Transient expansion of vestibular schwannoma following stereotactic radiosurgery

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Abstract
Object The authors prospectively analyzed volume changes in vestibular schwannomas (VSs) after stereotactic radiosurgery. Methods One hundred consecutive patients with unilateral VS treated with Gamma Knife surgery (GKS) at Chiba Cardiovascular Center between 1998 and 2006 were analyzed in this study. For each lesion the Gd-enhanced volume was measured serially every 3 months in the 1st year, then every 6 months thereafter, using volumetric software. The frequency and degree of transient tumor expansion were documented and possible prognostic factors were analyzed. Concurrently, neurological deterioration involving trigeminal, facial, and cochlear nerve functions were also assessed. Results The mean observation period was 65 months (range 25–100 months). There were 32 men and 68 women, whose mean age was 59.1 years (range 29–80 years). Tumor volumes at GKS averaged 2.7 cm3 (range 0.1–13.2 cm3), and the lesions were irradiated at the mean 52.2% isodose line for the tumor margin (range 50–67%), with a mean dose of 12.2 Gy (range 10.5–13 Gy) at the periphery. The tumor volume was increased by 23% at 3 months and 27% at 6 months. Tumors shrank to their initial size over a mean period of 12 months. The maximum volume increase was < 10% (no significant increase) in 26 patients, 10–30% in 23, 30–50% in 22, 50–100% in 16, and > 100% in 13. The peak tumor expansion averaged 47% (range 0–613%). A high-dose (≥ 3.5 Gy/min) treatment appears to be the greatest risk factor for transient tumor expansion, although the difference did not reach statistical significance. Transient facial palsy and facial dysesthesia correlated strongly with tumor expansion, but only half of the hearing loss was coincident with this phenomenon. Conclusions Transient expansion of VSs after GKS was found to be much more frequent than previously reported, strongly suggesting a correlation with deterioration of facial and trigeminal nerve functions.