PRESERVATION OF PAROTID FUNCTION AFTER EXTERNAL-BEAM IRRADIATION IN HEAD AND NECK-CANCER PATIENTS - A FEASIBILITY STUDY USING 3-DIMENSIONALTREATMENT PLANNING

Purpose: Radiation-induced xerostomia is a frequent complication and m ajor 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 a nd preserve salivary function, we have evaluated the use of 3-dimensio nal (3-D) treatment planning to spare the contralateral parotid gland in twelve patients undergoing radiation therapy for head and neck canc ers. Methods and Materials: In each case, beam's eye view displays wer e used to design beam and blocking arrangements that excluded the cont ralateral parotid. Ten patients were treated with 2 nonopposing obliqu e fields in the axial and non-axial plane while two patients required a non-axial, non-coplanar 3-field arrangement. These 3-D treatment pla ns 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 f or the 3-D plans compared with 18.8% (range, 2.0-36.6%) for the 2-D pl ans. This was due to improved identification of the target volumes and better design of blocked fields with beam's eye view treatment planni ng. 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 f ollow-up of 4 months, only 2 of 12 patients have complained of a dry m outh. Conclusion: These encouraging results suggest that this approach is feasible in many cases. 3-D treatment planning may allow the use o f parotid sparing techniques in patients who otherwise would not have been considered candidates using conventional radiotherapy techniques.