59 4 Introduction Swallowing is a physiological process formed by oral, pharyngeal and esophageal phases.1 It occurs due to neuromuscular actions involving sensitive cranial, motor and parasympathetic nerves.2 Its purpose is to transport food from the mouth to the stomach, promoting hydration and nutrition. In order to be successful at this, a number of rapid, coordinated and accurate events have to occur, such as soft palate elevation, vocal fold closure, pharyngeal muscle contraction, laryngeal elevation and anteriorization and epiglottis lowering.3 These mechanisms occur involuntarily after stimulation of sensory receptors, especially located in the oropharyngeal cavity.2 A lack of onset or delayed onset of these events can be a sign of dysphagia. Dysphagia is a significant toxicity resulting in difficulty in swallowing, caused by abnormalities in structure or function of cartilaginous, bony, muscular or neural anatomy involved in normal swallowing.4 Complications such as malnutrition, aspiration and subsequent pneumonia can occur.4 Dysphagia can not only lead to a reduction of intake, but a reduction in peoples’ activities and social interactions as well, with corresponding negative changes to quality of life.5 Dysphagia may occur in up to 44% of patients with head and neck cancer (HNC) treated with Radiotherapy (RT) and up to 84% of patients treated with surgery.6,7 In addition, up to 2 out of 3 HNC patients may present with dysphagia at the time of diagnosis, and silent aspiration is present in 14 to 18% of patients pre-treatment.8 RT related toxicity may consist of dysphagia caused by the irradiation of swallowing related normal tissues, fibrosis, edema, ulcers, vascular toxicity, and osteoradionecrosis.9,10 Chemotherapy can add to the effects of RT and cause edema, mucositis and fibrosis.4 Surgical resection of the soft palate, floor of mouth, or base of tongue can cause severe swallowing dysfunction as well,6 compromising lingual mobility, muscle strength, mastication, muscle action, and muscle coordination.4,5,8 The most common procedure to evaluate dysphagia, swallowing safety and efficiency in patients with HNC is based on video-endoscopy, such as Fibreoptic Endoscopic Evaluation of Swallowing (FEES).8,11 However, these procedures are time consuming and require special equipment. Therefore, the 100 mL Water Swallow Test (WST) was developed.11,12 This test requires minimal equipment, is easily accessed and provides quantitative measures of swallowing performance. It is therefore used as a standardized test for screening dysphagia.13 In addition, the WST may be better in reflecting swallowing in everyday life in comparison to FEES, because it allows the patients to selfselect the size of each bolus swallowed.11 In previous research, the WST was performed in neurological patients, where it had high inter-rater reliability, a difference on average of 2.4% between two measurements, when assessing videotaped swallowing movies.12,14 Besides, the WST has been validated using video fluoroscopy in patients with neurogenic dysphagia, with a sensitivity and specificity up to 85.5% and 91.7%.15 It showed no