A Study On Difference Of Setup Errors In CBCT On Different Registrations In Intensity Modulated Radiotherapy Of Nasopharyngeal Carcinoma.

Objective:To study the difference of setup errors for CBCT in different registration sites of intensity modulated radiotherapy of nasopharyngeal carcinoma.Methods:17nasopharyngeal carcinoma patients created by image guided IMRT were collected randomly and had CBCT before radiation therapy every week,113CBCT images were collected. Different registration sites were represented by bony landmarks. CBCT and CT image both had automatic and manual registration. Nasal septum and pterygoid process represents head,1to3cervical vertebra spine (C1-C3) represents upper neck and4to6cervical vertebra cervical spine(C4-C6) represents lower neck and then try to compare the positioning differences of three different regions after registration. According to the definition of Stroom:mean value of set up error of each patients is individual systematic error; standard deviation of set up error is individual random error; groups systematic error(∑)was retrieved from standard deviation of the individual systematic error; random error(8)was retrieved from standard deviation of the individual random error. Setup error is systematic error±random error. To analyse the set up error using SAS statistical software and give SNK test for set up error of head, upper neck and lower neck. Statistical result is of great significance when P<0.05.And then according to the van-Herk formula MPTV=2.5∑+0.7δto calculate MPTV.Results:1.The setup error of upper neck and lower neck at X and Z direction have statistically significant(P<0.05) and are not all equal. The setup error of upper neck and lower neck at Y direction have no statistically significant (P>0.05).Among which the setup error of upper neck and lower neck at X direction have statistically significant (P=0.0008). No upper statistically significant have found in upper neck and lower neck as well as head and upper neck(P=0.0654and P=0.0897).The setup error of upper neck and lower neck, lower neck and head, head and upper neck at Z direction all have statistically significant(P<0.0001P<0.0001P=0.0003).Setup error lower neck is greater than head and upper neck and upper neck greater than head.2. The standard deviation of systematic error(X) and random error (σ) in17cases at X、Y、Z direction are0.837mm and1.847mm;0.465mm and0.200mm;0.926mm and0.533mm, respectively.Their MPTV are3.386mm,1.303mm and2.688mm, respectively. Conclusion:1. There are setup errors at X, Y and Z direction of nasopharyngeal carcinoma patients receiving radiotherapy.2. The setup error of neck is greater than head in IMRT of nasopharyngeal carcinoma which should be treated differently.3.CBCT IMRT tends to reduce setup error and improve the accuracy of radiotherapy. Providing a basis for the correct settings of PTV and protecting the surrounding normal tissue in a better way.4. This study shows that our reference value of MPTV are3.4mm (left and right),1.3mm (head and feet),2.7mm(front and rear)5. If necessary, do CT check again and redo the plans and target delineation.