RÉSUMÉ
Objective:To investigate the effect of dwell time deviation constraint of inverse optimization on the quality and position error robustness of three-dimensional (3D) brachytherapy plans for cervical cancer.Methods:A total of 20 patients with cervical cancer receiving 3D brachytherapy treatment in Xiangya Hospital Central South University from August 2020 to August 2021 were retrospectively selected. All plans were designed using the Eclipse treatment planning system, and the dwell time deviation constraint parameter smooth value in the system were set to 0.00, 0.25, 0.50, 0.75, and 1.00, respectively. An inverse dose volume optimization algorithm was used to generate plans with various smooth values, and the optimization conditions were the same as the original clinical plans. Key dosimetric metrics and total dwell time differences were comparatively analyzed. The applicators were intentionally subjected to position errors (0.2-1.0 cm) in 6 directions (left-right, anterior-posterior, head-foot), and the effect of various smooth values on plan quality and robustness was assessed. There were 133 plans per case and 2 660 plans for 20 patients. The results were statistically analyzed using the Wilcoxon signed-rank nonparametric test.Results:As the smooth value was increased, the modulation factor was gradually decreased and the D 2 cm3 of the bladder and rectum was increased. Plans with smooth values of 0.25, 0.50, 0.75, 1.00 had modulation factors of 0.72±0.09, 0.63±0.08, 0.55±0.08, 0.51±0.06, respectively, lower than 0.75±0.05 of the plan with the smooth value of 0.00, and all differences were statistically significant ( P=0.004, 0.002, 0.002, 0.002). The bladder D 2 cm3 of plans with smooth values of 0.50, 0.75, 1.00 were (475.4±41.0) cGy, (483.7±46.2) cGy, and (489.0±46.8) cGy, respectively, higher than (469.8±41.8) cGy of the plan with the smooth value of 0.00, with statistically significant differences (all P=0.002). The rectum D 2 cm3 of plans with smooth values of 0.50 and 0.75 plans were (413.2±93.3) cGy and (418.6±96.4) cGy, both higher than (410.2±91.5) cGy of the plan with the smooth value of 0.00, with statistically significant differences ( P=0.006, 0.010). When positional errors were introduced, the high risk clinical target volume (HR-CTV) D 90% was close for different smooth plans at most positional errors, and the differences were not statistically significant. The organs at risk D 2 cm3 of plans with the smooth value of 0.00 was lower than those of plans with other smooth values, and for the bladder and rectum, the differences were statistically significant at most positional errors (all P<0.01). Conclusions:The dwell time deviation constraint parameter exerts significant effect on the plan quality, and the smaller the value of the constraint parameter, the higher quality of the plan. The dwell time deviation constraint parameter has slight impact on the positional error robustness of dosimetric indices of targets and key organs at risk.
RÉSUMÉ
Objective:To investigate the optimal bladder filling volume in the 3D brachytherapy of postoperative cervical cancer.Methods:Totally 111 early cervical cancer patients with positive incisal margins or insufficient safety boundaries were included. The normal saline 50, 60, 70, 80, 90, and 100 ml were filled into their bladders, and accordingly six groups were determined, and 66, 69, 66, 69, 72, 56 person-times in each group, respectively. The CT-based simulation positioning was performed. According to the ICRU 89 report, high-risk clinical target volume and organs at risk such as bladder and rectum were delineated. The Oncentra planning system was used to prepare the treatment program. The high-risk clinical target volume (HR-CTV), D90, and the D2 cm 3 and D1 cm 3 of organs at risk under different volumes were recorded. Results:Compared to the 60 ml group, the volume and dosage of HR-CTV in the groups of 50, 70, 80, 90, and 100 ml had no significant difference ( P>0.05). The D2 cm 3 and D1 cm 3 of the bladder and rectum of patients in these groups significantly decreased, and the difference was statistically significant ( tbladder = 3.21, 5.83, 2.89, 12.95, 7.96, Pbladder = 0.031, 0.010, 0.041, 0.000, 0.001; trectum = 2.94, 4.66, 2.53, 5.89, 4.13, Prectum = 0.037, 0.024, 0.049, 0.005, 0.028). The pairwise comparison among these groups except for the 60 ml group showed that the volume and dosage of HR-CTV and the D2 cm 3 and D1 cm 3 of bladder and rectum had no significant difference ( P > 0.05). Moreover, the D2 cm 3 and D1 cm 3 of sigmoid colon and small intestine of these groups had no significant difference ( P > 0.05). Conclusions:In the 3D brachytherapy of postoperative early cervical cancer, a bladder filling volume of 60 ml can ensure the volume and dose of HR-CTV and can protect the bladder and rectum compared with other filling volumes.