A split-parotid delineation approach for dose optimization in volumetric modulated arc therapy for nasopharyngeal carcinoma patients with parapharyngeal space invasion and level IIa cervical lymph node involvements

Objective: This study evaluated the potential benefit of a split-parotid delineation approach on the parotid gland in the treatment planning of patients with nasopharyngeal carcinoma (NPC). Methods: 50 patients with NPC with parapharyngeal space (PPS) and/or level IIa cervical node involvements were divided into three groups: PPS only, level IIa cervical node only and both. Two volumetric-modulated arc therapy plans were computed. The first plan (control) was generated based on the routine treatment-planning protocol, while the second plan (test) was computed with the split-parotid delineation approach, in which a line through the anterolateral margin of the retromandibular vein was created that divided the parotid gland into anterolateral and posteromedial subsegments. For the test plan, the anterolateral subsegment was prescribed, with a dose constraint of 25Gy in the plan optimization. Dosimetric data of the parotid gland, target volumes and selected organs at risk (OARs) were compared between the control and test plans. Results: The mean dose to the anterolateral subsegment of the parotid gland in all three groups was kept below 25Gy. The test plan demonstrated significantly lower mean parotid dose than the control plan in the entire gland and the anterolateral subsegment in all three groups. The difference was the greatest in Group 3. Conclusion: The split-parotid delineation approach significantly lowered the mean dose to the anterolateral subsegment and overall gland without greatly compromising the doses to target volumes and other OARs. The effect was more obvious for both PPS and level IIa cervical node involvements than for either of them alone.