基于iCBCT分析鼻咽癌自适应放疗过程中腮腺实际体积、位置和剂量的变化OA

Background and purpose:Radiation therapy,particularly intensity-modulated radiation therapy(IMRT),has become the standard radical treatment for nasopharyngeal carcinoma(NPC).It enhances local tumor control and reduces the radiation dose to organ at risk(OAR)through precise dose distribution,thereby improving patients'quality of life.However,during the treatment process,the volume and position of OAR such as the parotid gland may undergo significant changes,leading to deviations between the actual delivered dose and the planned dose,which increases the risk of adverse reactions.Adaptive radiotherapy(ART)effectively optimizes dose distribution by assessing and adjusting the treatment plan in real-time based on imaging.This study systematically analyzes the changes in the volume,position and radiation dose of the parotid gland with the accumulation of radiotherapy fractions,using iterative cone beam computed tomography(iCBCT)imaging data obtained from a Halcyon linear accelerator.The findings provide data support and a theoretical basis for optimizing radiotherapy practices for NPC and reducing parotid gland damage.Methods:NPC patients who were treated at Jiangsu Cancer Hospital from January 2018 to December 2022 were retrospectively enrolled.The body weight before and after treatment was recorded and compared.The parotid glands were contoured on the daily pre-treatment iCBCT images to analyze the absolute and relative volume reduction,shrinkage rate,and centroid displacement of the parotid glands.The correlation between medial displacement of the parotid glands and the extent of volume shrinkage was also assessed.Additionally,the actual cumulative radiation dose to the parotid glands over the entire treatment course was obtained using the Eclipse treatment planning system,and the correlation between dose variations and parotid gland volume changes or displacement was investigated.All statistical analyses were performed using the R language(version 4.5.1)within the RStudio integrated development environment(version 2025.05).Paired comparisons were conducted using the Wilcoxon signed-rank test,and Spearman rank correlation was employed for correlation analysis.Results:During radiotherapy,the overall trend in patient body weight was a decrease,with a statistically significant median reduction of 4.68%.Throughout the entire course of radiotherapy for NPC,the volumes of the left and right parotid glands decreased to 53.97%and 58.34%of their original volumes,respectively,and both glands exhibited positional changes to varying degrees.Specifically,the proportions of left parotid glands that moved medially,cranially,and anteriorly were 32(68.09%),25(53.19%)and 23(48.94%),respectively,while the corresponding proportions for the right parotid gland were 28(59.57%),35(74.47%)and 24(51.06%).Further analysis indicated that among patients with medial displacement,the shift was more pronounced within the first 15 treatment fractions;whereas for patients with cranial or anterior displacement,the difference in displacement before and after 15 fractions was not significant.Statistical test results also confirmed that the positions of both parotid glands along the X-axis had changed significantly by the 15th fraction compared to the initial plan.A positive correlation was observed between medial displacement and volume reduction of the parotid glands.The number of cases with an increase in mean dose(Dmean)and dose to 50%of the volume(D50)for the left and right parotid glands was 20(42.55%)and 17(36.17%),respectively.Among these cases,the left Dmean and D50 increased by 21.3%and 24.8%,while the right Dmean and D50 increased by 13.7%and 22.2%,respectively.The Dmean for both left and right parotid glands showed a mild increase in the early stage(before the 15th fraction),generally around 5%,with very few patients exceeding 120%.In the later stage(after the 15th fraction),the increase accelerated significantly,primarily ranging between 15%and 20%.By comparing the relative changes in Dmean and D50 across different fractionation intervals,we also found a notable relative increase in the delivered dose,primarily after the 15th fraction.For the left parotid gland,during fractions 16-20,21-25 and after the 30th fraction,Dmean increased by 2.2%(24.2 Gy),4.6%(24.8 Gy)and 6.9%(26.1 Gy)compared to the previous interval,respectively,while D50 increased by 2.1%(22.0 Gy),5.6%(22.5 Gy)and 9.6%(24.0 Gy),respectively.For the right parotid gland,during fractions 21-25 and after the 30th fraction,Dmean increased by 3.8%(25.3 Gy)and 4.1%(26.0 Gy)compared to the previous interval,while D50 increased by 6.5%(23.3 Gy)and 9.7%(24.5 Gy),respectively.The correlations between changes in Dmean and D50 for the left parotid glands and the medial displacement of their centroids were 0.72 and 0.70.The correlations between changes in Dmean and D50 for the right parotid glands and the medial displacement of their centroids were 0.63 and 0.67.Conclusion:During the entire radiotherapy process,the parotid glands decreased in volume and moved closer to the midline high-dose region.The changes in the radiation dose received by the parotid glands were related to the movement of their centroids toward the midline structures.A reduction in parotid gland volume did not necessarily lead to an increase in the radiation dose received by the parotid glands,and the main increase in radiation dose occurred after 15 fractions.Therefore,early replanning can help control the increase in radiation dose received by the parotid glands throughout radiotherapy.