ESTRO 36 Abstract Book

S173 ESTRO 36 _______________________________________________________________________________________________

point with maximal surface distance (MSD). A validated deformable image registration (DIR) propagated the POs from recurrence-CT to planning-CT. The distance from POs to the surface of the GTV was calculated and presented as mean distance from all four POs to the GTV. The patient specific GTV-CTV1 margin was calculated as median surface distance from GTV to CTV1. Difference between LRR distribution in groups with small and large CTV margins was evaluated using Kolmogorov-Smirnov test (p<0.05). Results In total 1,581 patients were identified and 297 had LRR within the first 3 years of follow-up; of those, 172 patients had CT-verified recurrent disease. Among them, 50% had GTV-CTV1 margin less than 5 mm and 50% larger than 5 mm. There was no difference in sex, tumour site, stage, tumour differentiation and p16-status between these two groups. After successful DIR, in total 192 recurrences were further analysed in the two margin groups; no significant difference in LRR distribution was found (p=0.6). Of the POs in the first and the second groups, 58% and 64% received 95% of the prescription dose, respectively (Figure 1).

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

Conclusion The presented data do not suggest any difference in distribution of loco-regional recurrences in relation to CTV margins. Such a difference could be expected if the CTV margin was a key component for loco-regional recurrence probability.