Invasive brain tumour after radiosurgery

Microsatellite analysis of recurrent vestibular schwannoma (acoustic neuroma) following stereotactic radiosurgery

HYPOTHESIS

Genetic and immunohistochemical studies may provide insight into the mechanisms of vestibular schwannoma (VS) recurrence following radiation therapy.

BACKGROUND

Stereotactic radiation therapy is an increasingly common alternative to microsurgical resection for the primary management of sporadic VS. The molecular mechanisms associated with recurrent vestibular schwannoma (VS) following radiation therapy are not known.

METHODS

Primary or irradiated VS tumors were fresh-frozen at the time of surgical resection and microdissected to undergo DNA extraction. Lymphocytic control DNA was isolated from blood obtained by venipuncture. Paired normal and tumor DNA specimens were analyzed for allelic loss by PCR amplification of polymorphic dinucleotide repeat sequences. Immunohistochemical studies were performed on paraffin-embedded, irradiated surgical specimens.

RESULTS

Using 16 polymorphic microsatellite markers, 20 of 26 non-irradiated VS demonstrated loss of heterozygosity (LOH) in at least one locus of chromosome 22q. In contrast, none of the four irradiated recurrent VS demonstrated LOH on chromosome 22q (p = 0.008). No allelic loss was seen in either the primary or irradiated VS utilizing markers mapping to chromosome 10. Deletions on chromosome 10 are seen in both benign and higher-grade meningiomas and intracranial malignancies associated with radiotherapy. Immunohistochemical studies were performed to detect the protein product of the NF2 gene, merlin, in the four irradiated VS. NF2 staining was not observed.

CONCLUSION

This study represents the first microsatellite and immunohistochemical analysis of recurrent VS following radiation therapy. Our preliminary observations suggest an alternative mechanism of NF2 inactivation that may correlate with radioresistance in VS.

Clinical and histopathologic features of recurrent vestibular schwannoma (acoustic neuroma) after stereotactic radiosurgery

OBJECTIVE

Stereotactic radiosurgery for vestibular schwannoma entails uncertain long-term risk of tumor recurrence and delayed cranial neuropathies. In addition, the underlying histopathologic changes to the tumor bed are not fully characterized. We seek to understand the clinical and histologic features of recurrent vestibular schwannoma after stereotactic radiation therapy.

STUDY DESIGN

Retrospective review.

SETTING

Tertiary referral center.

PATIENTS

Four patients who underwent microsurgical resection of vestibular schwannoma after primary stereotactic radiation therapy.

INTERVENTION

Patients were treated primarily with gamma knife radiosurgery or fractionated stereotactic radiotherapy followed by salvage microsurgery. Retrosigmoid craniotomy was used in all cases.

MAIN OUTCOME MEASURES

Histopathologic review. Preoperative and postoperative facial nerve function was assessed with the House-Brackmann scale.

RESULTS

We observed highly inconsistent radiation changes in the cerebellopontine angle and internal auditory canal. Fibrosis outside and within the tumor bed varied markedly, complicating microsurgical dissection. Light microscopy confirmed the presence of viable tumor in all cases. Histopathologic features were typical of vestibular schwannoma, and there was no significant scarring that could be attributed to radiation effect.

CONCLUSIONS

The variable fibrosis in the cerebellopontine angle and lack of radiation changes seen histopathologically in irradiated vestibular schwannoma suggest that a uniform treatment effect was not achieved in these cases. Although all four patients with preoperative cranial neuropathies were found intraoperatively to have fibrosis in the cerebellopontine angle, excellent preservation of facial nerve anatomy and function was possible with salvage microsurgical resection. Additional analyses are needed to clarify the histopathologic and molecular characteristics associated with vestibular schwannoma growth after stereotactic radiation.