Abstract: PURPOSE: In postoperative radiotherapy planning for tongue cancer, with a focus on protecting the spinal cord and mandible, this study aims to explore the application effectiveness of non-coplanar radiation fields in the protection of the spinal cord and mandible in radiotherapy planning by comparing the differences in dose distribution between coplanar and non-coplanar plans. The goal is to select a reasonable radiotherapy plan to address the possibility of tongue cancer recurrence requiring re-irradiation. METHODS: Fifteen postoperative tongue cancer patients admitted to the Department of Oromaxillofacial Head and Neck Oncology, Shanghai Ninth People's Hospital, Shanghai Jiao Tong University School of Medicine from February 2018 to February 2023 were selected as the study subjects. The actual treatment target volume was used as the calculation target, and the dose limits for normal tissues were reference to national standards. With other parameters fixed, only the constraint functions for the spinal cord and mandible were varied. Serial functions were used, and the limits ranged from low to high: 20, 25, 30, 35, and 40. The limits were automatically calculated in the Monaco treatment planning system, and various parameters of the plans were evaluated, including target volume(CI, HI, D98, D95, D5, D2) and the maximum dose to the spinal cord, mean dose to the parotid glands, larynx, and mandible in normal tissues. SPSS 20.0 software package was used for data analysis. RESULTS: The evaluation indicators for the non-coplanar plan were superior to those for the coplanar plan. For the coplanar plan, when the serial function limits were set at 20 and 25, the dose distribution trends of the target volume indicators(D98, D95, D5, D2) showed divergence, and the target volume homogeneity was poor. As the limits increased, the differences in the dose distribution curves of the two plans decreased, and the homogeneity of the target volume became relatively uniform. The statistical results showed that compared to the non-coplanar plan, the coplanar plan had poorer results in terms of the spinal cord, bilateral parotid glands, larynx, mandible, and the CI and HI of the target volume when the serial function limits were set at 20 and 25. The differences between the two groups were statistically significant (P<0.05), with the greatest difference observed at a serial function limit of 20, where the difference in the HI index was 7.7% and the CI difference was 14%. As the limits increased, the differences between the two groups gradually decreased, and most of the differences in the evaluation indicators were statistically significant(P<0.05). CONCLUSIONS: In postoperative radiotherapy planning for tongue cancer, when the constraints on the spinal cord and mandible are strict, there is significant difference in dose distribution between the two plan groups. It is recommended to choose non-coplanar angles when planning the radiation fields. As the limits increase, the differences decrease gradually. Although most indicators still show statistical differences, considering the potential positional errors associated with non-coplanar fields, the selection of planning fields should be based on patient-specific factors and machine accuracy.

Key words: Non-coplanar irradiation, Monaco treatment planning system, Tongue cancer, Radiotherapy