ESTRO 38 Abstract book

S333 ESTRO 38

pathological staging, tumor regression score, and local, distant relapse-free, and overall survivals. An inverse probability of treatment weighting (IPTW) analysis was performed to evaluate the association of MMR proficiency on surgical and clinical outcomes. The primary endpoint was downstaging defined as a lower T and/or N after than before neoadjuvant treatment. Results Among the 307 patients included, 21 (6.8%) had defective MMR (dMMR). Median follow-up was 36.7 months (95%CI: 34,7-39,7). dMMR patients were significantly younger than proficient MMR patients (pMMR) (60.4 y.o. (52.8-69.8) vs 45.4 y.o. (41.8-56.2), p<.0001) and trended towards a higher N stage (N1 patients: 14 (87.5%) for dMMR vs. 178 (64.0%) for pMMR, p=0.055). In an unmatched analysis, dMMR patients had a higher pathological downstaging rate (17 (85.0%) vs. 137 (52.7%), p=0.005) and had a lower rate of recurrence (10.0% vs. 30.5%, p=0.12). After IPTW matching, dMMR patients had a higher pathological downstaging rate (OR=5.77, 3.24-10.26, p<0.0001) and a longer recurrence-free survival (OR=0.29 (0.16-0.54), p<0.0001), although local recurrence free survival and overall survival did not differ significantly (HR=1.038 (0.475-2.27), p=0.925 and HR=0.65 (0.34-1.22), p=0.176 respectively).

Results Between 05/13 -05/18 132 CRC OM cases were treated and 95 patients had complete data for analysis. 70 (74%) metachronous OM. The median age was 67 years (range 36-89). 51 (54%) patients were male. Primary site of disease was rectum in 53 (56%), left colon in 22 (23%) and right colon in 15 (16%). OM sites treated: 27 liver, 56 lymph nodes (LN), 9 lung and 3 other. Median BED 10 was 79.2 Gy (range 37.5 – 151.2). Median FU was 13.7 mo (IQR 4.6 - 25.3). In-field local control at 1 year was 85.5% (95% C.I 76% – 96.2%). Median PFS for the cohort was 10.7 mo (95% C.I 8.1 – 15.2). The median PFS for the liver, lung and LN metastases were 6, 13 and 19 mo respectively. There was a significant difference in PFS based on metastasis location, with lymph node disease being associated with improved PFS (HR 0.34, 95% C.I 0.19 – 0.626, p = 0.0004; overall log rank, p = 0.0019). Lung was borderline. KRAS status was available for 47 patients (50%), 34 wild type and 13 mutant. WT KRAS status was associated with improved PFS (HR 0.44, 95% C.I 0.19 – 0.97, p = 0.04). No significant difference in PFS was seen when comparing groups by primary site, colon side, age, gender or whether presentation was with synchronous or metastatic disease. On MVA, metastasis location of lung and lymph node and KRAS WT or unknown status, remain statistically significant predictors of improved PFS. Conclusion In this cohort lymph node metastases appear to be associated with improved PFS compared to lung, bone or liver sites independent of KRAS status. KRAS WT is a good prognostic factor for PFS in treatment with SABR. These findings could be used to further refine the selection of CRC OM for treatment with SABR and incorporating systemic treatment. The excellent local control, regardless of KRAS subtype, suggests that OM PFS is driven by locoregional or widespread failure highlighting need for exploring biological differences. PV-0626 Mismatch Repair System Deficiency increases response to neoadjuvant chemoradiation in rectal cancer N. Meillan 1 , D. Vernerey 2 , J.H. Lefèvre 3 , G. Manceau 4 , M. Svrcek 5 , J. Augustin 6 , J. Fléjou 5 , O. Lascols 7 , J. Simon 1 , R. Cohen 8 , P. Maingon 1 , J. Bachet 9 , F. Huguet 10 1 Groupe Hospitalier Pitié-Salpêtrière-Charles-Foix- APHP, Radiation Oncology, Paris, France ; 2 University Hospital of Besançon, Methodology and Quality of Life Unit in Oncology, Besançon, France ; 3 Hôpital Saint- Antoine- APHP, Digestive Surgery, Paris, France; 4 Groupe Hospitalier Pitié-Salpêtrière-Charles-Foix- APHP, Digestive and Hepato-Pancreato-Biliary Surgery, Paris, France ; 5 Hôpital Saint-Antoine- APHP, Pathology, Paris, France ; 6 Groupe Hospitalier Pitié-Salpêtrière-Charles- Foix- APHP, Pathology, Paris, France; 7 Hôpital Saint- Antoine- APHP, Biology and Molecular Genetics, Paris, France ; 8 Hôpital Saint-Antoine- APHP, Medical Oncology, Paris, France; 9 Groupe Hospitalier Pitié-Salpêtrière- Charles-Foix- APHP, Hepato-Gastroenterology, Paris, France; 10 Hôpital Tenon- APHP, Radiation Oncology, Paris, France Purpose or Objective Defective mismatch repair system (MMR) has been shown to have a favorable impact on outcome in colorectal cancer patients treated with surgery or immunotherapy, adjuvant chemotherapy being discouraged unless there is nodal involvement. Its impact on radiosensitivity is unknown in rectal cancer patients. Material and Methods Patients treated for locally advanced rectal cancer between 2000 and 2016 were studied. MMR status was studied on the histological sample through PCR and immunohistochemistry. Reported points included age, sex, clinical and radiological tumor stages at diagnosis, modalities of neoadjuvant treatment, post-treatment

Conclusion MMR deficiency was associated with tumor downstaging after neoadjuvant chemoradiation as well with increase recurrence-free survival. dMMR patients could be good candidates for rectal preservation strategy. PV-0627 IL17F-rs641701 polymorphism as prognostic factor in rectal cancer after preoperative chemoradiation E. Palazzari 1 , E. Dreussi 2 , I. Maretto 3 , F. Navarria 1 , R. Innocente 1 , C. Belluco 4 , F. Matrone 1 , G. Fanetti 1 , A. Revelant 5 , M. Montico 6 , G. Toffoli 2 , S. Pucciarelli 3 , E. Cecchin 2 , A. De Paoli 1 1 IRCCS Centro di Riferimento Oncologico Aviano-National Cancer Institute, Radiation Oncology Department, Aviano, Italy ; 2 IRCCS Centro di Riferimento Oncologico Aviano-National Cancer Institute, Experimental and Clinical Pharmacology, Aviano, Italy ; 3 Padova University, Department of Surgical- Oncological and Gastroenterological Sciences- Section of Surgery, Padova, Italy ; 4 IRCCS Centro di Riferimento Oncologico Aviano-National Cancer Institute, Surgical Oncology Department, Aviano, Italy ; 5 IRCCS Centro di Riferimento Oncologico Aviano-National Cancer Institute and Padova University, Radiation Oncology Department, Aviano- Padova, Italy ; 6 IRCCS Centro di Riferimento Oncologico