SD) revealing excellent agreement for B 0.95 (0.92-.97, 0.01), moderate for PB-CTV 0.69 (0.32-0.89, 0.13), R 0.78 (0.54-0.88, 0.08), LF 0.77 (0.39-0.95, 0.19), RF 0.76 (0.38-0.96, 0.19), and poor for PB 0.51 (0-0.92, 0.21). Geographic location, location of practice, years of experience, and number of prostate bed cases contoured yearly were not significant predictors of PB-CTV variation. TROG and GUROC guideline users had significant variation in contouring of B (p Z 0.009), LF (p Z 0.01) and RF (p Z 0.02), and B (0.002) respectively. ANOVA revealed statistically significant variation in rectal contouring between Canadians and Ameri- cans (p Z 0.03). Conclusions: Results of this largest conducted prostate bed contouring challenge to date confirm that planning for post-prostatectomy radiation therapy is subject to significant inter-observer variability in target and OAR contouring in the radiation oncology community at large. A STAPLE consensus atlas has been proposed as a result of this study. Author Disclosure: V. Velker: E. Research Grant; 2011 Canadian Radiation Oncology Foundation / Sanofi e Aventis Research Innovation Award (CASARIA). A.V. Louie: E. Research Grant; 2011 Canadian Radiation Oncology Foundation / Sanofi e Aventis Research Innovation Award (CASARIA). J. Markham: None. G.B. Rodrigues: E. Research Grant; 2011 Canadian Radiation Oncology Foundation / Sanofi e Aventis Research Innovation Award (CASARIA). 2386 Comparison of Soft-tissue Matching on Daily Cone Beam Computed Tomography and Fluoroscopy-based Bony-structure Matching in the Target Localization for the IMRT of Prostate Cancer H. Sato, E. Abe, and H. Aoyama; Niigata University Medical and Dental Hospital, Niigata, Japan Purpose/Objective(s): To evaluate set-up accuracy with the use of cone- beam computed tomography (CBCT) compared with fluoroscopy-based bony-structure matching in prostate intensity-modulated radiation therapy (IMRT). Materials/Methods: Twenty patients with localized prostate cancer who received IMRT between November 2010 and February 2012 constitute the materials. All patients were positioned supine. After initial set-up based on skin marker, bony-structure matching was carried out by using fluoros- copy-based set-up system. After that, CBCT was taken and soft-tissue matching using prostate-rectal interface as land mark was carried out. A shift of coordinate of isocenter between CBCT-based set-up and fluoros- copy-based set-up was recorded in anterior-posterior (AP), superior- inferior (SI), and left-right (LR) axes and considered as a potential inter-fractional shift when bony-structured based set-up was employed. Post-treatment CBCT was also taken once a week in order to measure the intra-fractional shift. Results: Interfractional and intra-fractional shifts were analyzed using 580 pre-treatment CBCTs and 136 post-treatment CBCTs, respectively. Mean and standard deviation (SD) of the inter-fractional shifts if bony-structured based set-up was solely used was -0.93.2 mm in AP, -0.92.4 mm in SI, and 0.10.7 mm in LR axes. Mean and SD of the intra-fractional shifts was 0.41.6 mm in AP, 0.01.3 mm in SI, and -0.20.6 mm in LR axes. Conclusions: The application of CBCT-based prostate-rectal interface matching may reduce the inter-fractional shift compared to fluoroscopy- based bony-structure matching in all coordinate with a range in SD of 0.7mm to 3.2 mm. Author Disclosure: H. Sato: None. E. Abe: None. H. Aoyama: None. 2387 Assessment of Late Rectal Toxicity After I-125 Prostate Brachytherapy Using Equivalent Uniform Dose Y. Shiraishi, 1 T. Hanada, 1 T. Ohashi, 1 A. Yorozu, 2 and N. Shigematsu 1 ; 1 Department of Radiology, Keio University, School of Medicine, Tokyo, Japan, 2 Department of Radiology, Tokyo Medical Center, National Hospital Organization, Tokyo, Japan Purpose/Objective(s): To