Non-Coplanar vs. Coplanar Intensity-Modulated Radiation Therapy (IMRT) for Protection of Lip and Buccal Mucosa

Abstract

In this study, by comparing the coplanar and non-coplanar intensity-modulated radiation therapy (IMRT) treatment planning in treating tongue cancer, the significance of non-coplanar fields in the protection of lip and buccal mucosa was determined, and a reasonable solution was selected.48 tongue cancer patients treated from June 2019 to Feb 2021 were selected and divided into coplanar field group and non-coplanar field group randomly: the former one used divided nine fields on average while the latter one adopted seven fields meanly with two other fields which needed to turn the bed 270 degrees for making the angle of radiation ray tangent to the region of mucous membrane. The mucosal dose limit changed from 15 Gy to 45 Gy for comparison of the two treatment plans. The evaluating indicators including conformal index (CI), homogeneity index (HI), D5, D50, D98 of the target volume. The dose of normal tissues and organs including spine, throat, bilateral parotid gland, pharyngeal constrictor and oral mucosa (lip, upper and lower buccal mucosa) were calculated under different mucosal dose limit. The clinical manifestations of lip and buccal mucositis of 48 cases were recorded and graded by NCI-CTCAE 4.0. Corresponding statistical analysis were performed.The difference of CI, HI, D98, D50 and D5 between two groups in the target volume has the tendency to become smaller when the mucosal dose limit is less than 30 Gy, with significant statistical difference (P < 0.05). When the limit exceeds 30 Gy, there are still statistical differences in other indicators except CI (P < 0.05). In terms of normal tissue, a certain gap of doses on normal tissues between two groups exists when the mucosal limit is lower than 20 Gy, with significant statistical difference (P < 0.05). When the limit exceeds 20 Gy, there was no statistical difference. There are statistical differences (P < 0.05) in D98, D50, D5, CI and HI of the target area between the two groups when the mucosal function setting is less than 30 Gy. When the mucosal function setting exceeds 30 Gy, there is no statistical difference in CI, but there were still statistical differences in others (P < 0.05). In normal tissue, there is statistical difference (P < 0.05) between two treatment plans when mucosal function setting is smaller than 20 Gy; if it exceeds, there is no statistical difference. The patients of non-coplanar group showed better results in terms of the radiation-related toxicity of lip and cheek membrane: the incidence rate and classification of severity were much lower. Patients usually developed lip and buccal mucositis after the fourth week during the radiotherapy, and symptoms of most cases were classified mild. 21 cases ranked grade I while the remaining 3 ranked grade II.Compared with coplanar field, non-coplanar field radiotherapy can effectively reduce the exposure dose of lip and buccal mucosa. The application of non-coplanar treatment plan has good clinical significance and deserves to be promoted.