Rate of Acoustic Neuroma Growth by Volumetric Analysis Preradiation Does Not Predict Treatment Failure

Introduction: Stereotactic radiosurgery (SRS) is frequently used to treat acoustic neuromas (AN). The association between pretreatment tumor growth by volume with posttreatment changes have not been well characterized. This study aimed to assess the relationship between pretreatment changes in tumor volume with treatment response.

Methods: A retrospective chart review of acoustic neuromas treated SRS from 2011 to 2016 at University of Miami was performed. Volumetric analysis on all ANs with documented growth prior to treatment was performed using ITK-SNAP. Each successive patient MRI was compared with the patient’s initial tumor volume and prior MRI tumor volume to assess overall growth and step-wise growth, respectively. In assessment of treatment response, only patients with 5 years of follow-up were included. Statistics were performed with S.A.S 9.4 and Prism.

Results: A total of 22 SRS-treated ANs with documented growth were identified with volumes calculated from 187 MRIs. Mean patient age was 58.9 years (SD: 11.8) with 8 females (36%). Two patients had an NF2 diagnosis, 2 had prior radiation therapy, and 2 had prior surgical resection. Twelve patients received Gamma Knife (GK) treatment and 10 received Cyber Knife (CK). Average total observation time was 56 months (range: 10–151). Average follow-up after SRS treatment was 19 months (range: 1–54). Mean baseline tumor volume was 0.974 cm³ (range: 111–4,830). The mean growth observed prior to treatment was 346% baseline volume (range: 46–2,118%), with a mean growth rate of 26.82 mm³/month (range: 175.5–257.5). Individual pretreatment growth curves are shown in [Fig. 1]. Posttreatment individual step-wise growth curves are shown in [Fig. 2]. Many ANs initially increased in size following treatment but then stabilized or decreased at 1 to 2 years posttreatment. For tumors growing prior to treatment, growth rates were significantly altered by treatment (p = 0.0302, paired t-test). Rates of posttreatment swelling (increase in tumor size by 20% within the first year) trended toward significance of increased rates in CK (p = 0.06). The impact of pretreatment growth on posttreatment size changes was analyzed. The quartile of tumors with the fastest growth rates were compared with the quartile of tumors with the slowest rates. Posttreatment growth rates did not significantly differ between the fast and slow tumors (p = 0.8413). A nonparametric Spearman’s correlation test did not demonstrate a correlation between pre- and posttreatment growth rates (p = 0.7630). Three patients had a failure of treatment which was not predicted by pretreatment growth rate.

Conclusion: Stereotactic radiosurgery is an effective treatment for small acoustic neuromas. Volumetric analysis of ANs pre and posttreatment demonstrated that these tumors typically swell posttreatment, and then decrease in size. The rate of growth pretreatment is significantly different from posttreatment, but pretreatment rates are not correlated with posttreatment rates. Additionally, pretreatment growth rates did not predict treatment failure.