Identifying dose constraints for the parotid ducts to minimize patient-reported xerostomia: Is conventional mean dose sparing of the parotid glands sufficient?

Background And Purpose: The aim of this study was to identify dose constraints for the parotid ducts that limit patient-reported xerostomia and estimate whether these constraints are achieved during conventional parotid gland sparing radiation therapy (PGS-RT).

Methods And Materials: Thirty-eight oropharyngeal squamous cell carcinoma patients were treated prospectively on trial with MRI sialography-guided parotid duct sparing radiation therapy (PDS-RT). PDS-RT explicitly minimizes dose to the parotid ducts in addition to PGS-RT. Parotid duct dose constraints were identified that distinguished patients reporting high and low rates of xerostomia. Atlas-based parotid duct contours were generated on a retrospective cohort of similar patients where the parotid ducts were not contoured nor explicitly spared to estimate the dose received by the parotid ducts during PGS-RT.

Results: Patients whose intraglandular parotid ducts or total parotid ducts were planned for a mean dose < 14 Gy and < 12 Gy, respectively, reported significantly (p < 0.01) lower rates of xerostomia at 6 and 12 months post-RT. Patients receiving PDS-RT had average total and intraglandular duct doses of 11.6  and 13.6 Gy, respectively, compared to an estimated 23.8  and 22.1 Gy, for those receiving PGS-RT (p < 0.01). Only 6% (6/108) and 20% (22/108) of patients receiving PGS-RT were estimated to meet the dose constraints for the total ducts and intraglandular ducts, respectively.

Conclusion: Parotid duct dose thresholds exist that appear to distinguish patients with and without xerostomia. The identified dose thresholds are frequently not met in PGS-RT plans. In addition to reducing the dose to the parotid gland(s), parotid duct sparing may also further reduce xerostomia.