Abstract Book

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ESTRO 37

groups), the poor-responders had worse 5-year OS (93.8% vs. 60.8%, p= 0.035) and 5-year DFS (81.3% vs. 58.1%, p=0.105) compared to the good-responders. And there were no differences between the ypStage II group and the good-responders (TRG 3 or 4 ypStage III groups) in terms of the 5-year OS (86.0% vs. 93.8%, p=0.457) and 5- year DFS (79.9% vs. 81.3%, p=0.833). Multivariate analyses showed that grouping patients into 3 modified groups – ypStage 0/I, ypStage II/good-responders in ypStage III, and poor-responders in ypStage III – clearly divides patients and becomes a prognostic factor for OS (p=0.003) and DFS (p=0.004). Conclusion Combining histologic responses into staging system allows ypStage III rectal cancer patients to be clearly divided into two groups: good-responders and poor-responders. The TRG may be used to discern a high-risk group who might benefit from more intensive adjuvant treatments. EP-1480 Neoadjuvant radiation dose to the anal sphincter and postoperative bowel function in rectal cancer A.M. Couwenberg 1 , G. Van Veen 1 , H.M. Verkooijen 1 , W.M.U. Van Grevenstein 1 , M.P.W. Intven 1 1 UMC Utrecht, Radiotherapy, Utrecht, The Netherlands Purpose or Objective Sphincter-preserving rectal cancer surgery is associated with postoperative bowel dysfunction, known as the low anterior resection syndrome (LARS). LARS is characterized by incontinence for flatus and feces, frequency of, clustering of, and urgency of stools and is measured by the LARS score. One of the risk factors for developing severe LARS is neoadjuvant radiotherapy. This study determined the association between radiation dose to the anal sphincter and degree of bowel dysfunction after low anterior resection (LAR) in rectal cancer patients. Material and Methods We identified rectal cancer patients participating in a prospective colorectal cancer cohort treated with neoadjuvant chemoradiation (25x2Gy with concurrent capecitabine) or short-course radiotherapy (5x5Gy) followed by LAR between 2013 and 2016. Bowel dysfunction was measured with the LARS questionnaire and assessed at multiple time points after surgery. The average of all reported LARS scores after six months following surgery was categorized into no/minor LARS (range 0-29) and major LARS (range 30-42). The anal sphincter was delineated on pretreatment computed tomography (CT) scans. Mean dose (Dmean), and maximum dose (Dmax) were extracted and converted into biologically equivalent doses (EQD2) using α/β equals 3. Dose to the anal sphincter was compared between patients with no/minor and major LARS using the Mann- Whitney U test. Outcomes were stratified by low (<5cm) and mid/high (>5cm) tumour location. Results Eighty-eight patients were included with a median age of 64 years and 67% was male. Sixty-five (62%) patients received chemoradiation and 40 (38%) patients short- course radiotherapy. Median EQD2mean and EQD2max to the anal sphincter were 15.5Gy and 46.7Gy respectively. Major, minor and no LARS were reported in 41%, 25% and 34% of the patients respectively. EQD2mean to the anal sphincter was significantly higher in patients reporting major LARS (18.0Gy vs. 12.6Gy in patients with no/minor LARS p=0.042) (Figure). Major LARS was reported in 35% of the patients with an EQD2mean to the anal sphincter

of <20Gy, in 47% with an EQD2mean of 20-40Gy, and in 57% of the patients with an EQD2mean of >40Gy. No significant association was found between EQD2max and severity of LARS (43.8Gy in patients with no/mild LARS vs. 50.3Gy in patients with major LARS p=0.172). Stratification by low and mid/high tumour distance showed similar trends.

Conclusion Neoadjuvant radiation dose to the anal sphincter is associated with major bowel dysfunction after sphincter- sparing surgery in rectal cancer patients. These findings suggest that the dose to the anal sphincter should be limited in patients with rectal cancer planned for low anterior resection to reduce the risk of postoperative bowel dysfunction. EP-1481 DVH- and NTCP-based dosimetric comparison of different cranial margins for VMAT of rectal cancer H. Dapper 1 , M. Oechsner 1 , S. Münch 1 , S.E. Combs 1,2,3 , D. Habermehl 1,3 1 Klinikum rechts der Isar der Technischen Universität München, Department of Radiation Oncology, München, Germany 2 Deutsches Konsortium für Translationale Krebsforschung DKTK, Partner SiTe Munich, München, Germany 3 Helmholtz Zentrum München, Institute for innovative Radiotherapie iRT, Neuherberg/München, Germany Purpose or Objective Locally advanced rectal cancer patients usually undergo neoadjuvant chemoradiation (nCRT). In case of positive lymph nodes the cranial boarder of the clinical target volume (CTV) is mostly defined at the level of the bifurcation of the aorta. However, current studies have shown that relapse on cranial site of the planned target volume in neoadjuvant CRT treated rectal cancer patients is very rare. Especially if advanced diagnostic imaging shows no evidence of pathological lymph nodes above the primary tumor in lower or middle third rectal cancer patients, a reduction of cranial margins could be reasonable to reduce small bowel toxicity. Aim of this work was to compare dose distributions and expected toxicity of the small bowel for different cranial margins using a DVH- and NTCP-based approach. Material and Methods Ten patients with rectal cancer (T3N1/N2) of the middle or lower third and ≥3 cm longitudinal distance of macroscopic tumor to the cranial margins of the origin planned target volume (PTV) were retrospectively selected. All patients were planned with CT and MRI in prone position for a volumetric modulated arc therapy