Stereotactic radiosurgery toxicity in the treatment of intracanalicular acoustic neuromas: the Seattle Northwest gamma knife experience.

Patients with acoustic neuromas have several treatment options. The appropriate individual treatment decision and expected control rates and risks for the individual techniques have been outlined in several texts [1-4, 6-8]. This article describes radiosurgery toxicity in those patients with acoustic neuromas who have intracanalicular disease. 52 patients with 54 acoustic neuromas were treated between September 1993 and April 1997. 14 tumors were intracanalicular lesions, with a mean diameter <1 cm and volume <1 cm3. Dose to the periphery of the intracanalicular lesion extension ranged from 12-18 Gy (mean 16 Gy). The margin isodose was 40-60% (mean 47%). 32 isocenters were used to treat the 14 intracanalicular tumors (mean 2.3 isocenters per patient). At a mean follow-up of 18 months (range 1-39 months), 12/12 or 100% of the intracanalicular lesions demonstrated regression or no change in size on subsequent imaging. The following acute side effects were observed posttreatment in intracanalicular tumors: diminished hearing 14%, facial neuropathy 43%, trigeminal neuropathy 21%, balance disorder 14%, dizziness 7%, and headache 7%. Facial and trigeminal neuropathy, balance disorder, dizziness, vertigo and headaches were more common in patients with intracanalicular tumors than those with an extracanalicular extension. Although it has been suggested that small acoustic neuromas (i.e. <1 cm3) tolerate doses of 18 Gy with acceptable toxicity, when the lesion is located in the auditory canal a lesser dose may be warranted to minimize potential side effects. For now, our center has established a protocol that limits radiosurgical stereotactic intracanalicular peripheral doses to 12 Gy until further toxicity studies have been collected and reviewed.