Radiosurgery for Acoustic Neuromas

ABSTRACT & COMMENTARY
Source: Mendenhall WM, et al. Preliminary results of linear accelerator radiosurgery for acoustic schwannomas. J Neurosurg 1996;85:1013-1019.

Surgery is the treatment of choice for most patients with acoustic nerve tumors because of its high likelihood of achieving cure by total resection. A significant group of patients, however, are not good surgical candidates for various reasons including advanced age, medical illness, recurrent tumor after surgery, and tumor that is too extensive to be removed completely or safely. Patients in this group who are relatively asymptomatic and have a limited life expectancy can be followed without treatment. The remainder are candidates for radiotherapy with either conventional external beam radiation or with stereotactic radiosurgery via the gamma knife or the less expensive linear accelerator (Linac)-based radiosurgery.

The authors report on the results of Linac-based stereotactic radiosurgery in 56 patients with acoustic neuromas. Each patient was followed clinically and radiographically for at least one year or until death. The number of patients at three years was 45 and at five years, 12. Audiograms were not obtained post-treatment because almost all patients had no useful hearing in the affected ear before treatment. Radiation doses ranged between 10 and 22.5 Gy with 70% of patients receiving 12.5-15 Gy.

After treatment, 55 patients (98%) achieved local control defined as stable disease or tumor regression without evidence of regrowth. The five-year actuarial rate of local control was 95%. At the time of data analysis, seven patients had died of intercurrent illness, 48 patients had stable disease, and one patient was alive with tumor progression.

Fifteen complications developed in 13 patients: fifth and/or seventh cranial nerve palsy (7 patients), worsening of a pre-existing fifth and/or seventh cranial nerve palsy (5 patients), and hydrocephalus requiring a ventriculo-peritoneal shunt (3 patients). The risk of complications was related to both radiation dose and treatment volume and, therefore, to tumor size.

In this series, the results of Linac radiosurgery were comparable in terms of local control and complications to those obtained by others with the gamma knife.

COMMENTARY

The gamma knife was first used to treat acoustic neuromas in Sweden by Leksel (Noren G, et al. Acta Neurochir Suppl 1993;58:104-107) and in the United States at several centers (Flickinger JC, et al. Radiother Oncol 1993;27:91-98; Foote RL, et al. Int J Radiat Oncol Biol Phys 1995;32:1153-1160). The gamma knife has not come into widespread use because of its high cost and limited applications. Therefore, conventional external-beam radiation has been used in most centers to treat unresectable or recurrent acoustic tumors (Wallner KE, et al. J Neurosurg 1987;67:858-863). The disadvantages of external-beam radiotherapy are the relatively large volume of normal tissue included in the target volume and the time required to treat the patient. Therefore, the authors compared their results using Linac stereotactic radiosurgery with those reported for gamma knife and external-beam radiation.

In previous series using either gamma knife (Noren et al, Flickinger et al, and Foote et al) or external-beam radiation (Wallner et al), local control rates have ranged from 82% to 100%, cranial nerve injuries of varying severity developed in up to one-third of patients, and hydrocephalus developed in 3%. Serviceable hearing was preserved in approximately one-half of patients after radiotherapy. Limited data indicate that external beam therapy resulted in a high rate of local control at a lower risk of cranial neuropathy.

Therefore, the optimum non-surgical treatment for patients with small acoustic tumors, and, hence, a low risk of complications, is stereotactic radiosurgery using either the gamma knife, where available, or the Linac. Patients with large acoustic tumors have an increased risk of complications with stereotactic radiosurgery; therefore, if surgery cannot be performed, some form of fractionated radiotherapy may be the better choice in these cases. —jjc