| BackgroundNasopharyngeal carcinoma is a common malignant tumor of the head and neck,and is more prevalent in the Guangdong and Guangxi regions of China.Due to the specific biological behavior and anatomical structure of nasopharyngeal carcinoma,according to NCCN guidelines,the standard treatment for early-stage nasopharyngeal carcinoma is radiation therapy,while radiation therapy combined with chemotherapy is recommended for intermediate and advanced-stage nasopharyngeal carcinoma.Volumetric-Modulated Arc Therapy(VMAT)is a novel radiotherapy technique that has been shown to provide excellent target coverage and conformality in multiple cancer types,resulting in a significant improvement in survival prognosis for patients with nasopharyngeal carcinoma.However,in long-term survivors,problems such as dry mouth and dental caries caused by the impaired function of the parotid gland due to radiotherapy have seriously affected the quality of life of patients.Therefore,for patients with nasopharyngeal cancer,minimizing the dose to the parotid gland during physical planning is one of the main countermeasures to reduce the toxic side effects after parotid radiation therapy,including optimizing the beam direction,dose limit conditions,equivalent uniform dose optimization and other optimization methods.Studies have suggested that the critical dose threshold for parotid gland damage is between26-35 Gy,which is important when optimizing dose constraints to either limit the average dose of bilateral parotid lands to≤26Gy or limit the dose on one side to less than30 Gy.Currently,there are few studies that have explored the threshold of parotid limit for dose optimization in VMAT planning for nasopharyngeal carcinoma,and the dosimetric advantages of non-coplanar VMAT techniques for the parotid gland are still controversial and have some room for exploration,leaving room for further investigation.PurposeThe aim of this study is to explore the minimum value for parotid gland constraints during dose optimization in VMAT planning for nasopharyngeal carcinoma.Based on this research,further investigation will be conducted to explore the dosimetric advantages of non-coplanar VMAT planning in protecting the parotid glands,providing reference data for the use of non-coplanar VMAT technology in treating patients with nasopharyngeal carcinoma at different stages.Materials and methodsForty initial nasopharyngeal carcinoma patients who underwent radical intensity-modulated radiation therapy were included from Affiliated Cancer Hospital of Guangzhou Medical University and complete imaging data was collected.According to international guidelines on target areas and organs at risk prioritization,Monaco V5.1 TPS system was used to develop three coplanar VMAT plans(CoPlan2600,CoPlan2200,and CoPlan2000)for each patient with salivary gland constraint conditions of Dmean 26 Gy,22Gy,and 20 Gy,respectively.The dosimetric parameters of the target area and organs at risk were compared among the three plans to identify the minimum salivary gland constraint condition.Based on this,three sets of non-coplanar VMAT plans(NonCoPlan15°,NonCoPlan30°,and NonCoPlan90°)were developed for each patient with different couch angles,and the dosimetric differences between the coplanar plan and the non-coplanar plans at different couch angles were compared regarding target areas and organs at risk.Statistical analysis was performed using SPSS25.0 software,and paired T-tests were used to assess statistical significance with a P-value less than 0.05 indicating significant differences.ResultsCompared to the coplanar plan-2600 in the coplanar VMAT group,the CoPlan2200 not only showed no significant differences in target area and other organs at risk dosimetric parameters(P>0.05),but also demonstrated significant reduction in parotid gland dosimetric parameters.Although the coplanar plan-2000 compared to the coplanar plan-2600 showed significant reduction in parotid gland dosimetric parameters with statistical significance,and no significant differences in target area dosimetric parameters(P>0.05)were observed.However,a parameter introduced in this study(the area where the PCTV2 dose line over laps with the parotid gland)showed statistical differences compared to both the coplanar plan-2200 and coplanar plan-2600.Based on the first part of the study,non-coplanar fields were added.The a 15°non-coplanar plan was compared with the coplanar plan,which not only ensured the dose uniformity and conformity of the target area,but also significantly reduced the radiation dose to the parotid gland,with a statistically significant difference observed.In addition,several organs at risk,such as the lens,esophagus,and brainstem,also benefited from this plan.The30° and 90°non-coplanar VMAT plans were compared with the coplanar plan.Although there were benefits in parotid gland related parameters in the 30° and 90°non-coplanar plans,a statistical difference was observed compared to the coplanar plan in the PCTV2 parameter,indicating a possibility of target area dose deficiency.In particular,the90°non-coplanar plan exhibited poor dose uniformity,and the 30°non-coplanar plan required the longest treatment time.ConclusionThe coplanar VMAT plan for nasopharyngeal carcinoma with salivary gland constraint conditions optimized via the Monaco V5.1 planning system can set Dmean to 2200 c Gy,which ensures the dose to the target area while reducing the dose to the salivary gland,thereby possessing certain dosimetric advantages.Compared with the non-coplanar NonCoPlan30° and NonCoPlan90° plans,the NonCoPlan15° VMAT plan does not compromise the dose to the target area while reducing the dose to the salivary gland,improving the conformity and uniformity of the target area,although it has a slightly increased number of machine segments compared to the coplanar VMAT plan.In clinical practice,suitable radiotherapy techniques should be selected based on actual conditions.However,for patients who require salivary gland protection,this study still presents a feasible clinical treatment plan. |
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