RESUMO
Object To study the effects of the number of arcs in the treatment of cervical cancer with volumetric modulated arc therapy (VMAT) on the treatment plan, and to provide reference for the selection of the number of arcs in the clinical application.Methods CT images of 10 patients with cervical cancer were selected, it's the radiation therapist that delineated the target area and organs, the prescribed dose was 5040 cGy per 28 times to the target area, and single arc (Arc1), double arc (Arc2) and three (Arc3) plan were designed by RayStation planning system to compare the differences with the dosimetry characteristics of target and organs. The time required for planning optimization was recorded, and the difference between the monitor unit and the delivery time in the three plans was analyzed. A dose validation tool for PTW was used to verify the dose to analyze the influence of the number of arc on the passing rate of gamma verification.Results Single arc plan (Arc1), double arc plan (Arc2) and three arc plan (Arc3) for 10 cases of cervical cancer all proved to meet the clinical requirements. With the increase of arc number, the target area distribution got improved, V40, V45 and Dmean in the organs at risk of rectum, bladder, and femoral head gradually decreased in case of doses of 40 and 45 Gy, and V35, V40 and V45 in the normal tissue were lowered in case of the doses of 35, 40 and 45 Gy. With the increase of the arc number, the optimization time and the delivery time of the plan increased, while the difference between the monitor unit of the three plans was smaller. The passing rate of the three plans was more than 99.5％, and the difference was small.Conclusion Cervical cancer VMAT plan has the target dose distribution and the protection of organs at risk enhanced with the increase of the number of arcs, while the treatment time and cost increased. Therefore, it is necessary to take comprehensive considerations on the number of VMAT plan arcs.
RESUMO
Objective To analyze the setup error during tumor radiotherapy by linear accelerator cone beam CT(CBCT),and to determine the presence of correlation between subcutaneous fat thickness and setup error. Methods From May 2016 to June 2017 59 patients with pelvic tumors underwent CT scan after body positioning with thermoplastic model,CBCT images were obtained before treatment and registered automatically and manually along with localizable CT images. The displacement data at X(left-right), Y(head-foot) and Z(front-back) directions as well as pelvic subcutaneous fat thickness were recorded,and SPSS 18.0 was used for data statistics.Results The 59 patients had the mean setup errors at X,Y and Z directions being(1.07±2.18),(-0.07±5.00)and(1.15±2.38)mm respectively,and the subcutaneous fat thickness ranging from 0.3 to 2.9 cm with a mean value of 1.6 cm.Spearman analysis showed the correlations between the absolute value of the errors and the subcutaneous fat thickness being 0.15, 0.11 and 0.20 respectively at X, Y and Z directions, and there were no significant differences between the correlations(P>0.05).Conclusion The setup method verifies its feasibility by restraining its errors at X, Y and Z directions between -5 and +5 mm, and there is a weak correlation between the setup error and subcutaneous fat thickness.[Chinese Medical Equipment Journal,2018,39(5):64-67]