; for single-session radiosurgery, a frame or mask with fiducial verification is used, and the plan is built on a 1-mm slice dataset.
Treatment process
Small and medium tumors are treated with single-fraction radiosurgery to 12–13 Gy at the tumor margin, fractionated stereotactic radiotherapy to 21–25 Gy in 3–5 fractions, or conventionally fractionated 54 Gy in 30 fractions when larger volumes or better hearing preservation is needed. Each fraction takes under an hour. MRI is repeated at six to twelve months and then annually: local control is over 90% at ten years, with serviceable hearing preserved in the majority.
Key takeaways for students
- Growth arrest is the goal — most tumors never shrink dramatically.
- Lower marginal doses (12–13 Gy) improved cranial nerve outcomes versus older, higher-dose series.
- The cochlea is a critical organ at risk for hearing preservation.