| Objective:This study used linear acceleration airborne kilovolt cone beam CT（KV-CBCT）to collect anatomical images of the target area and surrounding tissue structure of breast cancer patients undergoing breast-conserving surgery,and compared with planned CT images.To observe the errors in the course of radiotherapy,and its effect on the target area and the surrounding normal tissues by software.Then to calculate the appropriate expansion boundary of the clinical target volume（CTV）and explore the necessity of correcting the positioning errors of patients by KV-CBCT before radiotherapy.Method:（1）Thirteen patients with breast-conserving surgery were selected for intensity modulated radiation therapy（IMRT）and enrolled in this study.These patients were scanned with KV-CBCT before treatment and three times a week for5 weeks,15 times per patient.The reconstructed images scanned by CBCT were compared with the DRR images generated by simulated positioning CT,and the error of data was obtained.The CBCT images were compared with CT treatment plan to analyze the effect of errors on target volume（CTV）,planned target volume（PTV）and evaluate the reasonable scope of the PTV boundary in patients undergoing breast-conserving surgery.According to overlapping volume and volume changing of target region in the process of treatment.Then to calculate DICE index and to analyze shape change error.（2）Six patients who underwent SIB-IMRT after breast-conserving surgery for left breast cancer were selected as the study subjects.After completing the preparatory work for treatment,each patient in the group underwent CBCT scans before the first,sixth,eleventh,sixteenth and twenty-first stages of treatment.A total of 5 times,30 sets of CBCT images were collected.Volume density allocation（BDA）was applied to the three-dimensional reconstruction image of KV-CBCT.The reconstructed image was registered with the simulated location,the target area was delineated on the reconstructed image,then mapped to the simulated location CT,and a new treatment plan PLAN-CBCT（1-5）was made.The average dose of target area and organ at risk on five CBCT images was derived from the treatment plan system.Compared with the original treatment plan-CT,the average dose of target area and organ at risk on five CBCT images was obtained.Result:The study shows that the X,Y and Z axis orientation errors were（2.1±1.8）mm,（2.6±1.9）mm and（3.2±2.5）mm,The rotational errors of Xr,Yr and Zr degrees are 1.8±2.3、2.3±1.5、and 2.9±3.2,respectively.Besides,Z-axis was the largest displacement deviation and axis rotation error and there is no obvious correlation between displacement deviation and axis rotation error.According to the formula MPTV,the range of the extensive boundary on the X,Y,Z axis were 4.5mm,5.3 mm,6.7mm,respectively.The GTV and PTV of the CBCT treatment plan cumulative values were lower than D5%,D95%,D98%and Dmean of CT treatment plan（P<0.05）.There were significant differences in conformity index CI and dose uniformity index HI between CT treatment plan and actual cumulative plan（P<0.05）.The heart,lung,V10 V20,V30,V40,and Dmean were significantly increased in the CBCT treatment plan（P<0.05）.The average of DICE index PTV1were 0.85±0.09,range 0.78-0.93.PTV2were 0.90±0.06,range 0.84-0.96,Variation DICE index of PTV1 were obvious.Conclusion:Using KV-CBCT image to simulate adaptive radiotherapy can reduce the positioning error of patients,improve the radiation dose and accuracy of patients with PTV,and reduce the radiation dose of normal tissue.KV-CBCT may play an important role in reducing the complications of radiotherapy and improving the quality of life of patients. |
