3rd ESTRO Forum 2015 S671 (μ±σ) of the volume identified by image analysis on the new image sets and by an experienced oncologist on the original image sets was 0.76±0.13 measured by the Dice similarity criterion (DSC). 17 cases had a DSC above 0.8; 17 cases had a DSC between 0.8 and 0.6; 2 cases had a DSC below 0.6. The figure shows the CT images from three bladder cancer patients. The Blue contour is the CTV; green is the CTV produced by thresholding of the original CT images; cyan is the CTV produced by thresholding of the image analysis enhanced CT image, which shows good agreement with the clinical contours using the BPJ-enhanced image data. Conclusions: The image analysis study presented for defining the CTV on low contrast CT images has the potential to be used as part of an adaptive radiotherapy system. Such a simple approach was not possible on the original image set. Although good agreement was found between the clinical and image analysis defined CTV this study serves only to provide pilot results. More extensive validation is required using CBCT and more observers. EP-1239 Hypofractionated split cours radiotherapy for elderly patients with prostate cancer and comorbidities M. Gawkowska-Suwinska 1 , R. Suwinski 2 , B. Smolska- Ciszewska 2 , K. Behrent 2 , T. Dworzecki 2 , G. Plewicki 2 1 Maria Sklodowska-Curie Memorial Cancer Center and Institute of Oncology, III Department of Radiotherapy and Chemotherapy, Gliwice, Poland 2 Maria Sklodowska-Curie Memorial Cancer Center and Institute of Oncology, II Department of Radiotherapy and Chemotherapy, Gliwice, Poland Purpose/Objective: Treatment of prostate cancer in frail elderly patients is challenging. Many of such patients are not candidates for surgery, brachytherapy or SBRT. Conventionally fractionated radiotherapy is frequently a choice, but duration of such treatment appears as an apparent challenge for frail individuals. To address these problems we designed a hypofractionated split course radiation therapy (HSC-RT) that, potentially, allows treatment in reduced number of fractions without compromising effectiveness. Because prostate cancer does not repopulate rapidly split course RT was allowed. Planned interruption may reduce acute reactions and may facilitate comfortable outpatient management of patients. Materials and Methods: Elderly frail patients with intermediate or high risk prostate cancer were considered as candidates for the present study. Radiation dose of 62,5 Gy in 2.5 Gy per fraction was given over 9 weeks with two planned gaps of 14 days each, after accumulation of 25 Gy and 50 Gy. Median age of the patients in the analyzed pilot series of 20 patients was 76 years (range 67-83), most patients had impaired general performance (ZUBROD 1-2) . A series of 130 consecutive patients (comparable risk characteristics, good PS, median age of 67 years) treated with conventionally fractionated radiotherapy (dose of 70-76 Gy, median 74 Gy) was used for comparison of biochemical outcome. Neoadjuvant ADT was used in both groups. PSA concentration and treatment tolerance was assessed at follow-up. Results: In general, HSC-RT was well tolerated, ( the highest RTOG score for bladder toxicity during treatment was 1 for 45% of patients, 2 for 15%, 3 for 5%, rectal toxicity was minimal with highest score 1 in 15 % of patients. There were not late rectal toxicity and late bladder toxicity was minima with highest score 1). Baseline median pretreatment PSA concentration was the same in both groups (15 ng/ml). However, in spite of planned treatment gaps, the PSA measurements at follow-up revealed somewhat favorable biochemical outcome in patients treated with HSC-RT, compared to those treated conventionally (CF). Median PSA concentration (ng/ml) at follow-up was lower in HSC-RT compared to CF (0.03 vs. 0.08 at 12 months; 0.03 vs. 0.13 at 24 months; 0.01 vs. 0.22 at 36 months). Median PSA nadir was 0.013 ng/ml for HSC-RT vs. 0,03 for CF. Conclusions: While the results of this study are limited by a small size and clinical heterogeneity of the analyzed group the outcome presented suggest that hypofractionated split course radiation therapy in frail elderly patients with intermediate/high risk prostate cancer is feasible and does not compromise biochemical control. EP-1240 Clinical outcomes of image guided adaptive radiotherapy (IGART) for hypofractionated treatment of bladder cancer S. Hafeez 1 , F. McDonald 2 , S. Lalondrelle 2 , H. McNair 1 , K. Warren-Oseni 2 , K. Jones 3 , V. Harris 2 , H. Taylor 4 , V. Khoo 4 , K. Thomas 5 , V. Hansen 1 , D. Dearnaley 1 , A. Horwich 1 , R. Huddart 1 1 Institute of Cancer Research & The Royal Marsden NHS Foundation Trust, Radiotherapy and Imaging, Sutton Surrey, United Kingdom 2 The Royal Marsden NHS Foundation Trust, Radiotherapy and Imaging, Sutton Surrey, United Kingdom 3 The Royal Marsden NHS Foundation Trust, Bob Champion Unit, Sutton Surrey, United Kingdom 4 The Royal Marsden NHS Foundation Trust, Radiotherapy and Imaging, London, United Kingdom 5 The Royal Marsden NHS Foundation Trust, Research Data Management and Statistics Unit, Sutton Surrey, United Kingdom Purpose/Objective: Anatomical variation in bladder position and size has driven on-line IGART strategies to prevent geographical misses. A ‘plan of the day’ IGART approach improves target coverage and reduces normal tissue irradiation. We report on the clinical outcomes of patients treated in a phase 2 study with hypofractionated radiotherapy using this technique. Materials and Methods: Fifty-five patients with T2-T4aN0-2 M0 bladder cancer not suitable for cystectomy or daily radiotherapy treatment were recruited between February 2009 and March 2014. A library of 3 plans were created (using small, medium and large PTV) from planning CT scans performed at 0 and 30 minutes, treating whole (empty) bladder to 36Gy in 6 weekly fractions. Cone beam CT (CBCT) imaging was performed prior to each fraction. Appropriate PTV was selected from the library for treatment delivery. Acute toxicity was assessed weekly during radiotherapy and at 6 and 12 weeks using CTCAE v3.0. Late toxicity was assessed at 6 months and 12 months using Radiation Therapy Oncology Group (RTOG) grading. Local control was assessed at 3 months preferentially with cystoscopy, or if this was not possible with imaging alone (CT/MRI). Local progression free