Application study of megavolt cone-beam computed tomography to intensity-modulated radiotherapy of nasopharyngeal carcinoma.

Objective To evaluate the interfractional three-dimensional(3D)setup error and propose optimum margin of planning target volume(PTV)in radiotherapy for nasopharyngeal carcinoma at risk using intensity modulated radiotherapy(IMRT)guided with cone-beam computed tomography(CBCT).Methods Twenty two cases of patients received radiotherapy of IMRT imaging-guided with megavolt MVision CBCT scans equipped in ONCOR Linear Accelerator.The initial five fractional CBCT volume-imaging and data obtained by CBCT scans after initial setup and after re-positioning were dimensionally matched with planned CT image.Results The interfractional setup errors on X,Y and Z axes were(-0.03±1.68)mm,(0.17±2.03)mm and(0.28±2.15)mm,respectively,for 22 nasopharyngeal carcinoma.Comparing with the pre-correction position,the post-correction setup errors decreased significantly on all three axes(P 0.05),whereas there were no statistically significant differences in five fractional setup errors(P 0.05).While without correction of CBCT setup errors,PTV margin in three dimension were La(tcm)-X:2.78 mm,Long(cm)-Y:3.22 mm,Ver(tcm)-Z:3.46 mm.while with correction of CBCT setup errors,PTV margin in three dimension were Lat(cm)-X:1.74 mm,Long(cm)-Y:1.94 mm,Vert(cm)-Z:2.13 mm.Conclusion The set-up margins were less than 4 mm in all directions.Measurement and correction of interfractional setup errors and PTV margins before fractional radiotherapy using megavolt MVision CBCT could help to improve the precision of IMRT radiotherapy for nasopharyngeal carcinomas.