Stereotactic radiosurgery is an effective alternative to surgical resection for the majority of patients with vestibular schwannomas (VS). However, after radiosurgery, the imaging characteristics of VSs are variable, and correct interpretation is critical to prevent unnecessary surgery for these patients.

A retrospective study of 208 consecutive patients with unilateral VS having radiosurgery between March 1990 and December 2001. Thirty (14%) patients had tumors that enlarged at least 2 mm after radiosurgery. The median follow-up after radiosurgery was 56 months (range 24-132 mo).

The median time to tumor enlargement was 9 months (5-60 mo). The median volume increase was 75%. A loss of central enhancement was noted in 28 (93%) patients. Six (20%) patients had new symptoms noted at the time of tumor enlargement including hemifacial spasm (n = 2), ataxia (n = 2), trigeminal neuralgia (n = 1), and facial numbness (n = 1). Additional treatment was performed at the time of initial enlargement in 3 patients (resection, n = 2; ventriculoperitoneal shunt, n = 1). In the 28 patients who did not undergo resection at the time of initial enlargement, three patterns were identified on later imaging. Sixteen (57%) patients showed eventual tumor regression (type 1), and eight (29%) patients had tumors that increased and remained larger but did not show progressive enlargement (type 2). Four (14%) patients showed progressive enlargement on serial imaging (type 3) and underwent additional treatment (resection, n = 3; stereotactic radiation therapy, n = 1).

Tumor expansion after VS radiosurgery rarely denotes a failed procedure, and the majority of patients only require further imaging. Approximately one third of tumors that enlarge will remain increased in size compared with the time of radiosurgery but will not show sequential growth. Additional tumor treatment should be reserved only for patients who demonstrate progressive tumor enlargement on serial imaging (2% in this series).