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P03.01.A Late pseudoprogression of vestibular schwannoma after robotic guided stereotactic radiosurgery - Implications for follow-up


 
 
 Stereotactic radiosurgery (SRS) may cause transient changes of morphology and volume in vestibular schwannomas (VS). This may lead to difficulties in distinguishing treatment-related changes (pseudoprogression) from tumor recurrence (true progression), especially at 12-24 months after treatment. Therefore, we investigated the time course of volume changes of VS after robot-guided SRS.
 
 
 
 We included all patients with unilateral VS who underwent single fraction robotic guided SRS using the Cyberknife® with a minimum follow-up (FU) of 24 months and MR images ≤3 mm slice thickness. Tumor volumes were measured on T1-weighted contrast enhanced images. Volume changes (percentage of tumor volume change compared to baseline) during FU were classified according to RANO criteria (“partial response” (PR) (≥65% decrease), “stable disease” (SD) (<65% decrease; <20% increase), or “progressive disease” (PD) (≥40% increase)). A new status “pseudoprogression” (PP) (>20% transient increase) was defined and divided into early (ePP, occurrence within first <12 months) and late (lPP, >12 months) PP.
 
 
 
 Overall 63 patients fulfilled the inclusion criteria. The median age was 56 years (range: 20-82) and the median initial tumor volume was 1.5 cm3 (range: 0.1 - 8.6). All patients received 13 Gy with an isodose level of 80%. The median radiological and clinical FU was 66 months (range: 24-103).
 We found PR in 36% (n=23), SD in 35% (n=22) and PP in 29% (n=18). The latter was separated in ePP in 16% (n=10) and lPP in 13% (n=8). The median time to peak in the ePP was six months (range: 4 - 10) and in the lPP 35 months (range: 14 - 61). The median time to return from peak to baseline was seven months in the ePP (range: 5 - 20) and 18 months (range: 6 - 33) in the lPP group. Using these criteria no PD was observed. Additionally, we did not find any significant impact of radiation parameters (coverage, nCi, prescription dose, maximal dose) or patient related parameters (tumor volume, age) on the onset of early and/or late PP.
 
 
 
 In our study, we demonstrated that any volume increase assumed to be PD turned out to be ePP or lPP. This might impact the management of VS treated with robotic SRS during FU in favour of further observation.
