CYSTIC ACOUSTIC SCHWANNOMAS - MR CHARACTERISTICS

PURPOSE: To evaluate the spectrum of MR characteristics of cystic acou stic schwannoma and to investigate its incidence. METHODS: We retrospe ctively reviewed the MR findings and clinical records of 16 patients w ith cystic acoustic schwannomas. In addition, the MR examinations of 4 11 consecutive patients referred for clinical suspicion of acoustic sc hwannomas were reviewed retrospectively to assess the incidence of aco ustic schwannomas with cystic lesions arising from the internal audito ry canal. RESULTS: Of the 16 acoustic schwannomas with MR evidence of intramural cysts, 11 tumors had single small cysts, and five had multi ple intramural cysts of variable size. Intramural cysts in 11 of the 1 6 tumors exhibited higher signal intensity than that of cerebrospinal fluid; the remainder were isointense to cerebrospinal fluid on both T1 - and T2-weighted images. All intramural cysts showed circumferential enhancement after contrast administration. Nine of the 16 cystic acous tic schwannomas also had MR evidence of extramural/arachnoid cysts. Si x of the extramural/arachnoid cysts had epicenters away from the dural interface, and the other three cysts were broadly based against the d ura. The incidence of cystic acoustic schwannomas was 11.3% and associ ation with extramural/arachnoid cysts 7.5%. CONCLUSION: Our series sug gests that cystic changes in acoustic schwannomas and the association with extramural/arachnoid cysts are not as rare as previously reported by other diagnostic methods. The high signal intensity of intramural cysts is probably related to necrotic material, blood, or colloid-rich fluid. The difference in the MR characteristics of extramural/arachno id cysts associated with acoustic schwannomas and those of typical ara chnoid cysts not associated with neoplasia may be related to higher pr otein and/or colloid contents secreted by the tumor. Most extramural/a rachnoid cysts had epicenters between the tumor and brain, suggesting that the most likely mechanism of formation is peritumoral adhesions. It creates a pseudoduplication caused by the trapping of fluid between the leptomeninges and the mass, resulting in an acquired type of arac hnoid cyst.