## 1 Introduction  
Oral cancer is one of the sixteen most common cancers worldwide [1]. Most patients are treated with chemotherapy, surgery, and radiotherapy. Ionizing radiation, which is used for treating head and neck cancers, produces oral side effects such as saliva quality variations, mucositis, and destruction of dental structures. In these cases, infections of dental origin in compromised patients are potentially lethal [2]. Radiation therapy techniques have steadily improved over the last few decades. Treatments now target the cancers more precisely, and more is known about setting radiation doses. These advances are expected to decrease the number of secondary neoformation pathologies and other complications that result from radiation therapy [3]. Oral fluid that lubricates the oral cavity, in case of xerostomia, hyposialia, and sicca syndrome (decreased or absent salivary flow measured by tests), may induce burning sensations. Decreased salivary flow and dry mouth (xerostomia) can have consequences on both mastication and speech. In addition, less saliva means less protection of the teeth and oral cavity. Hyposialia promotes the appearance of infections, caries, and alteration of the oral tissues. Hyposialvation can have serious negative effects on the patient's quality of life by affecting eating habits, nutritional status, phonation, taste perception, and prosthesis tolerance, as well. Xerostomia, caused by dysfunction of the salivary gland, is a common complication in oral cancer patients after radiotherapy [4], and it increases the risk of infections at the level of the oral cavity, causing the appearance of candida albicans. Saliva is presented as an aqueous, hypotonic solution, which protects all the tissues of the oral cavity. It is secreted by numerous minor salivary glands and predominantly the major salivary glands- the parotid, submandibular or submaxillary, and sublingual. Saliva is critically important in oral homeostasis [5]. Physiologic salivary functions include maintenance of oral soft and hard tissues' homeostasis, bacterial modulation, or support for other oral functions. Saliva assists in digestion (both in the mouth and gastro-intestinal tract), taste, and swallowing, as well [6; 7]. It plays an essential role in oral tissue lubrication and the adjustment of the salivary potential of hydrogen (pH) [8]. The effects of a loss or reduction in salivary function can be severe, and can have a significant impact on common activities [9]. Salivary secretory disorders, xerostomia and sialorrhea, can be caused by a wide range of factors, including drugs and radiotherapy [10]. There are few studies in the literature related to the alterations of saliva, in the context of systemic diseases [11]. The usefulness of saliva in the diagnosis of diseases of the oral cavity, the risk of caries, and the monitoring of oral pathology is an ever-growing area of active investigation [12; 13]. So, salivary tests can be used for diagnosis as well as to monitor the status of disease. According to the literature, patients with oral saliva pH values \(\leq\) 5.3 nine months after radiotherapy presented significantly higher risks for radiotherapy-related caries (RRCs) [14]. Another study found that patients suffering from radiotherapy-related caries (RRCs) displayed low levels of oral salivary pH values (4.3-5.0) for a long period after radiotherapy and had a slow rate of recovery. Patients without dental caries showed a decrease in saliva pH to 5.0-5.5 but recovered quickly to 5.5-6.0. The persistance of a steady state, which no longer decreased, indicated that increasing the oral saliva pH value to >5.3 could be a new target to prevent or reduce the occurrence of RRCs [13]. Some changes related to oral-systemic conditions, such as the increased prevalence of dental caries [14] and, consequently, oral infections, patients with xerostomia [15] represented a challenge for dental practitioners.