108 Chapter 5 Materials and Methods Study design and population This cross-sectional study included all patients who were treated by primary or adjuvant RT, CRT of BRT with curative intent for HNC at the Comprehensive Cancer Center of Maastricht University Medical Center (MUMC+) and Maastro Clinic between 2015 and 2019. Patients were excluded who were edentulous, did not need tooth extractions pre-RT, had a tooth extraction after RT instead of before, or neglected their teeth to such an extent that extraction of all remaining teeth was required. In addition, patients were excluded if they had previous head and neck RT, proton- or brachytherapy, or RT with palliative intent. Finally, patients with an unknown primary, centrally located, or bilateral proven tumor spread were excluded to allow for reporting of dose distributions in the jaws according to laterality: ipsi- versus contralateral. Data on age, sex, tumor location, tumor size (T), and lymph node status (N), as well as information on tooth extractions were extracted from the electronic health records by an experienced maxillofacial prosthodontist (DB). If a TNM classification was stated according to the 7th edition, it was converted to the TNM classification according to the 8th edition [17, 18]. This study was approved by the medical ethics committee of the MUMC+ (METC 2019-1241). The institutional review board of MUMC+ allowed us to invoke the institutional “no objection regulation”, so no patient informed consent was needed. Tumor location The patient cohort was divided into eight groups according to the anatomical region of the expected radiation fields: 1) larynx, 2) hypopharynx, 3) parotid region, 4) oropharynx, 5) oral cavity, 6) maxillary complex, 7) nasopharynx, and 8) other (Table 1). The group of patients with parotid gland tumors was supplemented with patients presenting with pre-auricular skin cancers with a radiation field including the parotid gland (due to metastasis or elective coverage) and formed the ‘parotid region’ group. Patients with a salivary gland tumor of the submandibular or sublingual gland were included in the ‘oral cavity’ group, because of the close anatomical relation with the mandible. Patients with a tumor in the maxillary region/hard palate were combined with patients with a nasal, paranasal sinus or nasal cavity tumor and formed: the ‘maxillary complex’ group.
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