Radiation therapy physics
Regional radiation dose susceptibility within the parotid gland: Effects on salivary loss and recovery
Xerostomia is one of the most likely late toxic effects of radiotherapy treatment in patients with head-and-neck cancers. Modern treatment techniques can incorporate knowledge of complication risk into treatment plans. To this end, the authors attempt to quantify the regional radiotherapy dose-dependence of salivary output loss and recovery in a prospective study.
Salivary output was collected from patients undergoing radiotherapy treatment for head-and-neck cancers at the BC Cancer Agency between February 2008 and May 2013. Regional dose-dependence (i.e., dose susceptibility) of loss and recovery is quantified using nonparametric (Spearman's rank correlation coefficients, local linear regression) and parametric (least-sum of squares, least-median of squares) techniques.
Salivary flow recovery was seen in 79 of 102 patients considered (p < 0.0001, Wilcoxon sign rank test). Output loss was strongly correlated with left- and right parotid combined dose φ = min (DL, 45 Gy) + min (DR, 45 Gy), and can be accurately predicted. Median early loss (three months) was 72% of baseline, while median overall loss (1 yr) was 56% of baseline. Fitting an exponential model to whole parotid yields dose sensitivities A3m = 0.0604 Gy−1 and A1y = 0.0379 Gy−1. Recovery was not significantly associated with dose. Hints of lateral organ sub-segment dose–response dimorphism were observed.
Sub-segmentation appears to predict neither loss nor recovery with any greater precision than whole parotid mean dose, though it is not any worse. Sparing the parotid to a combined dose φ of <50 Gy is recommended for a patient to keep ≈40% of baseline function and thus avoid severe xerostomia at 12 months post-treatment. It seems unlikely that a population's mean recovery will exceed 20%–30% of baseline output at 1 yr after radiotherapy treatment using current (whole-organ based) clinical guidelines.