ESTRO 36 Abstract Book

S171 ESTRO 36 2017 _______________________________________________________________________________________________

PV-0328 Factors associated with complete response after brachytherapy for rectal cancer; the HERBERT study. E.C. Rijkmans 1 , R.A. Nout 1 , E.M. Kerkhof 1 , A. Cats 2 , B. Van Triest 3 , A. Inderson 4 , R.P.J. Van den Ende 1 , M.S. Laman 1 , M. Ketelaars 1 , C.A.M. Marijnen 1 1 Leiden University Medical Center LUMC, Department of Radiotherapy, Leiden, The Netherlands 2 The Netherlands Cancer Institute, Department of Gastroenterology and Hepatology, Amsterdam, The Netherlands 3 The Netherlands Cancer Institute, Department of Radiotherapy, Amsterdam, The Netherlands 4 Leiden University Medical Center LUMC, Department of Gastroenterology and Hepatology, Leiden, The Netherlands Purpose or Objective The HERBERT study was performed to examine the feasibility of a high hose rate endorectal brachytherapy (HDREBT) boost after external beam radiotherapy (EBRT) in elderly patients with rectal cancer who were unfit for surgery. The primary results and long term clinical outcomes have been presented at ESTRO 2014 and 2016. With rising interest for organ preservation, the role of definitive (chemo)radiotherapy becomes increasingly important. This current analysis evaluates factors that are associated with a complete response to treatment. Material and Methods A dose finding feasibility study was performed from 2007 to 2013 in inoperable rectal cancer patients. Patients received 13x3 Gy EBRT followed by three weekly applications HDREBT of 5 to 8 Gy per fraction. Clinical target volume (CTV) for HDREBT was defined as residual scarring or tumor after EBRT. Clinical tumor response was evaluated based on digital rectal examination and endoscopy (MRI or biopsy was not routinely performed). Complete response was determined after serial assessments. Patient, tumor and treatment characteristics of complete responders (CR) were compared to non-complete responders (nCR) using Chi- square test and the independent samples t-test. Results Of the 38 patients included in the study 33 were evaluable for response evaluation. Seven were treated with 5 Gy per fraction, four with 6 Gy, 12 with 7 Gy and 10 with 8 Gy per fraction. In total 20 patients achieved a complete response. Baseline patient characteristics (age, ASA, WHO and co-morbidity) and tumor-characteristics (T-stage, N- stage, cranio-caudal length of the tumor and distance from anal verge) were not associated with response to treatment. A trend was observed in complete response between dose levels; 2/ 7 treated with 5 Gy per fraction; 1/4 with 6 Gy; 9/12 with 7 Gy and 8/10 with 8 Gy per fraction (p=0.05). The actual planned D98 (dose to 98% of the CTV) was however not significantly different between patient with a complete response and no complete response: 6.25 Gy (range 3.8-8.3 Gy) vs. 5.98 Gy (range 1.2-8.8 Gy) respectively (p=0.63). Endoscopic evaluation of response after EBRT was significantly associated with the overall response rate. Seven patients already had a CR after EBRT, whereas 13/21 patients (62%) with a partial response after EBRT achieved a CR. None of the five patients with stable disease achieved a complete response (p=0.002). Mean residual volume and thickness of residual scarring or tumor after EBRT were significantly lower in complete responders (see Figure). In addition, tumors encompassing less than 1/3 of the circumference were more likely to achieve a complete response than larger tumors (70% vs 17% respectively, p=0.025).

Conclusion Endoscopic response after EBRT and residual tumor thickness, circumference and volume at time of HDREBT were significantly associated with achieving a complete response. This demonstrates that careful selection of patients for organ preserving strategies can result in a very high success rate.

Proffered Papers: Head and Neck

OC-0329 Does margin matter? Distribution of loco- regional failures after primary IMRT for Head &Neck cancer R. Zukauskaite 1 , C.R. Hansen 1 , C. Brink 1 , C. Grau 2 , E. Samsøe 3 , J. Johansen 1 , E. Andersen 3 , J. Petersen 2 , J. Overgaard 4 , J. Eriksen 1 1 Odense University Hospital, Department of Oncology, Odense, Denmark 2 Aarhus University Hospital, Department of Oncology, Aarhus, Denmark 3 Herlev Hospital, Department of Oncology, Copenhagen, Denmark 4 Aarhus University Hospital, Department of Experimental Clinical Oncology, Aarhus, Denmark Purpose or Objective Head and neck squamous cell carcinoma (HNSCC) often presents as a local or loco-regional disease. Margins are often added around the gross tumour volume (GTV) during the planning of curative radiotherapy to cover microscopic disease. However, there is little evidence available for the optimal size of the high dose clinical target volume (CTV1) margin. Until 2013, different margins from GTV to CTV1 were allowed according to the national treatment guidelines in Denmark, varying from 0 to up to 10 mm. The objective of this study was to analyse loco-regional recurrence pattern in a large cohort of patients with HNSCC treated with curatively intended IMRT. We aimed at evaluating how the location of CT verified loco-regional recurrences (LRR) were influenced by different CTV1 margins. Material and Methods Patients with larynx, oro-/hypopharynx or oral cavity HNSCC treated with primary IMRT during 2006–2012 in three centres were retrospectively identified from national database. Treatment was given according to DAHANCA guidelines, primarily 66-68 Gy in 6 fractions/week with concomitant Nimorazole and weekly cisplatin in loco-regionally advanced cases. The GTV-CTV1 margin was primarily produced by volumetric expansion that varied from 0-10 mm and eventually modified according to anatomy. The origin of recurrence was estimated for all loco-regional treatment failures with diagnostic CT or PET/CT images available. Assuming that loco-regional recurrences arise from a few surviving cancer cells, the possible points of LRR origin (PO) were identified on diagnostic scans by two independent observers, and calculated as mass mid-point (MMP) and a