CHEN Shengqing, BAI Jing, LI Xiaoming, WU Zhonglai, ZHANG Zhiyu, SUN Jinxia
Objective: To investigate the difference between the same isocenter plan and dual-center plan when using cone-beam CT (CBCT) for breast cancer breast-conserving surgery with chest wall and supraclavicular fields combined radiotherapy for reference of clinical treatment. Methods: Ten patients with the same isocenter and 10 patients with double isocenters were collected after breast-conserving surgery with three-dimensional conformal radiation therapy (3D-CRT) irradiation of the chest wall and supraclavicular field, and the patients applied CBCT to record the positional error data of X (left and right), Y (head and foot), and Z (front and back) in the three linear directions of the center points (once a week) to keep the same conditions of the original plan, and the centers were corrected using the positional error to generate error plans. The centroid coordinates were corrected using the positioning errors and brought into the planning system to generate an error plan. Under the premise that 90% of the target volume of the original plan reached 90% of the prescribed dose, the changes in the actual dosimetric parameters of the planning target volume (PTV) and organs at risk and the differences in the therapeutic parameters of the two types of single-center and dual-center radiotherapy plans caused by the errors were evaluated. Results: Compared with single-center and double-center error plans, the maximum dose Dmax (t=-4.744, P＜ 0.001), mean dose Dmean (t=-2.288, P=0.025), percentage of volume encompassed by 90% of the prescribed dose V90% (t=-3.216, P=0.002), and 110% of the volume encompassed by 110% of the prescribed dose (Z=-3.838, P＜0.001), and hotspot dose D1cc (t=-6.051, P＜0.001) were lower in the single-center group than those in the double -center group, and the difference was statistically significant (P＜0.05). In terms of organs at risk, the percentage of volume encompassed by 500 cGy dose in the contralateral lung, V5 (Z=-2.373, P=0.018), Dmean (Z=-2.534, P=0.011), spinal cord Dmax (=-4.417, P＜0.001), and thyroid Dmean (t=-4.535, P＜0.001) were higher in the two-center group than those in the single-center group, the difference was statistically significant (P＜0.05), and the differences in the remaining parameters were not statistically significant. In terms of treatment parameters, the treatment time (t=-2.621, P=0.010) was higher in the two-center group than in the single-center group, and the difference was statistically significant (P＜0.05), while the difference in the parameters of the number of machine jumps was not statistically significant. Conclusion: The same isocenter radiotherapy plan of chest wall field combined with supraclavicular field after breast conserving surgery for breast cancer is superior to the double isocenter radiotherapy plan in reducing the error leading to high dose in the target area, dose hotspots and protection of organs at risk.
