115 6 caused more symptoms.3 Other research mainly focused on limited mouth opening (trismus) as outcome measure, which is also correlated to mastication.22 It was found that trismus is significantly related to tumor stage, the use of RT and the use of free tissue reconstruction. Patients with stage 3 and 4 tumors, and patients receiving RT or a reconstruction had a smaller mouth opening.22 The relation between chewing function and stage 4 tumors was described previously as well.23 These risk factors are in line with the results found in this research, except for choice of treatment, which was not found in this study. Strengths and limitations Strengths of our study were the prospective study design, the use of the LMM checklist with recommendations for reporting multilevel data and analyses,24 and the high test-retest reliability of the MAT as found in previous research.15 Limitations were the low number of patients at follow-up, which limited the number of factors that could be explored with the LMM, and the relatively large drop-out and missing values. These missing data might have affected the analyses, because it is unknown how these patients would have performed on the MAT. Although the LMM is better at handling missing values in comparison to other regression analyses, these regression models do not take into account the number of deaths as competing risk.25 Although no significant correlations were found between the factors used in the LMM, treatment and tumor stage did differ between different tumor sites, as seen in Table 1: patients with an oropharynx tumor most often received RT or CRT, while patients with an oral cavity tumor most often received surgery or surgery followed by RT or CRT. In addition, oropharynx tumors were most often stage 4 tumors, while hypopharynx and larynx tumors were most often stage 1 tumors. Therefore, the association found between MAT outcome and tumor site is, to a lesser extent, also caused by treatment modality and tumor stage. Because of the low number of patients in this study, no interactions between treatment, tumor stage and tumor site could be explored in the LMM. The mean values indicate a decrease in masticatory function especially 3 and 6 months after treatment, and a return to baseline at 12 and 24 months after treatment. However, the cut-off values indicate masticatory dysfunction especially 3 and 24 months after treatment. Impairment after treatment varies greatly between patients; it is affected by site and extent of the tumor, age, irradiation site and dose, extent of tumor resection, and reconstruction procedures.26 Acute toxicity after treatment (e.g., mucositis, xerostomia, tooth loss) causes a decrease in masticatory function, which slowly recovers over time. However, long term treatment effects may persist even beyond 5 years after treatment,27 which may explain the masticatory dysfunction of 36% at 2 years after treatment. Although
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