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Objective:To analyze the correlation between the monitor units and pass rate of plan dose verification in the volumetric intensity modulated arc therapy (VMAT) plan.Methods:VMAT plans for 20 patients with nasopharyngeal carcinoma (NPC) and 30 patients with cervical cancer who underwent radiotherapy at Liuzhou Workers' Hospital from January to October 2020 were retrospectively chosen. The Detector 1500 array and Octavius 4D phantom from German PTW company were used for dose measurement. The pass rates of dose verification of relevant plans were analyzed under the conditions of 3%/2 mm and 2%/2 mm. The correlation between the monitor units and pass rate of plan dose verification in VMAT plans was assessed by Pearson's bivariate correlation analysis.Results:Under the condition of 3%/2 mm, the correlation coefficients between the monitor units and gamma pass rate were -0.873 ( P<0.001), -0.800 ( P<0.001), -0.781 ( P<0.001), -0.493 ( P=0.006) for NPC_1Arc, NPC_2Arc, NPC_1Arc+NPC_2Arc and Cervix_2Arc, respectively. Under the condition of 2%/2 mm, the correlation coefficients between the monitor units and gamma pass rate were -0.842 ( P<0.001), -0.770 ( P<0.001), -0.748 ( P<0.001) and -0.531 ( P=0.003) for NPC_1Arc, NPC_2Arc, NPC_1Arc+NPC_2Arc and Cervix_2Arc, respectively. Conclusion:Significant negative correlation can be observed between the monitor units and plan dose verification pass rate in VMAT plan.
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Objective:To study the differences in the dosimetry and delivery time between helical tomotherapy (HT) and volumetric modulated arc therapy (VMAT) in postoperative radiotherapy of synchronous bilateral breast cancer (SBBC) and to explore the feasibility of clinical application of the HT technology.Methods:A retrospective analysis was conducted for nine SBBC patients who underwent modified radical postoperative radiotherapy in the Fourth Affiliated Hospital of Guangxi Medical University from February 2017 to May 2022. For each patient, the Precision planning system was used to formulate the HT plan, and the RayStation planning system was employed to develop the VMAT plan. All plans obtained were divided into HT group and VMAT group according to plan type. The paired sample t-test was used to compare the dosimetric parameters of the target volume and organ at risk (OAR) and delivery time between the two radiotherapy technologies. Results:The plans of both groups could meet the clinical treatment requirements. The coverage ( D95% and V100%), conformity index (CI), average dose ( Dmean), and median dose ( D50%) of the target volumes in the HT group were better than those in the VMAT group, and the differences were statistically significant ( t=-3.21, -3.39, -5.03, 3.76, 4.97, P < 0.05). The differences in the maximum dose ( D2%), minimum dose ( D98%), high dose volumes ( V107% and V110%), and homogeneity index (HI) of the target volumes between the two groups were not statistically significant ( P > 0.05). Regarding the OAR, the V20 and Dmean of both lungs in the HT group were significantly lower than those in the VMAT group, but the irradiation volume V5 of both lungs in the HT group was significantly higher than that in the VMAT group ( t=-3.01, 3.83, -2.81, P < 0.05). Moreover, V20, V30, V40, and Dmean of heart and the V20 and Dmean of liver in the HT group were significantly lower than those in the VMAT group, with statistically significant differences ( t=3.76, -2.83, -2.74, 5.93, 4.57, 4.48, P < 0.05). There was no significant difference in the radiation doses to other OARs (spinal cord, thyroid gland, and humerus head, P > 0.05). The delivery time of the HT group was significantly higher than that of the VMAT group ( t = 11.32, P < 0.05). Conclusions:Compared with VMAT, HT has greater dosimetric advantages, and can provide higher target coverage, conformability and average dose, and significantly reduce the overall radiation doses to both lungs, heart, and liver in OARs. However, the irradiation volume V5 at low-dose areas of both lungs and the delivery time in the HT group are higher than those in the VMAT group, but still meet the clinical treatment requirements. Therefore, it is feasible to apply the HT technology to the modified radical postoperative radiotherapy of SBBC.
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Objective@#To evaluate the dosimetric effects of set-up errors on nasal NK/T cell lymphoma by introducing set-up errors into the radiotherapy planning system for dose reconstruction.@*Methods@#Ten patients with nasal NK/T cell lymphoma were recruited. A non-coplanar volumetric modulated arc therapy plan was designed for CT image and clinical target area of each patient. After the completion of the plan, the set-up errors were introduced into the radiotherapy plan by changing the ISO of the treatment, and dose calculation was performed to reconstruct the dose distribution.@*Results@#With the increase of system set-up errors, the dose of target was decreased and the order affected by set-up errors in different directions was: left-right direction> head-foot direction> front-rear direction. When the translational set-up errors in each direction were -3 mm to 3 mm and the rotating set-up errors were -3° to 3°, the range of dose change in all targets was less than ±3%. When the set-up errors in all directions were ≤ 3 mm, the dose of organ at risk was less than or similar to the prescribed dose. When the set-up errors were> 3 mm, the doses of lens, spinal cord, parotid gland and optic nerve gradually exceeded the prescribed dose. Only when the rotating set-up errors were ≥ 3°, the dose of lens exceeded the prescribed dose. Special attention should be paid to the influence of the greater set-up errors in the left and right direction on lens, spinal cord and parotid gland, as well as on the spinal cord due to the larger set-up errors in the front and rear direction. After the actual set-up errors were introduced from our department, it exerted slight effect on the irradiation dose of GTV and CTV, which was less than ±2%. In a few cases, the dose of organ at risk potentially exceeded the prescribed dose limit, and special attention should be diverted to overdose of the lens and optic nerve.@*Conclusions@#The set-up errors will result in target dose deficiency and overdose of organ at risk in nasal NK/T cell lymphoma, especially upon the set-up errors in the left and right direction. The effect of 3 mm and 3° set-up errors on target and organ at risk is limited. Therefore, it is recommended to maintain the single direction set-up errors within 3 mm and 3°. The actual set-up errors introduced from our department exert little effect on the target dose, but a small number of organs are at risk of exceeding the prescribed dose limit. It is necessary to increase the evaluation of the extension region of organ at risk.