Analysis of risk factors associated with radiosurgery for vestibular schwannoma

Object. The aim of this study was to identify factors associated with delay ed cranial neuropathy following radiosurgery for vestibular schwannoma (VS or acoustic neuroma) and to determine how such factors may be manipulated t o minimize the incidence of radiosurgical complications while maintaining h igh rates of tumor control. Methods. From July 1988 to June 1998, 149 cases of VS were treated using li near accelerator radiosurgery at the University of Florida. In each of thes e cases, the patient's tumor and brainstem were contoured in 1-mm slices on the original radiosurgical targeting images. Resulting tumor and brainstem volumes were coupled with the original radiosurgery plans to generate dose -volume histograms. Various tumor dimensions were also measured to estimate the length of cranial nerve that would be irradiated. Patient follow-up da ta, including evidence of cranial neuropathy and radiographic tumor control , were obtained from a prospectively maintained, computerized database. The authors performed statistical analyses to compare the incidence of posttre atment cranial neuropathies or tumor growth between patient strata defined by risk factors of interest. One hundred thirty-nine of the 149 patients we re included in the analysis of complications. The median duration of clinic al follow up for this group was 36 months (range 18-94 months). The tumor c ontrol analysis included 133 patients. The median duration of radiological follow up in this group was 34 months (range 6-94 months). The overall 2-year actuarial incidences of facial and trigeminal neuropathi es were 11.8% and 9.5%, respectively. In 41 patients treated before 1994, t he incidences of facial and trigeminal neuropathies were both 29%, but in t he 108 patients treated since January 1994, these rates declined to 5% and 2%, respectively. An evaluation of multiple risk factor models showed that maximum radiation dose to the brainstem, treatment era (pre-1994 compared with 1994 or later) , and prior surgical resection were all simultaneously informative predicto rs of cranial neuropathy risk. The radiation dose prescribed to the tumor m argin could be substituted for the maximum dose to the brainstem with a sma ll loss in predictive strength. The pons-petrous tumor diameter was an addi tional statistically significant simultaneous predictor of trigeminal neuro pathy risk, whereas the distance from the brainstem to the end of the tumor in the petrous bone was an additional marginally significant simultaneous predictor of facial neuropathy risk. The overall radiological tumor control rate was 93% (59% tumors regressed, 34% remained stable, and 7.5% enlarged), and the 5-year actuarial tumor con trol rate was 87% (95% confidence interval [CI] 76-98%). Analysis revealed that a radiation dose cutpoint of 10 Gy compared with more than 10 Gy presc ribed to the tumor margin yielded the greatest relative difference in tumor growth risk (relative risk 2.4, 95% Cl 0.6-9.3), although this difference was not statistically significant (p = 0.207). Conclusions. Five points must be noted. 1) Radiosurgery is a safe, effectiv e treatment for small VSs. 2) Reduction in the radiation dose has played th e most important role in reducing the complications associated with VS radi osurgery. 3) The dose to the brainstem is a more informative predictor of p ostradiosurgical cranial neuropathy than the length of the nerve that is ir radiated. 4) Prior resection increases the risk of late cranial neuropathie s after radiosurgery. 5) A prescription dose of 12.5 Gy to the tumor margin resulted in the best combination of maximum tumor control and minimum comp lications in this series.