The Exploration Of Reducing The Exposure Dose Of Trachea By Intensity Modulated Radiotherapy In Nasopharyngeal Carcinoma

Background and Purpose:Intensity modulated radiotherapy(IMRT)has become the mainstream of radiotherapy for nasopharyngeal carcinoma due to its good target area conformant and dose uniformity,which can improve the efficacy and protect the brain,eyes,optical nerves,inner ear,parotid gland and other organs at risk.However,It should be noted that intensity modulated radiotherapy for nasopharyngeal carcinoma does not provide a special block for the trachea as previously did with three-dimensional conformal radiotherapy.If the trachea is not given adequate dose limitation instructions during the planned production,it is easy to make the trachea be exposed to a higher dose and develop more serious tracheal radiation,Clinical manifestations are cough,expectoration,dyspnea,wheeze,severe cases include early cough and rest influence combination with sore throat,nausea,and vomiting,the patient suffers from increased pain,resulting in malnutrition.Tumor cells are hypoxic due to their high metabolism;However,malnutrition can aggravate the hypoxia of tumor tissues and decrease the sensitivity of tumor cells to radiation,thus affects the therapeutic effect of radiotherapy.Materials and methods:Intensity modulated radiotherapy(IMRT)plans were collected from 31 cases of initially treated nasopharyngeal carcinoma(NPC)patients from 2017-2 to 2018-9 in the radiotherapy department of the Affiliated Cancer Hospital of Guangzhou MedicalUniversity.The treatment plan is collected from the Monaco treatment planning system.Patients in the study ranged from 17 to 67,with an average age of 45.There were 20 males and 11 females.According to TNM staging standard of UICC/ AJCC8 th edition.The quantities T1,T2,T3 and T4 patients were 2,9,15 and 5,respectively.N1,N2 and N3 patients were 5,16 and 10,respectively and ?,?,?A period patients were 6,16 and 9 respectively.While setting the dose limit in the planning system,the trachea Dmean limit of the same patient was successively lowered from 4500 cGy to 4000 cGy,3500cGy,3000 cGy and 2500 cGy,and the target area and other organ at risk limit conditions remained unchanged.Each patient was made 5 corresponding radiotherapy plans respectively.The optimal plan was selected for dose evaluation.Results:The decrease of tracheal limit Dmean was positively correlated with the change of tracheal,esophageal and laryngeal/hypopharyngeal Dmean(r values were 0.936,0.505,0.199,and P values were 0.000,0.000,0.005,respectively).That is,as the tracheal limit decreased from 4500 cGy to 4000 cGy,3500cGy,3000 cGy and 2500 cGy,the Dmean of tracheal exposure showed a significant decreasing trend(4252.3cGy,3827.9cGy,3324.0cGy,2792.4cGy and 2284.4cGy,respectively).Meanwhile,esophageal Dmean also showed a gradually decreasing trend(4378.1cGy,4229.8cGy,4106.1cGy,3839.8cGy,3546.9cGy),and hypopharyngeal/laryngeal Dmean also showed a gradually decreasing trend(4500.9cGy,4495.7cGy,4459.5cGy,4462.5cGy,4384.5).There was no correlation with the changes of Dmean,Vd100% and Vd107%indexes of GTVnx and GTVnd in the target area(p>0.05),there was no correlation with the Vd100% change of CTV1 and CTV2(p >0.05).There was no correlation between the down-regulation of tracheal Dmean and the dose changes of other organs at risk(spinal cord,brainstem,temporal lobe,parotid gland,cochlea,oral cavity,eyeball,optic nerve,optic chiasm,temporomandibular joint and mandible)(P >0.05).The comparison of pGTVnx,pGTVnd and pCTV1 uniform index in the target areas of trachea with different dose limits showed that the P values were all > 0.05 and there was no statistical difference,that is,the uniformity of pGTVnx,pGTVndand pCTV1 in the target areas of trachea with different dose limits did not change;However,the P value of the pCTV2 uniform index was 0.04,which showed a statistical difference,that is,the uniformity of pCTV2 would be affected.There was no statistical difference in the conspecific index of pGTVnx,pGTVnd,pCTV1 and pCTV2 in the target areas of different dose limits of trachea,with P values of 0.781,0.183,0.289 and 0.436,respectively.After stratification of lymph node metastasis,it was found that whether unilateral or bilateral,whether the lower cervical lymph node was metastatic or not,and whether the N stage changed from N1-3,the actual exposure dose of trachea would decrease with the decrease of tracheal limit value,and the target dose and the rest of the organs at risk were not significantly affected by the exposure dose.Conclusion:By lowering tracheal limit threshold,tracheal exposure dose can be significantly reduced without affecting the target dose.Although pCTV2 uniform index will be affected,other target area conformal index,uniform index are not affected.At the same time,the dose of other organs at risk is not increased,and the dose of esophagus and lower throat can be reduced.It can be concluded that it is feasible to reduce the tracheal limit Dmean value from 4500 cGy to 2500 cGy in IMRT for nasopharyngeal carcinoma.