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16 CHAPTER 1 used interchangeably in literature. It is estimated that the prevalence of primary sarcopenia in the general population is 5-13% for people aged 60-70 years, and up to 50% for those aged 80 years or above. 29 Secondary sarcopenia is due to chronic systemic inflammation, malnutrition and immobilization. 28 It is suggested that for cancer patients, a chronic systemic inflammatory state caused by the tumor microenvironment leads to the presence of secondary sarcopenia. 28 Low skeletal muscle mass is associated with adverse outcomes in oncological patients and in particular when a state of both low skeletal muscle mass and a disproportional surplus of fat mass is present (sarcopenic obesity). 30–34 Head and neck cancer patients are at risk for low skeletal muscle mass (secondary sarcopenia) due to tumor site which leads to dysphagia and difficulty of swallowing, leading to malnutrition and a catabolic state. At diagnosis, up to 50% of patients with HNSCC present with signs of malnutrition. 35 SKELETAL MUSCLE MASS INVESTIGATION There are several methods to measure body composition and skeletal muscle mass. These methods include ‘dual-energy X-ray’-absorptiometry (DEXA) scan, bioelectrical impedance analysis (BIA) and modern imaging techniques including Computed Tomography (CT) and Magnetic Resonance Imaging (MRI). 36,37 BIA is based on the difference in electrical conductance of the different body compartments; muscle mass has a high water content and therefore low electrical resistance, whereas fat mass has a lower water content and higher resistance. Both DEXA and BIA are generally low cost and easy to use. These diagnostic tools are how - ever confounded by alterations in hydration, edema and food intake. Therefore, its use in assessing body composition of patients with cancer is not favored. Both CT and MRI allow for the detailed assessment of all body compartments including skeletal muscle mass. In 2004, Shen et al. showed a high correlation between the cross-sectional skeletal muscle area on a single MRI slice at the level of the third lumbar vertebra (L3) and whole body total skeletal muscle volume as measured on whole body MRI. 38 In contrast to skeletal muscle mass mea- surement on MRI, in which skeletal muscle mass measurement is fully manually performed, measurement on CT imaging can be performed using semi-automatic software programs and with predefined Hounsfield unit range that are muscle specific. In most studies, a Hounsfield unit range between -29 and +150 is accepted as being skeletal muscle mass. 39 For abdominal imaging, the area of the psoas, erector spinae, quadratus lumborum, transversus abdominis, external and internal obliques and rectus abdominis muscles on level L3 are segmented on a single axial-slice (figure 2).