ESTRO 38 Abstract book

S427 ESTRO 38

Radiohterapy Unit, Campobasso, Italy ; 8 Fondazione di Ricerca e Cura "Giovanni Paolo II", Medical Physics Unit, Campobasso, Italy ; 9 Ospedale Bellaria, Radiotherapy Department, Bologna, Italy Purpose or Objective Aim of this study was to evaluate the pathological complete response (pCR) rate in locally advanced rectal cancer (LARC) after adaptive high-dose neoadjuvant radiation therapy (RT) with concomitant plus sequential boost based on [18F]FDG-PET/CT performed two weeks Patients (pts) with biopsy proven LARC were included. Primary end-point was pCR rate. Secondary objectives included acute and late toxicity. The sample size was calculated based on the Simon’s two-stage design. All pts underwent [18F]FDG-PET/CT at baseline (PET0) and after 2 RT weeks (PET1). Intensity modulated RT (IMRT) technique was delivered with concurrent capecitabine. The Gross Tumor Volume (GTV) was defined using a gradient-based delineation method and the maximal standardized uptake values (SUVmax) were recorded. The dose to rectum, mesorectum, and pelvic lymph nodes was 45 Gy (1.8 Gy/fr). A simultaneously integrated boost (SIB) was delivered to GTV+2 cm margin with a total dose of 50 Gy (2 Gy/fr). A sequential boost was delivered to GTV+5 mm margin with a total dose of 5 Gy in 2 fractions (2.5 Gy/fr) for a total dosed of 55 Gy using cone-beam CT. Pathological response was scored based on the American College of Pathologist Tumor Regression Grading (TRG). Toxicity was scored according to the CTCAE v4.03 scale. Results Eighteen pts (13 M, 5 F; median age 58.1 years) were enrolled. According to UICC TNM Staging (8th edition) classification the clinical stage was: T2N1M0 (2 pts), T3N1M0 (11 pts), T3N2M0 (2 pts), and T4N1M0 (3 pts). Nine-10 weeks since the end of neoadjuvant treatment all pts except one, who refused surgery after evidence of clinical complete response, underwent surgical resection. Six pts (35.3%) showed TRG=0 (pCR), 5 pts (29.4 %) TRG=2, and 6 pts (35.3%) TRG 3. GTV measured at PET0 was 22 mL (range: 1.2-90.9) and at PET1 9.8 mL (range: 0-62.4) (p<0.05). Median GTV reduction was 60.2 % (range: 0-100): 61.5 % (range: 13.9-100) for responders (pCR) pts and 59.5% (range: 0-94.5) for non-responders (p=0.91). Median SUVmax of the rectal lesions was 15.1 (range 5.7-45) at PET0 and 8.3 (range: 0-20.4) at PET1 (p=0.098). Median SUVmax reduction was 40.4% (range: 0-100): 50% for responder pts and 35.2% for non-responders (both ranges: 0-100) (p=0.39). Most pts showed G1 GI and GU toxicity. Only one pt had acute GI G3 toxicity. Conclusion Despite the delivery of the sequential boost on a reduced volume, adapted based on an early 18F-FDG-PET/CT, the pCR rate was 35.3%. This regimen was well tolerated. PO-0818 Comparison of three different approaches for bowel delineation in patients with rectal cancer. E. Rijkmans 1 , B.B.D. Otto 1 , J.R.N. Van der Voort van Zyp 2 , R.TH. Zinkstok 1 , F.P. Peters 1 , C.A.M. Marijnen 1 1 Leiden University Medical Center, Department of Radiation Oncology, Leiden, The Netherlands ; 2 University Medical Center Utrecht, Department of Radiotherapy, Utrecht, The Netherlands Purpose or Objective Acute gastro-intestinal (GI) toxicity is one of the main dose-limiting toxicities in radiotherapy for rectal cancer. Previous studies showed a dose-response relationship for small bowel and acute GI toxicity. However, different contouring methods have been used, and there is no consensus on which is best suitable for clinical practice. This study aims to compare different bowel delineation after the start of RT. Material and Methods

Conclusion Hypofractionated, adaptive RT concomitant with oxaliplatin and fluoropyrimidines is feasible with acceptable G3 toxicity and provides a very interesting response rate. There could be room for further dose escalation on the residual tumor with the aim of increasing pCR and/or cCR rates. PO-0817 PHASE II study about adaptive high dose radiotherapy in high risk rectal cancer A. Guido 1 , V. Panni 1 , L. Fuccio 2 , L. Giaccherini 3 , D. Cuicchi 2 , P. Castellucci 4 , S. Fanti 4 , F. Coppola 5 , F. Di Fabio 6 , G. Poggioli 2 , G. Macchia 7 , F. Deodato 7 , V. Picardi 7 , M. Boccardi 7 , S. Cilla 8 , A. Arcelli 1 , G.P. Frezza 9 , A.G. Morganti 1 , S. Cammelli 1 1 Radiation Oncology Center- University of Bologna, Department of Experimental- Diagnostic and Specialty Medicine- DIMES, Bologna, Italy ; 2 S. Orsola-Malpighi Hospital- University of Bologna, Department of Medical and Surgical Sciences- DIMEC, Bologna, Italy ; 3 AUSL- IRCCS, Radiation Oncology Unit, Reggio Emilia, Italy ; 4 Nuclear Medicine Unit- University of Bologna, Department of Experimental- Diagnostic and Specialty Medicine- DIMES, Bologna, Italy ; 5 S.Orsola-Malpighi Hospital, Department of Diagnostic Medicine and of Prevention, Bologna, Italy ; 6 S. Orsola-Malpighi Hospital, Department of Medical Oncology, Bologna, Italy ; 7 Fondazione di Ricerca e Cura "Giovanni Paolo II",