Purpose: Radiation-induced xerostomia is a frequent complication and major cause of morbidity in head and neck cancer patients. The severity of xerostomia is related to radiation dose and the amount of parotid tissue included in the irradiated volume. To reduce this side-effect and preserve salivary function, we have evaluated the use of 3-dimensional (3-D) treatment planning to spare the contralateral parotid gland in twelve patients undergoing radiation therapy for head and neck cancers.
Methods and materials: In each case, beam's eye view displays were used to design beam and blocking arrangements that excluded the contralateral parotid. Ten patients were treated with 2 nonopposing oblique fields in the axial and non-axial plane while two patients required a non-axial, non-coplanar 3-field arrangement. These 3-D treatment plans were also compared with conventional 2-dimensional (2-D) plans. The 2-dimensional plans were designed independently of the 3-D treatment planning information using the orthogonal radiographs and hard copies of the computed tomography scans.
Results: An average of 1.8% (range, 0-7%) of the target volume was underdosed with the 95% isodose level for the 3-D plans compared with 18.8% (range, 2.0-36.6%) for the 2-D plans. This was due to improved identification of the target volumes and better design of blocked fields with beam's eye view treatment planning. Furthermore, the mean dose to the opposite parotid was 3.9 Gy for the 3-D plans vs 28.9 Gy for the conventional plans. With a minimum follow-up of 4 months, only 2 of 12 patients have complained of a dry mouth.
Conclusion: These encouraging results suggest that this approach is feasible in many cases. 3-D treatment planning may allow the use of parotid sparing techniques in patients who otherwise would not have been considered candidates using conventional radiotherapy techniques.