Salivary Glands

A healthy individual produces up to 1.5L of saliva per day, with the major salivary glands contributing 80–90% of the total volume. The parotid glands secrete serous saliva, the submandibular glands produce mixed seromucous saliva, and the sublingual and minor salivary glands secrete mucous saliva. Minor salivary glands account for 70% of mucin production. The parotid glands generate the highest volume of stimulated saliva, whereas the submandibular glands produce the most unstimulated saliva.¹³

Salivary gland cells, despite being well-differentiated and slow-replicating, exhibit high radiosensitivity for reasons that remain unclear.^1, Radiotherapy induces acute degeneration and necrosis of acinar cells, leading to an 80% reduction in salivary output within the first two weeks of conventional treatment. Consequently, saliva becomes scant, thick, viscous, and either ropy or frothy. If any acinar recovery occurs, it may take 12 to 18 months following therapy completion.¹³

Both qualitative and quantitative alterations in saliva contribute to reduced lubrication, impaired lavage and cleansing of oral tissues, and decreased immunoglobulin levels, leading to diminished antibacterial, antiviral, and antifungal activity. These changes further compromise mucosal integrity, causing patients to experience difficulty with swallowing, chewing, speaking, and wearing prostheses. Additionally, hypogeusia or ageusia may develop, potentially resulting in poor nutrition and weight loss.¹³

If radiation exposure is sufficiently high, acinar cell regeneration fails, leading to fibrosis and atrophy of glandular tissue. While the generally accepted threshold for irreversible damage is a mean dose of 60 Gy, some cases have implicated mean doses as low as 26 Gy. In most instances, patients undergoing head and neck radiotherapy will experience a lifelong significant reduction in salivary flow, resulting in xerostomia (Figure 9).¹³

Figure 9.

Figure 9.

Xerostomia and chronic candidal infection are the most frequently reported long-term side effect of head and neck radiotherapy.

The loss of saliva’s buffering capacity and the decrease in the salivary immunoglobulin levels will increase cariogenic oral microflora, leading to “radiation caries”. 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

Figure 10.

Figure 10.

A distinctive form of rampant caries, termed “radiation caries”, is frequently observed following head and neck radiotherapy.