153 8 Discussion This large 2-year prospective cohort study (n=603) identified factors associated with worse swallowing as measured with the SWAL-QOL in patients with HNC and all treatment modalities. In this study, it was shown that patient-reported problems with swallowing increased from baseline to 3 months after treatment, and slowly decreased from 6 months onwards with return to baseline levels at 2 years after treatment in patients with HNC. Based on the SWAL-QOL cut-off score of ≥14, which indicates swallowing problems in daily life, 53% of patients had problems before treatment, which increased to 70% at 3 months after treatment, and decreased to 59% at 6 months after treatment, 50% at 12 months after treatment, and 48% at 24 months after treatment. After treatment, the subscale eating duration showed the most problems and did not return to baseline. Therefore, this subscale was used in a LMM to identify factors associated with a worse eating duration, to indicate which patients could benefit from preventive strategies and rehabilitation during and after treatment. Eating duration was associated with sex, smoking, weight loss, tumor site, treatment, tumor stage, and timing of assessment. In addition, the interactions of timing of assessment with tumor site and treatment modality were significant, indicating that the course over time differed for different tumor sites and different treatment modalities. Comparison with literature Based on a cut-off score on the total SWAL-QOL score of ≥14 points,26 a previous crosssectional study of patients with HNC (n=52) found a deviant score in 79% of patients, which is higher than the 70% found in this research at 3 months after treatment. One explanation could be that almost 60% of the patients in that study were treated with 3D conformal RT, in which salivary glands were not spared. After 2005, IMRT was introduced, enabling a significant reduction of dose to the salivary glands.26 Since then, IMRT has been further optimized, sparing, e.g., parotid glands, pharyngeal constrictor muscles, and the supraglottic larynx.37 Another cross-sectional study with healthy controls (n=111, mean age=56 years, 44% male) showed that mean scores of all subscales were between 3.7 (social functioning) and 10.4 (fear of eating).38 Although these healthy controls were slightly younger and a higher percentage of females responded to the questions, it strongly indicates that most of the patients with HNC already experience swallowing problems before treatment (Figure 2), and that these problems remain, even 2 years after treatment. A prospective cohort study from 2021 investigated factors associated with swallowing and social eating (n=2458) as measured with the EORTC QLQ-H&N35, and found that multimodality treatment, oropharynx tumors, age, sex, living alone, low socio-economic status