Morphological changes of vestibular schwannomas after radiosurgical treatment: pitfalls and diagnosis of failure.

Rationale: Evaluate the morphological changes following radiosurgery in order to better define failure parameters.

Methods: 332 non-neurofibromatosis type 2 vestibular schwannomas not previously treated surgically or radiosurgically were subjected to Gamma Knife radiosurgery between 1992 and 2004 at the Gamma Knife Center in Marseille with at least three sequential MRI scans after radiosurgery. Five length measurements were systematically obtained.

Results: Mean follow-up was 4.6 years. Transient loss of contrast enhancement appeared in 213 patients (68%). Significant increase was present at 6 months in 178 patients. In 74 patients, the volume at 3 years was still higher than on the day of radiosurgery but remained stable. Failure occurred in 16 patients. Most showed progressive growth at all MRI controls after radiosurgery but late failure after initial response was possible.

Conclusions: Sequential MRI scans after radiosurgery are necessary. A progressive and continuous growth at 3 years is essential to make diagnosis of failure.