发泡胶固定下鼻咽癌调强放疗颈部摆位误差的剂量学影响OA

Objective To investigate the characteristics of interfractional cervical setup errors and their dosimet-ric impact on the planning target volume(PTV)in nasopharyngeal carcinoma(NPC)patients undergoing intensity-mod-ulated radiation therapy(IMRT)with foam-based immobilization,and to provide evidence for optimizing PTV margin ex-pansion.Methods Clinical data of 60 NPC patients treated with IMRT between October 2023 and May 2024 were retro-spectively analyzed.All patients were immobilized using a thermoplastic head-neck-shoulder mask combined with foam fixation.According to image verification frequency,patients were divided into group A(n=30,verification every other fraction)and group B(n=30,verification at the 10th,20th,and 30th fractions).Kilovoltage(kV)images were ac-quired using an on-board imager(OBI)system,and cervical setup errors in the vertical(VRT),longitudinal(LNG),and lateral(LAT)directions were measured using a 2D-3D registration method.Error distribution models were estab-lished based on all setup data,and dose reconstruction was performed to simulate the delivered dose,evaluating dosimetric effects on the clinical target volume(CTV)and organs at risk(OARs).Results A total of 1,620 image registrations were analyzed,including 1,350 in group A and 270 in group B.No significant differences in baseline characteristics were observed between the two groups(P＞0.05).Cervical setup errors demonstrated dynamic changes over the treatment course,with significantly larger errors in the late treatment phase(＞20 fractions)compared with the early phase(P＜0.05).In both groups,random errors(σ)in the VRT and LAT directions exceeded 3 mm.Dosimetric analysis showed that setup errors of 3-5 mm did not result in significant changes in CTV dose coverage(D95％)in most scenarios.How-ever,when setup errors were≥5 mm,both the maximum dose(Dmax)and mean dose(Dmean)of the gross tumor vol-ume of lymph nodes(GTVnd)were significantly reduced(P＜0.05).Dose constraints for OARs remained within clini-cally acceptable limits across all simulated scenarios.Based on comprehensive error analysis,recommended cervical PTV margins were 4.5 mm in the VRT and LAT directions and 3.5 mm in the LNG direction.Conclusion During IMRT for NPC,interfractional cervical setup errors increase in the middle to late treatment stages,likely due to anatomical changes.Although most setup deviations are manageable with current immobilization techniques,large errors may lead to insufficient target dose coverage.Increased image guidance in later treatment stages and optimized PTV margin expansion are recom-mended to balance target coverage and OAR protection.