ESTRO 36 Abstract Book

S864 ESTRO 36 _______________________________________________________________________________________________

a non-linear conversion between tracer uptake and pO2, using a threshold of 10 mmHg. Assuming a clonogenic cell density of 10 8 per cm 3 in the CTV, the HTV doses required to achieve 95% local control (LC) were calculated based on a previously developed radiobiological model (Toma-Dasu et al 2009, 2012) accounting for the dynamic tumour oxygenation due to changes in acute hypoxia not visible in PET images. The total doses were calculated assuming that the treatment involves 24, 30 or 35 fractions. Results The non-linear conversion function and hypoxic threshold of 10 mmHg resulted in hypoxic subvolumes identified in five out six patients. Three out of six patients had a hypoxic subvolume > 3cm 3 . In two of the patients, the delineated HTV was not entirely confined within the primary CTV. For a treatment delivered in 30 fractions, the prescribed dose required to achieve 95% local control for the two patients with the largest HTVs of 32.74 and 38.29 cm 3 respectively were 75.52 and 75.67 Gy, both corresponding to an EQD2 of almost 79 Gy10. For the third patient with a smaller HTV of only 12.37 cm 3 , the total dose in 30 fractions for 95% LC was 72.35 Gy. If the total dose would be delivered in 35 fractions instead, the prescribed doses would increase with about 2.2% of the dose prescribed in 30 fractions for all three cases. The relative decrease in the total dose if the total dose will be delivered in only 24 fractions is about 3.5% for all three HTVs. For all patients and for all treatment fractionation schemes the dose levels required for achieving 95% tumour control probability accounting for local changes in the oxygenation of the tumour are below the levels of dose boosts proved to be feasible to be delivered without extra dose burden to the OARs on a previous study carried out on the same patients. Conclusion HX4-based delineation of hypoxic target volumes and calculation of required boost doses for a predefined tumour control probability appears to be feasible. HX4 is thus a potentially suitable tracer for the purpose of treatment individualisation in NSCLC. EP-1603 Atlas of complication incidence to explore dosimetric contributions to osteoradionecrosis L. Humbert-Vidan 1 , S. Gulliford 2 , V. Patel 3 , C. Thomas 1 , T. Guerrero-Urbano 4 1 Guy's & St Thomas' NHS Foundation Trust, Radiotherapy Physics, London, United Kingdom 2 The Institute of Cancer Research and Royal Marsden NHS Foundation Trust, Joint Department of Physics, London, United Kingdom 3 Guy's & St Thomas' NHS Foundation Trust, Oral Surgery, London, United Kingdom 4 Guy's & St Thomas' NHS Foundation Trust, Clinical Oncology, London, United Kingdom Purpose or Objective Mandibular osteoradionecrosis (ORN) is one of the most severe complications in patients with head and neck cancer undergoing radiation therapy (RT). Potential risk factors include primary tumour site and stage, radiation dose, pre- and post-RT dental extractions and mandibular surgery, chemotherapy, dental hygiene, smoking or alcohol. This pilot study aims to assess the contribution of radiation dose to the mandible to the incidence of ORN and investigates the effect of different risk factors using the atlas of complication incidence (ACI) method to summarise dose-volume histograms and toxicity data. Material and Methods This retrospective study included 80 patients with head and neck cancer with a median age of 62 (range 35-86) treated with radical IMRT. Primary tumour sites included a majority of oropharynx cases (42), oral cavity (26),

Conclusion Proper analysis of pixel-based data mining showed that lung pixel density outside the GTV did not predict for survival. The method we proposed allows pixel-based data mining based on distance to an organ. For such analysis, one should be well aware of confounding variables such as tumour size and mediastinal attachment. EP-1602 Treatment planning individualisation based on 18F-HX4 PET hypoxic subvolumes in NSCLC patients E. Lindblom 1 , A. Dasu 2 , J. Uhrdin 3 , A. Even 4 , W. Van Elmpt 4 , P. Lambin 4 , I. Toma-Dasu 5 1 Stockholm University, Medical Radiation Physics- Department of Physics, Stockholm, Sweden 2 The Skandion Clinic, The Skandion Clinic, Uppsala, Sweden 3 RaySearch Laboratories AB, RaySearch Laboratories AB, Stockholm, Sweden 4 Maastricht University Medical Center, Department of Radiation Oncology- GROW-School for Oncology and Developmental Biology, Maastricht, The Netherlands 5 Stockholm University and Karolinska Institutet, Medical Radiation Physics- Department of Physics and Department of Oncology and Pathology, Stockholm, Sweden Purpose or Objective Pre-treatment functional imaging of tumour hypoxia enables the identification of patients at greater risk of treatment failure, and, potentially, allows individualisation of treatment to overcome the increased radioresistance of hypoxic tumours. Treatment individualisation based on tumour hypoxia aims at identifying and prescribing higher doses to radioresistant hypoxic subvolumes based on the relative uptake of hypoxia-specific tracers. This study aimed to perform hypoxic target volume delineation and dose-prescription calculation for non-small cell lung cancer (NSCLC) patients using a novel hypoxic PET tracer, 18 F-HX4. Material and Methods Six non-small cell lung cancer (NSCLC) patients imaged with 18 F-HX4 PET/CT were included in the study. The hypoxic target volumes (HTV) were determined based on