predictor of improved local control was maximum dose >26 Gy. Additionally, BRAF positive metastases appeared to exhibit statistically significant improvements in local control outcomes when compared to BRAF negative and unknown metastases (pZ0.004). Other factors, including tumor location, morphology, volume, prescription dose, and RAS mutation status did not appear to significantly influence local control outcomes. The mean initial volumetric response following SRS was 0.86 (SD 0.37). Tumors with volume <1cc appeared to have improved initial volumetric response when compared to tumors with volume  1cc. Additionally, BRAF mutation positive metastases appeared to have improved initial volumetric response when compared to BRAF negative metastases. Other tumor parameters, including morphology, location, RAS, and c-KIT did not appear to have statisti- cally significant effects on initial volumetric response. Prescription dose and maximum dose also did not reveal a significant effect on initial volumetric response. Conclusion: Improved local control appears to be seen with increased maximum dose as well as BRAF positive mutation status. Tumors with volume < 1cc and BRAF positive mutation status also appear to improve initial volumetric response following SRS for brain metastases in mela- noma patients. Author Disclosure: S. All: None. H. Patel: None. A. Keller: None. N.R. Ramakrishna: None. 2147 Are Hippocampi Considered Organs at Risk During Stereotactic Radiation Therapy for Brain Metastases? A. Fiorentino, U. Tebano, N. Giaj Levra, S. Fersino, R. Mazzola, F. Ricchetti, G. Sicignano, R. Ruggieri, and F. Alongi; Radiation Oncology Division, Sacro Cuore Don Calabria Hospital, Cancer Care Center, Negrar-Verona, Italy Purpose/Objective(s): Hippocampal-dependent neurocognitive functions, including learning, memory and spatial information processing, could be affected by brain radiotherapy. Aim of the present study is to evaluate the dose to omolateral and contralateral hippocampus (O-H, C-H, respec- tively) during Stereotactic Radiotherapy (SRT) or Radiosurgery (SRS) for brain metastases (BM). Materials/Methods: Patients eligible for SRS/SRT treatment had a number of BM <5, with a size 30mm, Karnosky Performance Status (KPS)  80 and a life expectancy over 3 months. Gross Tumour Volume (GTV) was delineated by the fusion between Magnetic Reso- nance Imaging and Computed Tomography. A Planning Target Volume (PTV) was obtained from GTV by adding a 2-mm isotropic margin. The total dose ranged between 18-27 Gy in 1-3 fractions. For each BM, a volumetric modulated arc therapy plan was generated with one or two arcs and hippocampus sparing was not considered during optimizations phase. For the dosimetric evaluation of O-H and C-H, the D median , D mean , D 0.1cc and the V 1Gy , V 2Gy , V 5Gy and V 10Gy were analyzed. Results: From April 2014 to December 2015, 81 BM in 41 patients were treated with SRS/SRT and selected for the present analysis. The average value of PTV dimension and hippocampus volumes were (5.8 + 9.5) cc and (1.1 + 0.3) cc, respectively. For the O-H, the average values of D median ,D mean and D 0.1cc were (1.5 + 3.65) Gy, (1.54 + 3.6) Gy, (2.2 + 4.7) Gy, respectively, while the V 1Gy ,V 2Gy ,V 5Gy and V 10Gy values were (25 + 40) %, (18.9 + 35) %, (8.9 + 25.3) % and (2.1 + 11.8) %, respectively. For the C-H, the average D median ,D mean and D 0.1cc were (0.7 + 1.5) Gy, (0.7 + 1.4) Gy, (0.9 + 1.8) Gy, respectively, while the average values of V 1Gy ,V 2Gy ,V 5Gy and V 10Gy were (18 + 35) %, (10.2 + 27.7) %, (2.8 + 15.4) % and (1.4 + 11.6) %, respectively. The differences between O-H and C-H, in terms of received dose, were statistically significant (pZ0.03). Moreover, the PTV dimension (>5cc or >6cc) did not influence the dose of hippocampus (pZ 0.06; 0.2, respectively). Conclusion: During SRT/SRS treatments for BM, hippocampus received a very low dose and its clinical significance seems to be negligible, even if it is still under investigation. However, considering the increasing use of SRS/SRT for multiple BM, including also patients with up to 10 BMs, the dose to hippocampus need to be seriously evaluated in the treatment planning. Author Disclosure: A. Fiorentino: None. U. Tebano: None. N. Giaj Levra: None. S. Fersino: None. R. Mazzola: None. F. Ricchetti: None. G. Sicignano: None. R. Ruggieri: None. F. Alongi: None. 2148 Proton Therapy Reirradiation in Difficult-to-Treat Recurrent Glioblastoma D. Amelio, 1 D. Scartoni, 1 P. Farace, 1 L. Widesott, 1 S. Lorentini, 1 S. Vennarini, 1 F. Fellin, 1 S. Brugnara, 2 F. Maines, 2 M. Schwarz, 1 and M. Amichetti 1 ; 1 Proton Therapy Center, APSS, Trento, Italy, 2 Medical Oncology Dept, APSS, Trento, Italy Purpose/Objective(s): To report preliminary results of re-irradiation with proton therapy (PT) with or without chemotherapy (CHT) in difficult-to- treat recurrent glioblastoma (rGBM): patients (pts) were selected for PT due to the large tumor size or proximity to dose-limiting organs at risk that previously had received near-maximum dose tolerance during the first radiation course. Materials/Methods: Between January 2015 and January 2017 twenty pts with rGBM were re-irradiated with PT. All pts had been previ- ously treated with Stupp regimen. Fifteen (75%) were re-irradiated at first relapse/progression, five at the second/third one. Six patients (30%) were re-irradiated after partial tumor resection. Median age and Karnofsky performance status at re-irradiation were 56 years and 90%, respectively. Median time between prior radiotherapy and PT was 13 months. Target definition was based on CT, MR, and 18F- DOPA PET imaging. Gross Tumor Volume (GTV) included any area of contrast enhancement after contrast medium administration plus any pathological PET uptake regions. Clinical Target Volume (CTV) was generated by adding to GTV a 3-mm uniform margin manually corrected in proximity of anatomical barriers. Median CTV volume was 47 cc (range, 13-153 cc). All pts received 36 GyRBE in 18 fractions. PT was delivered with or without chemotherapy as follows: four (20%) pts (Group 1) also received concomitant TMZ (75 mg/m2/ die, 7 days/week); 3 (15%) pts (Group 2) also received concomitant (as above) and adjuvant TMZ (150-200 mg/m2/die, 5 days/month); 5 (25%) pts (Group 3) received PT only; 8 (40%) pts (Group 4) received PT followed by CHT (different regimens/drugs). All pts were treated with active pencil beam scanning PT. Registered side effects were graded according to Common Terminology Criteria for Adverse Events (CTCAE) version 4.0. Treatment response was assessed according to Response Assessment in Neuro-Oncology (RANO) criteria. Survival and progression-free survival after re- irradiation were calculated from initiation of PT until tumor pro- gression or death (by any cause), using the Kaplan Meier method. Results: All pts completed the treatment without breaks. There were no grade 3 or higher acute toxicities. One pts developed TMZ-related grade 1 neutropenia. There were no grade 3 or higher late toxicities. During follow-up three pts (15%) developed radionecrosis (diagnosed at imaging) with mild symptoms controlled with steroids. The median progression-free survival (PFS) was 6.3 months, while 6-month PFS rate was 60%. The median PFS was 6.8, 4.3, 5.4, and 5.5 for Group Volume 99  Number 2S  Supplement 2017 Poster Viewing E63