Temporal Dynamics of Pseudoprogression After Gamma Knife Radiosurgery for Vestibular Schwannomas-A Retrospective Volumetric Study

BACKGROUND: The optimal observation interval after the radiosurgical treatment of a sporadic vestibular schwannoma, prior to salvage intervention, is unknown. OBJECTIVE: To determine an optimal postradiosurgical treatment interval for differentiating between pseudoprogression and true tumor growth by analyzing serial volumetric data. METHODS: This single-institution retrospective study included all sporadic vestibular schwannomas treated with Gamma Knife radiosurgery (Eketa AB, Stockholm, Sweden; 12-13 Gy) from 2002 to 2014. Volumetric analysis was performed on all available pre- and posttreatment magnetic resonance imaging scans. Tumors were classified as "stable/decreasing," "transient enlargement", or "persistent growth" after treatment, based on incrementally increasing follow-up durations. RESULTS: A total of 118 patients included in the study had a median treatment tumor volume of 0.74 cm(3) (interquartile range [IQR] = 0.34-1.77 cm(3)) and a median follow-up of 4.1 yr (IQR = 2.6-6.0 yr). Transient tumor enlargement was observed in 44% of patients, beginning at a median of 1 yr (IQR = 0.6-1.4 yr) posttreatment, with 90% reaching peak volume within 3.5 yr, posttreatment. Volumetric enlargement resolved at a median of 2.4 yr (IQR 1.9-3.6 yr), with 90% of cases resolved at 6.9 yr. Increasing follow-up revealed that many of the tumors initially enlarging 1 to 3 yr after stereotactic radiosurgery ultimately begin to shrink on longer follow-up (45% by 4 yr, 77% by 6 yr). CONCLUSION: Tumor enlargement within similar to 3.5 yr of treatment should not be used as a sole criterion for salvage treatment. Patient symptoms and tumor size must be considered, and giving tumors a chance to regress before opting for salvage treatment may be worthwhile.