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348 CHAPTER 17 study this concerned 23% at six-month follow-up and 20% at twelve-month follow-up. Inci - dence rates of trismus in other studies including all head and neck cancer localizations treated with surgery and/or radiation vary, but oropharyngeal localization of the tumor consistently seems a risk factor. 11–16,39 This is probably because treatment of the oropharynx causes fibrosis in the mastication musculature. 15 This hypothesis is also supported by our results showing that patients with tumor localizations within the oropharynx other than base of tongue have trismus more often. Apparently, despite trismus preventing measures in our preventive rehabilitation program, trismus is still a prevalent problem in this cohort. Therefore, extra measures could be taken to prevent and treat trismus, for example, by selecting high risk patients for more intensive guidance, and emphasizing the need for trismus prevention stronger, prior to treatment. The consistent use of mouth opening exercises (e.g., with tongueblades or TheraBite®) in this pa - tient group might have been advantageous. 40 The lack of reimbursement for TheraBite® in the Netherlands, preventing regular use of this medical device in our patient population, is noteworthy in this respect. With respect to speech and voice outcomes, according to our results, observer-rated intel - ligibility was deteriorated at six-month follow-up and stayed stable up until twelve-month follow-up. Subjective speech outcomes, however, deteriorated up until six months and re - turned to baseline levels at twelve-month follow-up. This is most likely because patients get used to the altered speech. Vainshtein et al. found the same trend in patient-reported voice quality, which decreased maximally at one month after treatment and recovered to baseline after twelve to eighteen months. 41 In an earlier study from our institute, Jacobi et al. found comparable results. They reported that computer analyzed articulation and sound quality was impaired in head and neck cancer patients after RT+, especially with oral and oropharyngeal cancer sites. 42 Our results suggest that patients treated with concomitant systemic therapy have more func - tional limitations than patients treated with RT alone. This might be due to the toxicity of sys - temic therapy, but might also be because of the higher tumor stages, and therefore also larger radiotherapy fields. Only 17 (16%) of the 108 included patients were treated with cetuximab based RT+ and therefore there is a high risk of atypical sampling and conclusions on functional outcomes relative to RT only or cisplatin-based RT+ based on these analyses should be made with caution. A recently published randomized study concluded that the degree of toxicities, including dysphagia, between cisplatin and cetuximab in HPV positive OPC was comparable. 5 In our cohort, although HPV status was not associated with trismus and speech outcomes, patients with HPV positive tumors had less objective and subjective functional impairment. However, patients with HPV positive tumors also hadmore favorable baseline characteristics, including higher pretreatment SMI (as also reported by Chargi et al. 43 ), lower T classification, were more often treated with RT only and less often had a modified diet before treatment.