151751-Najiba-Chargi

84 CHAPTER 5 DISCUSSION This is the first study describing cut-off values for SMMmeasured on head and neck CT imaging or MRI at the level of C3 in patients with head and neck cancer. This study provided gender and BMI-specific cut-off values of themean (SD) quantity of SMM (skeletal muscle area and skeletal muscle mass index). For male patients with a BMI < 25 kg/m 2 , a CSMI ≤ 6.8 cm 2 /m 2 was defined and with a BMI ≥ 25kg/m 2 a CSMI ≤8.5cm 2 /m 2 was defined for low SMM. For female patients with a BMI < 25 kg/m 2 , a CSMI ≤5.3 cm 2 /m 2 was defined and with a BMI ≥ 25kg/m 2 a CSMI ≤ 6.4 cm 2 /m 2 was defined for low SMM. Various techniques can be used to quantify muscle mass, however not all of these modalities are routinely used in the clinical setting of HNC patients. Variable costs and sometimes limited availability determine which technique is better suited to the specific setting. In the manage - ment of HNC patients, CT and MRI of the head and neck are the most widely used imaging mo - dalities for routine diagnostics and clinical decisionmaking. MRI is considered one of the most accurate methods for analyzing quantitative and qualitative changes in body composition and is associated with an error in quantifying muscle that ranges between 1.1% and 4.4%. 23 CT, like MRI, is also considered as a highly precise imaging modality in investigating human body composition and has a reported precision error of about 1.4% for tissue areas. 21 Both scanning methods are able to distinguish muscle mass from fat. CT imaging can reveal fat infiltration within muscle by identifying areas in the range of −190 to −30 Hounsfield units. 24 Currently, MRI and CT are considered to be accuratemethods for quantifyingmusclemass, due to their abilities to separate fat fromother soft tissues. Therefore, this study included routinely performed CT and MRI imaging, which makes the results applicable to clinical practice. CT and MRI have already shown to have a significant agreement in measuring SMM and therefore can be used interchangeably in assessing SMM at the level of C3. 22 Using other software pro - grams than the software program used in the current study (slice-O-matic) may give slightly different results, but these differences are not clinically relevant. A previous study showed that the measurement of skeletal muscle area has an excellent inter-software agreement and therefore results of studies using different software programs may reliably be compared. 25 Reference values for SMM at the level of C3 for a heathy (non-HNC) Caucasian population are lacking, but reference values for SMM analysis at the level of L3 have been reported. 19 Van der Werf et al. included 420 healthy Caucasian kidney donors. 19 They found that SMI was 1.31-fold higher in men than in women. Previous studies also show that men have a significantly higher amount of skeletal musclemass than women. 26 In our study, we found a significant correlation between SMI and gender and between SMI and BMI. Therefore, gender and BMI-specific cut-off values were provided in this study. Van der Werf et al. determined cut-off values based on the 5 th percentile of SMM in healthy in- dividuals. These 5 th percentile cut-off values for low SMM (at L3) corresponded with the cut-off