131 7 Discussion Overall, swallowing function as measured by number of swallows needed to drink 100 mL of water, worsened from diagnosis to 3 months after treatment, after which it returned to or below baseline level in patients with head and neck cancer (Table 2). Swallowing dysfunction increased from diagnosis (19%) to 3 months after treatment (22%), after which it returned to or below baseline level (14%). Age and treatment modality were significantly associated with the course of swallowing function. Swallowing function was worse in older patients. Swallowing function of patients receiving surgery as primary treatment in particular was worse 3 months after treatment compared to baseline and remained worse up to 24 months. Patients treated with (C)RT did not show this worsening after treatment. Instead, their swallowing function improved after treatment. The clinical relevance of the LMM results can be clarified by taking into account the smallest detectable change (SDC) found in previous research. The SDC for the number of swallows was 0.79 points, indicating that the difference between two measurements has to be at least 0.79 points to be a real difference and not a measurement error.14 When looking at the estimates in Table 3, all results meet this condition except age; one year older does not contribute to a worse swallowing function, however, a difference of more than 11 years will. Comparison with literature In previous research, other factors associated with worse swallowing function were sex (female), tumor stage (T3 and T4), the addition of chemotherapy as treatment modality, and oropharynx tumors.2,18,19 These factors did not contribute to worse swallowing function in our study. In addition, the size of the radiation field, accelerated fractionation, neck irradiation, type of surgery, and normal tissue changes such as edema, neuropathy, fibrosis, and mucositis might influence the WST outcome as well.2,5,19 The sample size of the current study was too small to also include these factors. For example, only 16 patients received surgery followed by (C)RT. It is therefore recommended to repeat this study with a larger sample size, and include more factors in the LMM analysis. This study did not find an effect of RT treatment on swallowing function. This might be explained by the fact that patients treated with RT nowadays often receive IMRT, in order to spare the swallowing muscles.20 The next step should therefore be to investigate the effect of dose to the swallowing organs at risk (OAR) on swallowing function in order to see the effect of OAR sparing.
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