1. Purpose/Objectives: A percentage of patients receiving stereotactic radiosurgery (SRS)/fractionated stereotactic radiotherapy (FSRT) for Schwannomas initially respond with a transient volumetric enlargement. 1,2 The objective of this study is to evaluate possible predictive factors of transient MRI volumetric changes. Results: Change in tumor size was classified as:  Enlargement  Transient enlargement  Direct shrinkage  Alternating enlargement and regression Nine of the cases (36%) had transient enlargement or alternating enlargement and regression based on post-treatment imaging. The response in original tumor size ranged from 17%-653%. Patients with prior surgical intervention, radiation dose greater than 16Gy, or SRS were more likely to be represented in this population. Blue = Single fraction >16Gy, Brown = FSRT All patients with alternating tumor volume had a history of administered steroids. Conclusions: Results still mandate close post-treatment MRI surveillance for volumetric increase of Schwannomas following SRS/FSRT. Of special concern for transient/alternating increase in volume may be those patients having had prior surgical intervention, higher radiation doses or a single fraction treatment. Volumetric Increase in Schwannomas on Surveillance Brain MRIs Following Stereotactic Radiosurgery: a Review of Predictive Factors Krisha J. Opfermann, MD; S. Lewis Cooper, MD; Joseph M. Jenrette, III, MD Department of Radiation Oncology, Hollings Cancer Center, Medical University of South Carolina, Charleston, South Carolina, United States Methods: IRB approved retrospective review was undertaken of 25 schwannoma patients treated with definitive radiotherapy at our institution IRB approved retrospective review was undertaken of 25 schwannoma patients treated with definitive radiotherapy at our institution (1990-2009). Patient course consisted of:  Initial MRI  Radiation Oncologist/Neurosurgery consultation  Radiation Therapy:  Follow-up brain MRIs Median: 28 months (2.9mos-158mos) Volumetric changes were measured by a single reviewer on T1-contrasted images. Enlargement based on a >13% increase in volumetric tumor size*. 1,3 * For patients without volumetric data; + 0.2cm change in max diameter. Patients: Patient characteristics, post-treatment symptoms and management were reviewed: References: 1. Nakamura H, Jokura H, Takahashi K, Boku N, Akabane A, Yoshimoto T. Serial Follow- up MR Imaging after Gamma Knife Radiosurgery for Vestibular Schwannoma. American Journal of Neuroradiology 2000; 21:1540-1546. 2. Hirato M, Inoue H, Nakamura M, Ohye C, Hirato J, Shibazaki T, Andou Y. Gamma knife radiosurgery for acoustic schwannoma: early effects and preservation of hearing. Neurologia Medico-Chirurgica(Tokyo) 1995; 35:737-741. 3. Meijer O, Weijmans E, Knol D, Slotman B, Barkhof F, Vandertop W, Castelijns J. Tumor-Volume Changes after Radiosurgery for Vestibular Schwannoma: Implications for Follow-Up MR Imaging Protocol. American Journal of Neuroradiology 2008; 29:906 –10. FSRTSRS Linac-based Gamma-knife Time Frame 1/1990-1/19991/1999-1/20111/2011- Current Radiation Source Photon (6MV) Arcs/Beams 193 Cobalt-60 Sources Fractions 311 Gamma-knife treatment plan generated for radiotherapy of an acoustic neuroma. red line = cochlea, blue line = tumor contour, yellow = 12.5Gy IDL, inner green line = 8Gy IDL, outer green line = 3Gy IDL Patient Characteristics General Characteristics Tumors(n)* 25 Median Age(yrs) 49 range(13-82) Sex(n) male12 female13 Race(n) white19 black4 other2 Previous Surgery(n) 0/24 Previous RT(n) 0/24 Steriods Prior(n) 4/23 Co-morbidities HTN(n) 5/22 DM(n) 1/22 NF-2(n) 3/23 Tumor Characteristics Site of Schwannoma(n) acoustic20 Side of Schwannoma(n) left12 right13 Cystic Component(n) 11/23 Median Max diameter(cm)† 2.12 (1-4.27) Median Volume(cm 3 )† 3.12 (0.58-14.77) Treatment Isocenters(n) 1 (1-3) Rx IDL(%)† 100 (73 - 100) Steriods given within 48hrs 1/21 FSRTSRS Patients(n) 619 Median Total Dose(Gy) 22.5 (21-24)18 (12-25) *1 patient with NF-2 had bilateral tumors treated at differing times. † Data limited for up to 7 patients Volume changes (cm 3 )