ESTRO 36 Abstract Book

S320 ESTRO 36 _______________________________________________________________________________________________

8 Aalborg Hospital, Department of Oncology, Aalborg, Denmark Purpose or Objective Danish national guidelines (GL) for head and neck cancer radiotherapy (RT) have been available since 1990 and were revised in 2013. One of the major revision points was the change of GTV to CTV margins from mainly anatomically driven expansions to symmetric geometric expansion. The purpose of this study was to analyse the consistency of generated CTV contours by the new approach and to evaluate the impact on CTVs and any differences between four centres involved in RT before and after the guideline revision. Material and Methods Prior to the GL change in 2013, four centres were asked to delineate CTV1, CTV2 and CTV3 of a stage IV oropharynx patient with three prescription levels: 66 Gy, 60 Gy and 50 Gy in 33 fractions. The contours of the tumour GTV and the lymph node GTV were provided together with the organs at risk (OAR). Each centre made a RT plan from the CTVs. After the new GL was implemented, the centres were asked to repeat the CTV contouring and the dose planning. Likewise, the centres were asked in 2016, three years after the GL, to re-contour and re-plan to test the consistency over time of the GL. The difference in contouring was evaluated by the difference in CTV volume, Dice Similarity Coefficient (DSC) and average minimum surface distance (MSD) between the CTV contours. The difference in dose plans was evaluated by mean dose to OAR and dose-specific treated volumes (V 62.7Gy , V 57Gy , V 47.5 Gy and V 25Gy ) . The statistical difference was tested with a paired two-sided Student's t-test (p<0.05). Results The contours from GL 2004 were less uniform and showed large volume differences (Table 1). The MSD showed a mean difference of 0.6 cm and a relatively large standard deviation (SD) of 0.45 cm for the CTV1. The GL 2013 provided a more operational margin expansion and hence resulted in a high DSC and very similar volumes, however with a mean increase of 40% and 32% for the CTV1 and CTV2, respectively. The re-contouring in 2016 was similar to 2013 indicating that the interpretation of GL 2013 is not affected by time.

irradiated volume (figure 1), however improved planning and familiarity of the GL reduced this difference in the 2016 plans.

Conclusion The GL 2013 showed more uniform CTV1 and CTV2 contouring between centres, which was reproduced in 2016. The more geometric GTV to CTV expansion allows for an easier operational delineation which leaves less room for misinterpretation. This transforms into more uniform treatment plans and very similar irradiated volumes across all centres. PO-0614 The prognostic role of 18F-FDG PET/CT in head and neck cancer and the importance of HPV status J.M. Moan 1 , E. Malinen 2 , J.G. Svestad 3 , C.D. Amdal 1 , T.V. Bogsrud 4 , E. Dale 1 1 Oslo University Hospital, Department of Oncology, Oslo, Norway 2 University of Oslo, Department of Physics, Oslo, Norway 3 Oslo University Hospital, Department of Medical Physics, Oslo, Norway 4 Oslo University Hospital, Department of Nuclear Medicine, Oslo, Norway Purpose or Objective Standardized uptake value (SUV) and related parameters derived from 18F-FDG PET/CT prior to radiochemotherapy of head and neck cancer have been shown in several studies to correlate with survival. We wanted to validate this finding in our own patient cohort, but also to see the PET parameters together with clinical risk factors We retrospectively reviewed 225 patient cases from 2007 to 2014 with complete sets of 18F-FDG PET/CT and potential clinical risk factors (age, sex, ECOG status, Charlson comorbidity status, pack years of smoking, TNM stage, tumor differentiation, tumor site, HPV DNA status (tested for oropharyngeal cancers [OPC]), treatment duration, days on nimorazole and numbers of weekly cisplatinum. All patients received radiotherapy with 68-70 Gy in 2 Gy fractions. Patients older than 70 years or with comorbidity did not receive concomitant cisplatinum (26%). Clinical endpoints were overall survival (OS), local control, regional control, distant control, and disease-free survival (DFS). We investigated the image parameters; 1) Gross tumor volume (GTV) based on CT and PET, 2) PET tumor volume delineated by the nuclear medicine specialist, 3) metabolic tumor volume (MTV), 4) total including HPV status. Material and Methods

The similarity of the CTV targets in 2013-16 resulted in more uniform dose plans, however, the different planning approaches resulted in only slight difference to the OAR, and the SD of mean doses did not improve significantly. The SD of the irradiated volume improved with the GL 2013 and further improved in 2016 dose plans. It is obvious that the larger CTVs of the GL 2013 increased the