ESTRO 36 Abstract Book

S684 ESTRO 36 _______________________________________________________________________________________________

54.1%-99.9%), we stratified pts into two subgroups to test a possible correlation with pCR. Results Forty-eight pts were treated from March 2002 to October 2011 [M/F: 32/16; median age: 70.5, range: 43-84]. The median follow-up was 61.5 months (range: 2-136). Ten pts achieved a pCR (20.8%). No significant differences were observed based on Ki67 value (lower Ki67: 62.5% pCR; higher Ki67: 37.5% pCR); Fisher's Exact test: p=0.345). One-year, 3-year, and 5-year cumulative DFS were 97.9%, 86.0%, and 79.8%, respectively. One-year, 3-year, and 5- year cumulative OS were 97.8%, 93.5%, and 84.1%, respectively (fig.1).

radiotherapy (EBRT) field were retrospectively identified. Patients treated with prior pelvic brachytherapy were excluded. Details on primary site, previous EBRT and re- irradiation dose, toxicity, local control and survival were obtained on review of the hospital records. Acute (≤3 months) and late toxicity data were retrospectively collected and assessed using CTCAE v4.0. Local and distant progression free survival were calculated from the date of SBRT to the date of radiological progression using standard RECIST criteria (v1.1). Results 22 patients (9 prostate and 13 rectal cancer) received pelvic re-irradiation with SBRT between 25.07.2011 and 20.04.2016. Median age was 66 years (range 30-85 years). Median follow up was 18.5 months (range 3-58 months). The median time between primary EBRT and SBRT re- irradiation was 26 months (range 4-162 months). Median SBRT dose was 30Gy/3# (range 24Gy-44Gy/3-5#). Prior EBRT dose ranged from 20Gy/4#-70Gy/35# (median BED 63.72, range 30-84 assuming α/β = 10). 55% patients reported no measurable toxicity. No patients experienced ≥Grade 3 toxicity.10/22 (45%) patients experienced acute Grade 1/Grade 2 toxicities including fatigue, sciatica, nausea, diarrhoea, rectal haemorrhage and urinary symptoms. Only 1 patient experienced late toxicity (asymptomatic pelvic insufficiency at 21 months post SBRT, not requiring intervention). The 1 and 2 year LC rate were 90%, 85% and DPFS rate 92%, 71% respectively. OS at 1 and 2 years were 91% and 71% respectively. Conclusion Pelvic re-irradiation with SBRT is well tolerated and effective at controlling local disease. No ≥Grade 3 toxicity has been observed to date in our patient cohort although longer term follow up is needed. Further research to establish safe maximum cumulative doses to OARs for pelvic re-irradiation is warranted. EP-1274 Impact of concomitant radiotherapy boost in locally advanced rectal cancer: dose escalation M.A. Estornell 1 , D. Martinez 2 , V. Morillo 3 , M. López 1 , M. Soler 1 , J.L. Monroy 1 , A.V. Navarro 1 , A. Soler-Rodriguez 1 1 Hospital Universitario de la Ribera, Radiation Oncology Department., Alzira, Spain 2 University Hospital.Valladolid, Department of Medical Physics and Radiation Protection., Valladolid, Spain 3 Provincial Hospital. Castellon., Radiation Oncology Department, Castellon., Spain Purpose or Objective The standard preoperative radiation dose for locally advanced rectal cancer (LARC) is 45-50.4 Gy in 25-28 fractions. The aim of this study is to analyze the correlation between escalation radiotherapy dose, pathological complete clinical response (pCR) and downstaging rate or even its relation with other parameters of interest as toxicity, surgical margins, locoregional recurrence-free survival (LRFSD), distant metastasis free survival (DMFS) and overall survival (OS). The efficacy of the dose escalation in terms of pathological tumor response was evaluated as main end- point. Material and Methods Between 2000 and 2013, 287 patients were treated with preoperative chemoradiotherapy and surgical resection for LARC in our hospital. 233 patients underwent the standard chemoradiation schedule (median age 67 years; stage III 73.3%, stage IV 1.7%; 41.1% low third rectum; 45- 50.4 Gy; 1.8-2 Gy/fraction) and 54 patients were treated with simultaneous integrated boost-SIB (median age 66 years; stage III 74.5%, stage IV 1.8%; 21.8 % low third rectum; 45 Gy to the pelvis volume with a 2.17 Gy SIB on the tumor and macroscopical nodes).

Conclusion In this retrospective analysis no significant correlation was observed between Ki67 expression and pCR rate after preoperative chemoradiation. The small sample size and heterogeneity in neoadjuvant and adjuvant treatment could explain this result. EP-1273 Stereotactic body radiation therapy (SBRT) for pelvic re-irradiation Y. Augustin 1 , C. Chaw 1 , N. Van As 1 , K. Aitken 1 1 Royal Marsden Hospital Trust & Institute of Cancer Research, Radiotherapy, London, United Kingdom Purpose or Objective Re-irradiation of recurrent pelvic disease remains a challenging clinical problem. SBRT is an attractive modality for re-irradiation which is being increasingly utilised in carefully selected patients. However there remains uncertainty around clinical toxicity, efficacy and maximum safe cumulative doses to pelvic organs at risk (OARs). The primary aim of this retrospective study was to evaluate treatment related toxicity in patients receiving SBRT with the Cyberknife platform for pelvic re-irradiation at our institution. Secondary objectives were to evaluate efficacy by analysis of local control (LC), distant progression free survival (DPFS) and overall survival (OS) parameters. Material and Methods Patients receiving re-irradiation with SBRT to pelvic targets within a previously irradiated external beam