Effects of gantry acceleration limitations on VMAT plans / 中华放射医学与防护杂志

Objective:To study the effects of gantry acceleration limitations of a linear accelerator (linac) on the dosimetry of volumetric modulated arc therapy (VMAT) plans, machine efficiency, and dose verification result of VMAT plans and to explore the optimal selection of gantry motion models in the Pinnacle treatment planning system.Methods:Ten cases of nasopharyngeal carcinoma, non-small cell lung cancer, sigmoid adenocarcinoma with retroperitoneal lymph node metastasis, and invasive ductal carcinoma of the breast were each selected for this study. Then two models were set up in the Pinnacle v9.10 treatment planning system, namely the one allowing gantry acceleration and the one limiting gantry acceleration. The same field arrangement, optimized target parameters, and optimized weights of VMAT plans were adopted in the two models, in order to analyze the dosimetric variations in targets and organs at risk (OARs) and compare the differences in treatment time and gamma passing rates.Results:The treatment time of the enrolled patients under the model allowing gantry acceleration was significantly lower than that of the patients under the model limiting gantry acceleration was adopted ( t=-6.751, -0.209, -19.523, -28.999; P< 0.05) and decreased by 15.27%, 18.07%, 19.71%, and 28.75%, respectively. Meanwhile, the conformity and uniformity of target areas were affected, while there was no statistical significance in the gamma passing rates in the validation of VMAT plans ( P>0.05). For the cases of nasopharyngeal carcinoma (NPC), the maximum dose to brainstem PRV increased by 1.25%. For the cases of lung cancer, the maximum dose to the spinal cord and lung V20 increased by 1.19% and 1.21%, respectively, while lung V5 decreased by 1.21%. For the cases of sigmoid adenocarcinoma with retroperitoneal lymph node metastasis, the mean doses to bilateral kidneys, livers, small intestine, and colon all increased. For the cases of breast cancer, lung V10 on the opposite side of cancer increased by 1.66% and the mean dose to the lungs on the same side of cancer decreased by 7.45%. Conclusions:The model allowing gantry acceleration allows the treatment time to be significantly shortened and the treatment efficiency improved. Although this model had the shortcomings such as affecting the conformity and uniformity of target areas to a certain extent and increasing the doses to some OARs, clinical requirements for dosimetry were still met. Therefore, it is recommended to use the model allowing gantry acceleration in the Pinnacle planning system.

Monte Carlo dosimetry of a new (90)Y brachytherapy source.

PURPOSE: In this study, we attempted to obtain full dosimetric data for a new (90)Y brachytherapy source developed by the College of Chemistry (Sichuan University) for use in high-dose-rate after-loading systems. MATERIAL AND METHODS: The dosimetric data for this new source were used as required by the dose calculation formalisms proposed by the AAPM Task Group 60 and Task Group 149. The active core length of the new (90)Y source was increased to 4.7 mm compared to the value of 2.5 mm for the old (90)Sr/(90)Y source. The Monte Carlo simulation toolkit Geant4 was used to calculate these parameters. The source was located in a 30-cm-radius theoretical sphere water phantom. RESULTS: The dosimetric data included the reference absorbed dose rate, the radial dose function in the range of 1.0 to 8.0 mm in the longitudinal axis, and the anisotropy function with a θ in the range of 0° to 90° at 5° intervals and an r in the range of 1.0 to 8.0 mm in 0.2-mm intervals. The reference absorbed dose rate for the new (90)Y source was determined to be equal to 1.6608 ± 0.0008 cGy s(-1) mCi(-1), compared to the values of 0.9063 ± 0.0005 cGy s(-1) mCi(-1) that were calculated for the old (90)Sr/(90)Y source. A polynomial function was also obtained for the radial dose function by curve fitting. CONCLUSIONS: Dosimetric data are provided for the new (90)Y brachytherapy source. These data are meant to be used commercially in after-loading system.

Study of the influence of uniform transverse magnetic field on the dose distribution of high energy electron beam using Monte Carlo method / 生物医学工程学杂志

In the present work, Monte Carlo simulations were employed to study the characteristics of the dose distribution of high energy electron beam in the presence of uniform transverse magnetic field. The simulations carried out the transport processes of the 30 MeV electron beam in the homogeneous water phantom with different magnetic field. It was found that the dose distribution of the 30 MeV electron beam had changed significantly because of the magnetic field. The result showed that the range of the electron beam was decreased obviously and it formed a very high dose peak at the end of the range, and the ratio of maximum dose to the dose of the surface was greatly increased. The results of this study demonstrated that we could change the depth dose distribution of electron beam which is analogous to the heavy ion by modulating the energy of the electron and magnetic field. It means that using magnetic fields in conjunction with electron radiation therapy has great application prospect, but it also has brought new challenges for the research of dose algorithm.