The effect of parotid gland‐sparing intensity‐modulated radiotherapy on salivary composition, flow rate and xerostomia measures

Objectives To describe parotid gland (PG) saliva organic and inorganic composition and flow rate changes, after curative intensity‐modulated radiotherapy (IMRT) for head and neck cancer (HNC), and analyse the relationship between PG saliva analytes and xerostomia measures. Methods and Materials Twenty‐six patients recruited to five prospective phase 2 or 3 trials which assessed toxicity and efficacy of IMRT by HNC subsite, provided longitudinal PG saliva. Salivary flow rate, and subjective and objective xerostomia measures were prospectively collected and saliva tested for inorganic and organic analytes. Statistical comparisons of longitudinal analyte changes and analysis for a relationship between dichotomized xerostomia score and saliva analytes were performed. Results One hundred and forty‐two PG saliva samples from 26 patients were analysed. At 3–6 months after IMRT, stimulated and unstimulated saliva showed significantly decreased flow rate, total protein (TP) secretion rate, phosphate concentration and increased lactoferrin (LF) concentration. Stimulated saliva alone had elevated LF secretion rate and beta‐2‐microglobulin (B2M) concentration with decreased calcium (Ca2+) and magnesium (Mg2+) concentrations and Ca2+ secretion rate. At >12 months, under stimulated and unstimulated conditions, increased LF concentration and decreased Mg2+ and phosphate concentration persisted and, in stimulated saliva, there was decreased potassium (K+) and Mg2+ concentration. Unstimulated TP secretion rate was lower in the presence of high‐grade xerostomia. Otherwise, no relationship between xerostomia grade and PG salivary flow rate, TP and Ca2+ secretion rate was found. Conclusion Fewer significant differences in PG saliva analytes >12 months after IMRT indicate good functional recovery. Residual xerostomia after IMRT will only be further reduced by addressing the sparing of subsites of the PG or other salivary gland tissues, in addition to the PG.