The healthy patient produces up to 1.5L of saliva per day.13 The major salivary glands produce 80-90% of the total volume.13 Saliva varies from purely serous (parotid glands) to mixed seromucous (submandibular glands) to predominately mucous (sublingual and minor salivary glands). Minor salivary glands produce 70% of the mucins.13 Most stimulated saliva is produced by the parotids and most unstimulated saliva is produced by the submandibular glands.13
For reasons not clearly understood, the well-differentiated and slow replicating salivary gland cells are highly radiosensitive.13 Radiotherapy causes acute degeneration and necrosis of acinar cells. Salivary output may be reduced by as much as 80% within the first 2 weeks of conventional radiotherapy.13 The saliva will be scant, thick, viscous and ropy or frothy in character. Acinar recovery, if any, may occur about 12 to 18 months following completion of therapy.
Qualitative and quantitative salivary changes lead to reduced lubrication; reduced lavage and cleansing of oral tissues; decrease in immunoglobulin levels, i.e., reduced antibacterial, antiviral, and antifungal activity; and further loss of mucosal integrity. Patients may experience difficulty swallowing, chewing, speaking, and wearing prostheses, which along with hypogeusia or ageusia may lead to impaired nutrition and weight loss.13
If the radiation dose is sufficient, regeneration of acinar cells fails leading to fibrosis and atrophy of glandular tissue.13 While a mean dose of 60 Gy is the accepted threshold for producing irreversible damage, in some cases mean doses of as little as 26 Gy have been implicated.13 As a general rule, following head and neck radiotherapy, patients will experience significant reduction in salivary flow, i.e., xerostomia, for the remainder of their life (Figure 9).13
Xerostomia and chronic candidal infection are the most frequently reported long-term side effect of head and neck radiotherapy.
Loss of saliva’s buffering capacity and decreased salivary immunoglobulin levels increase the cariogenic oral microflora, i.e., Streptococcus mutans, lactobacillus and Actinomyces, and lead to “radiation caries”.13 Radiation caries begin on plaque forming surfaces and areas of exposed dentin resulting in circumferential lesions at the cementoenamel junction and smooth surface caries on cusp tips and incisal edges. (Figure 10).13
A distinctive form of rampant caries, termed “radiation caries”, is frequently observed following head and neck radiotherapy.