A statistical analysis was performed on the volumes. POSTER TITLE: Gross tumor volume assessment on CT and PET in dental implants or teeth filling introduced artifacts on CT in patients with head and neck cancers. PRESENTER NAME:Mrs Bhudevi Soubhagya.N.Kulkarni1, Dr Rohit Singareddy1, Dr M.Chandrashekhar2, Dr NVN. Madhusudan Sresty1,Dr A.K.Raju1,Dr Alok Kumar3.1Basavatarakam indo American cancer hospital and research centre, Hyderabad, Telangana 500034.2JNTUH, Hyderabad, Telangana, 50085.3 Netaji Subhas Chandra Bose Cancer Research Institute, Kolkata, West Bengal, 700016. INTRODUCTION : For routine imaging in radiotherapy computed tomography (CT) most often used as the primary modality for planning. CT provides the structural information and the necessary electron density information for the calculation of dose. But patients with dental implants and teeth filling introduce the artifacts on the primary CT study sets. So GTV contouring becomes a challenging. The Radiation Therapy Oncology Group RTOG in combination with other branches has given clear guidelines to use different modalities if available in conjunction with CT to delineate the target. Positron emission tomography (PET) with the glucose analog [18F] flouro-2-deoxy-D-glucose (FDG) has been accepted as a valuable tool for the staging of lung, cervix, prostate and head and neck cancers. KEYWORD: Gross tumour volume(GTV),Computed tomography(CT),Positron emission tomography(PET). AIM: To investigate the difference in gross tumour volumes for dental implanted patients with head and neck cancers using CT and PET based imaging and a dosimetric evaluation on planning target volumes. MATERIALS AND METHODS: 10 patients with dental implants enrolled in the study were diagnosed to have head and neck cancer were recruited from Jan 2015 to June 2016. All patients treated with radical intent of head and neck IMRT . Patients received either concurrent or neoadjuvant chemotherapy with radiotherapy. Patients were immobilized with vacuum molded thermoplastic white orfit mask immobilization devices. All the patients underwent computed tomography (CT) on Discovery STE PET/CT scanner (GE Healthcare, Waukesha, WI) with 512x512 pixels at 120Kvp and 3 mm slice width in supine position. Intravenous contrast was used in all patients to visualize lymph nodes. Patients were injected with a standard dose of 10 mCi FDG and left in the designated “quiet room” in the radiation oncology suite for an uptake period of 1 hour. Then, patients were scanned for the PET signal. Scanned images for all the patients were transferred from PET-CT through Digital Imaging and Communication (DICOM) to Varian Eclipse Treatment Planning System (Version -10 Varian Medial Systems) for further planning purpose. CT based gross tumour volume was delineated on Varian eclipse version 10.0. After PET fusion .the best window level with standard uptake value (SUV) maximum was adopted to contour the GTV on PET independently. The GTV on CT and PET generated volumes were expanded to clinical and planning target volumes (CTV and PTV). A statistical analysis was performed on the volumes. RESULTS: Variable  Mean SD Minimum Median Maximum CT 29.918 12.307 9.77 29.475 53.80 PET 35.932 16.119 13.58 34.860 63.04 Pearson's r 95% CI  N DF  p-value  0.665 0.061 to 0.912 10.00 8 0.0360 DISCUSSION: The 70% of the GT volume were under estimated by CT as compared to PET GT volumes. However the change in GTV did not alter the phase I and II volumes but the phase iii volume was very significant. The average percentage of deviation of GTV volume for phase iii was found to be 24.6. So it becomes more important as we treat the tumour volume in phase iii which will be missed due to the artifacts introduced in the CT images. 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