Pseudoprogression of vestibular schwannomas after fractionated stereotactic radiation therapy

Abstract

Local tumor control after radiation of vestibular schwannomas (VS) is assessed by follow-up magnetic resonance imaging (MRI) of the brain. Transient increases in tumor volume on MRI due to radiation can mimic tumor progression but are related to radiation treatment effects known as “pseudoprogression”. Our study aims to determine the presence and impact of pseudoprogression in VS patients treated with fractionated stereotactic radiotherapy. We analyzed 18 patients with VS treated with fractionated radiotherapy with at least 24 months of follow-up with T1-weighted gadolinium-enhanced MRIs. Mean follow-up time was 42 months. Pre- and post-treatment tumor volumes were calculated using OsiriX® software. Events were defined as volume changes ≥10 %. Tumor control rate was 94 %. Forty-four percent of tumors demonstrated increases in volume on follow-up, but all but one patient had transient increases in volume with subsequent regression. Mean tumor volume increased by 35 % with the average time to regression of 24 months. Adverse side effects were more frequent in patients demonstrating pseudoprogression. There were no differences in outcomes based on pre-treatment tumor size or cystic-type versus solid-type tumors. Fractionated stereotactic radiation therapy is an effective treatment for VS. Follow-up imaging with T1-weighted gadolinium-enhanced MRI demonstrated pseudoprogression in a third of patients occurring up to 36 months post-treatment before regression. Therefore, repeat imaging and clinical follow-up of VS should be done to rule out pseudoprogression and avoid unnecessary salvage treatment.