Abstract: Three-dimensional conformal radiotherapy (3D-CRT) is based on an extensive use of modem medical imaging techniques. Delineation of the gross tumor volume and organs at risk constitutes one of the most important phases of conformal radiotherapy procedures. ¹⁸F-fluorodeoxyghcose (¹⁸F-FDG) PET possesses greater sensitivity and accuracy in detecting diseased lymph nodes, is an important staging examination for patients considered for radiation treatment with curative intent, ¹⁸F-FDG PET has an important role in delineation of gross tumor volume for patients treated with three-dimensional conformal radiotherapy.