The Characterisation of Therapeutic Radiation-induced Caries; the Histopathological, Ultrastructural and Microbial Changes Affecting Its Prevention and Treatment.

1. Introduction

Head and neck cancer is the sixth most common cancer in the world representing 4-5% of all malignancies . Head and neck cancer (includes oral, nasal cavity, salivary glands, paranasal sinus, pharynx, and larynx) is one of the top 10 most diagnosed cancers in the UK . According to Cancer Research UK, there were 7591 newly diagnosed with oral cancer in 2013 in the country, representing the 14th most common cancer in the UK.

Radiotherapy (RT) is the first choice treatment for the majority head and neck cancer patients, which is a localized treatment to control the tumour . More than 50% of head and neck cancer patients receive RT delivered in fractions of 2 Grays (Gy) daily during 5 days per week until the end of their therapy dosage (mean cumulative dose 55-77 Gy). RT stimulates cellular changes that destroy the tumor cells, but may affect dramatically the healthy oral tissues altering the homeostasis of this complex environment. RT is the cause of a wide range of adverse oral tissue reactions such as permanent damage of the major and minor salivary glands (hyposalivation, xerostomia), modification of connective and vascular tissues and a reduction the healing capacity especially in mucosal and mineralised tissues (mucositis, caries lesion, osteoradionecrosis). In order to try to diminish the adverse side effects of the RT three-dimensional (3D) computed tomography (CT) planned radiotherapy known as intensity-modulated radiotherapy (IMRT) has been developed .

There are acute (mucositis) and late-onset adverse effects/sequelae that RT induces in healthy oral tissues . Radiation-induced dental caries is the most common late effect, but its origin and nature is not clear. RT related caries have a rapid onset as a result of a combination of factors such as shift in oral microbial community composition favouring acidophilic bacteria, hypo-salivation, poor oral hygiene and dietary alterations . The patients often have little/no pain, even in the most severe cases. Histological features show that the onset and development of caries is similar to non RT-related ones but the sites affected are different. Indeed RT-related caries is more frequent on the cervico-labial surface/ incisal edge of canines and incisors while non RT-related ones are more frequent in the interproximal/ occlusal areas of the posterior teeth (molar and premolars).

Also, caries associated with RT has a rapid circumferential progression which may lead to tooth fracture. It is characterized by a localized lesion to the incisal or occlusal surfaces which diffuse and progress in relation to irregular and generalized erosion of the incisal edge of the tooth. In addition, mechanical changes (increased elastic modulus and stiffness) within the tooth structure increase the wear of the incisal and occlusal areas and could lead to complete fracture of the tooth.