الفهرس 
IntroductionOnly 14 pages are availabe for public view
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Abstract
Head and neck cancer (HNC) covers a wide spectrum of heterogeneous diseases that originate in the head and neck region, including cancers originating from the oral cavity, nasopharynx, oropharynx, larynx, and hypopharynx.
Each subtype within this group is associated with unique etiology, epidemiological trends, and therapy.
As a major histological type of HNC, head and neck squamous cell carcinoma (HNSCC) is the sixth most common cancer worldwide.
The contemporary standard of care in non-surgical, curative-intent management of head and neck squamous cell carcinoma (HNSCC) is definitive radiotherapy (RT) with or without concurrent chemotherapy.
Traditionally, HNSCC had been treated with conventional RT techniques which generally comprised of simple field arrangements, typically parallel-opposed portals with or without matching low anterior neck field or anterolateral wedge pair based on two-dimensional (2D) fluoroscopic imaging with no major emphasis on shielding of normal tissues. Such conventional techniques led to considerable morbidity, such as dryness of mouth, sticky saliva, swallowing dysfunction, and subcutaneous fibrosis with resultant negative impact upon healthrelated quality-of-life (QOL) in long-term survivors.
Over the years, technological advances in treatment planning and delivery based on computed tomographic (CT) imaging have resulted in progressive conformation of radiation dose to the target tissues while sparing adjacent organs-at-risk (OARs) ushering in the era of three
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dimensional conformal RT (3D-CRT). In the last decade or so, intensity-modulated radiation therapy (IMRT), an advanced form of high-precision conformal RT that uses non-uniform beam intensities calculated through computer-controlled optimization to achieve the desired dose-distribution has largely supplanted older radiation techniques (2D-RT/3D-CRT) based on consistent high quality evidence demonstrating significant reduction in radiation-induced xerostomia.
Moreover, grouping both imaging modalities, Positron Emission Tomography (PET) and CT, together in one scanner machine allows the addition of functional data into anatomical images. Fluorodeoxy-D-glucose PET combined to CT (FDG-PET/CT) has become an important diagnostic tool for HNC evaluation, and is applied in various clinical settings, ranging from the detection and staging to tumor response assessment and post-therapy follow-up.
The use of positron emission tomography with fluor-18 fluorodeoxyglucose (FDG-PET) in clinical practice for patients with head and neck squamous cell carcinoma (HNSCC) has expanded rapidly, with implications for diagnostic staging, radiotherapy planning, adaptive radiotherapy, and post-therapy evaluation. The implementation of FDG-PET/CT in radiation treatment planning not only has consequences for target volume definition and dose prescription but is also associated with an increased overall survival in patients with HNSCC. FDG-PET/CT-guided gradient dose prescription provides a window of opportunity for treatment de-intensification of the neck in order to decrease treatment-related toxicity without compromising oncological outcome. Further, interim FDG-PET/CT during radiotherapy can be useful to assess metabolic tumor response and enables opportunities for adaptive treatment strategies. The goals
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are to increase treatment effectivity in poor responders and reduce unnecessary toxicity in patients with good early tumor response. Further prospective trials investigating adaptive radiotherapy based on interim PET-evaluation are needed, especially regarding human papilloma virus-negative HNSCC and patients treated with primary radiotherapy.