The treatment of malignant neoplasms in the head and neck region usually uses radiotherapy, due to the tissue growth characteristics of the disease and the difficulty of surgical removal close to anatomical sites with noble and difficult to access structures (Moore et al., 2020). Free radicals from the ionization of water molecules, such as oxygen and hydrogen peroxide, would lead to tissue damage (Fang et al., 2002). Furthermore, radiation therapy also alters healthy tissues that are in the irradiation field. One of the widely studied side effects refers to changes in irradiated dental structures, mainly enamel and dentin in the coronal region (Gonçalves et al., 2014, Qing et al., 2015, Reed et al., 2015, Qing et al., 2016, Rodrigues et al., 2018, Lu et al., 2019, Miranda et al., 2021, Douchy et al., 2022). Few studies have analyzed the effect of radiotherapy on root dentin. These studies showed a decrease in the hardness of root dentin after irradiation (Velo et al., 2018, Campi et al., 2019), as well as changes in chemical composition (Miranda et al., 2019, Pelloso et al., 2022).

The root canal is an anatomical area filled by pulp tissues, which maintains the vascularization and innervation of the tooth. When there is a need for endodontic treatment, the canal is filled with endodontic cement and gutta-percha and, later, with cement to fix some type of intraradicular post (Guldener et al., 2017). Understanding changes in root structures, as well as in root canal walls and/or intraradicular dentin properties, is of the utmost importance to determine the influence of radiotherapy on the risk of future disease and the success of endodontic and restorative treatments that may subsequently be required (Velo et al., 2018). Analysis of dentin within the root canal can elucidate chemical and structural changes that justify compromised adhesion in irradiated teeth, since resin materials and endodontic cements are commonly used in teeth with extensive coronal destruction (Pelloso et al., 2022). To the best of our knowledge, only one previous study has evaluated the properties of irradiated intraradicular dentin (Campi et al., 2019).

In addition, most studies in this area employ in vitro sample irradiation, which does not always accurately replicate the oral environment of cancer patients, including factors such as pH, salivary flow, and diet (Miranda et al., 2019). In clinical reality, head and neck radiotherapy also needs to cross a series of anatomical structures such as muscles, mucosa, and alveolar bone until it reaches the dental tissues (Seyedmahmoud et al., 2018). The use of tooth samples from irradiated patients seems to be the best option to find more reliable results regarding what happens clinically in the patient (de Siqueira Mellara et al., 2014, Gonçalves et al., 2014, Qing et al., 2015, Reed et al., 2015, Liang et al., 2016, Novais et al., 2016, Qing et al., 2016, Rodrigues et al., 2018).

Thus, the aim of this study was to analyze the chemical composition and microhardness of intraradicular dentin of teeth extracted from head and neck irradiated patients, divided into three thirds of the root.