evaluate the predictive value of equivalent uniform doses (EUD) for late rectal toxicity after I-125 brachytherapy for prostate cancer. Materials/Methods: From 2003 to 2006, consecutive 321 patients with prostate cancer were treated with I-125 monotherapy. In total, 319 patients whose CT-based postimplant dosimetry had been performed one month after the implant were analyzed. The prescription dose was set at 145 Gy. Dose-volume histograms (DVHs) of the rectum retrieved from a treatment planning system were converted to equivalent dose-based DVHs in order to account for differences in radiation treatment modality. Using the method developed by Kutcher et al, EUD for rectum were calculated from both the equivalent dose-based DVHs (EUDeq) and the physical dose-based DVHs (EUDphys). Late complications were recorded using the Radiation Therapy Oncology Group scale. Results: The average EUDeq to the rectum was 65.4 Gy (range, 33.9- 115.9 Gy). The average EUDphys to the rectum was 103.7 Gy (range, 53.7-183.8 Gy). EUDeq were exactly proportional to EUDphys. EUD were correlated with known postoperative dosimetric parameters of the rectum, such as V100, D30, D10 and D1. Especially, D1 had a strong correlation with EUD. EUDeq of 3 patients developing Grade 2 late rectal toxicity were 99.1 Gy, 93.3 Gy, and 68.3 Gy. D1 of the same patients were 211 Gy, 167 Gy and 162 Gy. Average EUDeq of 47 patients developing Grade 1 late rectal toxicity and 269 patients with no late rectal toxicity were 67.3 Gy and 64.8 Gy, respectively (not statis- tically significant). Conclusions: EUD of the rectum after I-125 brachytherapy for prostate cancer were calculated. Conversion of physical doses in DVHs to equiv- alent doses enabled a comparison of EUD between the standard fraction- ated 3D-CRT and I-125 brachytherapy. Although few patients developed Grade 2 late rectal toxicity after I-125 monotherapy, their rectal EUD were higher. EUD may be a more useful predictive indicator of late rectal toxicity than known postimplant dosimetric parameters. Author Disclosure: Y. Shiraishi: None. T. Hanada: None.T. Ohashi: None. A. Yorozu: None. N. Shigematsu: None. 2388 The Effect of Prostate MRI on Cancer Staging and Radiation Therapy Treatment Recommendations B.E. Terakedis, M.E. Heilbrun, W.T. Lowrance, D.C. Shrieve, and J.D. Tward; University of Utah Huntsman Cancer Hospital, Salt Lake City, UT Purpose/Objective(s): To evaluate the role of MRI in the clinical staging of prostate cancer in the definitive and salvage settings and to identify changes in treatment recommendations. Materials/Methods: Between November 2008 and November 2011, 114 patients referred for radiation therapy (RT) consultation underwent a prostate-protocol MRI of the prostate and pelvis. Charts were retro- spectively reviewed for demographic and clinical details including Gleason score, PSA level, MRI findings, clinical stage, treatment recommendations prior to and after completion of MRI, and treatment modality. Results: Eighty-six patients were evaluated for definitive treatment, 26 for salvage treatment, and 2 for RT planning in the adjuvant setting. The median Gleason score was 3+4, and the mean pre-treatment PSA levels were 13.1 and 1.6 for the definitive and salvage groups, respectively. Prostate cancer risk group prior to MRI was low in 23 (20%), intermediate in 38 (33%), high in 51 (45%), and unknown in 2 (2%) patients. MRI was performed after RT consultation in 68 patients and before consultation in 18 patients. The pre and post-MRI T stages for the definitive treatment Poster Viewing Abstract 2388; Table Primary tumor CT stage for patients prior to and after MRI of the prostate T1c T2a T2b T2c T3a T3b T4 Unknown Pre-MRI 43 18 11 6 5 2 1 0 Post-MRI 30 16 8 3 11 11 1 6 Volume 84  Number 3S  Supplement 2012 Poster Viewing Abstracts S363