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.