ESTRO 2021 Abstract Book

S1030

ESTRO 2021

days (occurred in 15 patients,27%) there is a reduction in LC, although it is not statistically significant(p=0,192). The most commonly prescribed RT dose was 56 Gy (median RT dose 56 Gy). According to the RT dose, LC was lower among patients who received less than 56 Gy at primary tumor(HR=0.44, p=0.088). There was no significant difference in OS for patients who received <56 Gy or 56 Gy compared with >56 Gy. This results is also confirmed in the analysis by subgroups. RT dose >56 Gy was associated with greater gastrointestinal toxicity, although it was not statistically significant(p=0.0092). Conclusion In clinical practice, it remains to be determined what is the optimal RT dose in anal cancer. In our study, we found that RT dose <56 Gy was associated with less LC but in absence of significance. This could be linked to the small number of patients. Prospective randomized studies designed with the aim of defining the appropriate dose are required. PO-1248 Preoperative radiotherapy with Intensity-Modulated SIB for rectal cancer: a systematic review A. Surgo 1 , R. Carbonara 1 , F. Gregucci 1 , M.P. Ciliberti 1 , I. Bonaparte 1 , A. fiorentino 1 1 Ospedale Generale Regionale "F. Miulli", Radiation Oncology, Acquaviva delle Fonti (BA), Italy Purpose or Objective The present review investigates the impact of preoperative radiotherapy (RT) with simultaneous integrated boost (SIB) on pathological complete response (pCR) in locally advanced rectal cancer. Materials and Methods A systematic review according to PRISMA model was performed. A literature search via PICO in MEDLINE/PubMed and EMBASE databases was independently conducted by two authors in May 2020. The following keywords were used with different arrangements: (P) Rectal cancer/adenocarcinoma, (I) Preoperative/neoadjuvant radiotherapy AND boost/SIB, (O) Pathological complete response. Clinical trials enrolling patients aged >18 years, affected by rectal adenocarcinoma, in clinical stage cT2-T4 cN0-N2 and suitable for preoperative chemoradiation were specifically included. Furthermore, only studies assessing intensity modulated (IMRT/VMAT) RT were included. The primary outcome was pCR; data on survival outcomes, toxicity, sphincter preservation rate and R0 resection were also collected and analyzed. Results A total of 64 studies were identified. After the PRISMA-based study selection, 8 clinical trials (prospective/phase II) were included in a qualitative synthesis. A total of 297 patients received preoperative RT to the pelvis with a SIB to the primary tumor. Different daily fractionation schedules were used: total dose to the pelvis ranged between 41.8 Gy (1.9 Gy/fr) and 50.6 Gy (2.3 Gy/fr) while SIB total dose ranged between 55 Gy (2.2 Gy/fr) and 58.75 Gy (2.35 Gy/fr). Different chemotherapy schemes and surgical approaches were admitted. Median follow up in 5 of 8 included studies ranged between 16 and 61 months. Pathological complete response was assessed according to different Tumor Regression Grading scales (Mandard’s, Dworak’s, ypT0N0 rates), reaching the maximum observed value of 38%. Good survival outcomes (1-, 3- or 5-year DFS, OS) were observed. Sphincter preservation was improved (up to 66%), with a 100% R0-resection rate achieved in 3/4 studies. Satisfactory toxicity profiles were observed, with only one study stopped for G3 toxicity (gastrointestinal, anemia, fatigue, pain). Conclusion The encouraging rates of pCR combined with satisfactory toxicity outcomes may support the use of preoperative intensity-modulated SIB to primary rectal tumor in clinical practice. Further considerations on the appropriate timing for post-RT surgery and anatomo-pathological results assessment are still required. PO-1249 Lateral Pelvic Nodal Boost During Short Course Radiation Therapy for Locally Advanced Rectal Cancer C. Hassanzadeh 1 , F. Fallahian 2 , G. Low 3 , A. Roy 4 , R. Chin 4 , K. Pedersen 5 , M. Mutch 3 , S. Glasgow 3 , L. Henke 4 , S. Badiyan 4 , H. Kim 4 1 Washington University in St. Louis, Department of Radiation Oncology, Saint Louis, USA; 2 Saint Louis University, Department of Surgery, St. Louis, USA; 3 Washington University in St. Louis, Department of Surgery, St. Louis, USA; 4 Washington University in St. Louis, Department of Radiation Oncology, St. Louis, USA; 5 Washington University in St. Louis, Department of Medical Oncology, St. Louis, USA Purpose or Objective The management of lateral pelvic lymph nodes (LPLN) in locally advanced rectal cancer is controversial, with limited data indicating if prophylactic resection or neoadjuvant therapy results in improved outcomes. There are no data regarding the management of LPLN in the setting of short course radiation (SCRT) with non- operative management. We evaluate a novel, definitive approach to addressing LPLN incorporating a simultaneous integrated boost to manage clinically suspicious LPLNs in patients receiving SCRT followed by chemotherapy (SCRT-CH) in a non-operative management paradigm. Materials and Methods Patients with locally advanced rectal adenocarcinoma who were treated with SCRT-CH with non-operative intent were included. All primary tumors were biopsy confirmed and disease staged with pelvic MRI. SCRT was delivered to the pelvis using intensity modulated RT to a dose of 25Gy in 5 daily fractions with a simultaneous integrated boost to 35Gy in 5 fractions to clinically enlarged, radiographically suspicious LPLNs. Patients then underwent consolidation chemotherapy with the goal of using mFOLFOX. Patients with clinical partial response (cPR) to SCRT-CH underwent total mesorectal excision (TME). No patients underwent lateral pelvic nodal dissection at time of TME. Progression free survival (PFS), distant metastasis free survival (DMFS), overall survival (OS), and local failure free survival (LFFS), including primary site and boosted LPLN nodal failures, were assessed. Results Between June 2017 and January 2021, 30 patients were treated with non-operative intent. Median follow-up