## 1 Introduction  
In 2020, around 930,000 cancers of the head and neck region (ICD10: C00-13 and C32) were diagnosed worldwide [1], and radiotherapy plays a key role in their treatment. Despite the improved techniques, almost all of these patients suffer from a variety of side effects, such as mucositis, dysgeusia, and--most prominently, especially in the long term--hyposalivation [1]. These side effects lead to changes in behavior and consequently to changes in oral health. A change in diet, direct radiation damage to the teeth, and, above all, a lack of saliva are considered causal factors and co-factors for rampant dental caries [2]. For large portions of the population, there is international consensus on how to prevent dental caries [3; 4; 5]. However, it is not clear whether these strategies are also effective after radiotherapy in the head and neck area. There is a lack of information on this topic in the guidelines on supportive treatment for cancer [6; 7]. Numerous reviews provide advice on dental care after radiotherapy, usually summarizing a few primary studies, expert opinions, and personal experiences [8; 9]. Among these is the iatrogenic radiation exposure of restorative materials during head and neck radiation therapy. High-energy radiation can be hazardous to teeth and dental restorations because of the high incidence of head and neck cancer and the efficacy of radiotherapy as a main or adjuvant treatment [10]. The microstructure of hard dental tissue has been demonstrated to be negatively impacted by therapeutic radiation dosages [11]; however, it is unknown what clinical consequences might arise from this microstructural damage. It is undeniable that patients receiving head and neck radiation have much shorter dental restoration lifespans [12], but it is unclear how many of these restoration failures are attributable to the radiation exposure of hard dental tissues and restorative materials [13] or radiation-related changes in the oral flora and decreased salivary flow [14]. Therapeutic radiation has been investigated for a variety of outcomes; however, the findings in the literature are contradictory. Almost every potential effect on different material properties has been reported, including no influence on mechanical and chemical properties [15; 16], positive effects such as increased diametral tensile strength and micro-hardness values of resin-modified glass ionomer cement, and negative effects like the decreased flexural strength of glass ionomer cements [17; 18; 19] or the decreased micro-hardness of micro-hybrid and nanohybrid resin-based composites [20; 21]. The varied responses of the studied materials to radiation are reflected in these inconsistencies, as dental restorative materials differ in their composition, structure, and characteristics. Variations in radiation techniques, such as the use of single-dose or fractionated irradiation, exacerbate this heterogeneity. Even though there is no solid proof yet that radiation therapy harms commonly used restorative materials, research into how radiation affects restorative materials with different compositions and characteristics is necessary. Cention Forte, an \"alkasite\" material, and Equia Forte HT, a glass hybrid, are two newly developed restorative materials that are particularly suitable for patients undergoing head and neck radiation therapy. Cention Forte incorporates three distinct types of glass fillers--standard inert barium aluminosilicate glass, calcium barium aluminofluoxysilicate ionomer glass, and basic calcium fluorosilicate glass--into a methacrylate matrix [22]. This material has the ability to raise the pH, precipitate hydroxyapatite on its surface, and release calcium and fluoride ions. Equia Forte HT, a glass hybrid, consists of highly reactive nano-sized fluoroaduminosilicate glass and high-molecular-weight polyacrylic acid, resulting from the development of high-strength restorative glass ionomer cement [23].