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Objective:To investigate the workflow, feasibility and advantages of respiratory navigator-guided stereotactic body radiation therapy (SBRT) of liver malignancies on the magnetic resonance linear accelerator (MR-linac).Methods:Clinical data of 10 patients with liver cancer treated with respiratory navigator-guided SBRT on the MR-linac from September to December 2021 were analyzed retrospectively. All patients underwent CT and MR simulated localization, and plain, enhanced and 4D CT scan, and T 1 3D MR and T 2 3D MR images were collected. The expiratory 4D CT was chosen to design the reference plan. The T 2 3D navigator MR image (end-exhalation) was collected before treatment, the target position was adjusted or the target shape was modified in combination with the real-time monitoring 2D MR image and appropriate online adaptive planning process was selected. Then, the ability of CT, T 2 3D and T 2 3D navigator MR images to display the tumor was evaluated by naked eye. The changes of target volume were calculated. Dosimetric differences between the adaptive and reference plans were compared. The efficacy and adverse reactions of patients were evaluated. Results:In the free breathing state, the T 2 3D navigator MR image was significantly better than T 2 3D MR image to clearly display the tumor and its boundary. The adaptive plans of adapt-to-position (ATP) and adapt-to-shape (ATS) adopted by 10 patients was 37 times and 22 times respectively. The tumor subsided significantly in 3 patients. The average target conformal index (CI) of the adaptive plans was no different from that of the reference plans, but the gradient index (GI) was higher ( P<0.05), especially in the ATS plans. Compared to the reference plans, the normal liver V 5 Gy, V 10 Gy and D mean were almost the same, but the average MU was increased with a significant difference in the ATP adaptive plans ( P<0.05). The average of MU, segments and normal liver D mean and V 10 Gy in the ATS adaptive plans were lower than those in the reference plans, and the liver V 5 Gy was slightly increased. Seven patients were evaluated after 1 month and 3 months. The local control of lesions was promising. Toxicities were mild and no grade 3 or higher toxicities were observed. Conclusion:Respiratory navigator on MR-linac improves the visual clarity of tumors and online MR images, and shows its advantages to guide the adaptive precision radiotherapy of liver tumors, especially in SBRT.
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Objective To investigate the differences between Monte Carlo (MC) calculated doseto-water (Dw) and dose-to-medium (Dm) for lung cancers treated with intensity-modulated radiotherapy (IMRT).Methods A total of 10 lung carcinoma patients with 5-field IMRT treatment plans were stratified sampling randomly selected for this study,which were performed on Monaco treatment planning system (TPS) with MC algorithm.Using the patients' own CT images as quality assurance (QA) phantoms,two kinds of QA plan were calculated,one was the Dm,and another was the Dw plan.Dose volume histogram (DVH) parameters and the subtraction of two plans were used to evaluate the spatial distribution of the difference between the Dm and Dw.Results Differences between dose-volume indices computed with Dm and Dw for the PTV65 and PTV50 doses (D50％,D98％ and D2％) were-0.3％,-0.2％,0.3％ and 0.1％,-0.6％,0.4％,respectively,of which the D50％ of PTV65 and D98％ of PTV50 had statistical difference (t =-2.536,-3.776,P ＜ 0.05).For normal tissues,spinal cord,heart,lung and esophagus,the D50％ differences between Dm and Dw were 0.3％,1.1％,-0.2％ and -0.1％,of which the Dm of spinal cord and heart were slightly lower than the Dw (t =2.535,3.254,P ＜ 0.05).For the D2％ of the normal tissues,the differences were 0.3％,-0.6％,-0.7％ and 0.6％,the differences were statistically significant (t =2.311,-4.105,-3.878,6.214,P＜0.05).All the differences were within 2％.Meanwhile planned subtraction analysis showed the differences between the Dm and Dw varied very much with the other body parts of the patient,especially for bone tissues,and the two doses were significant difference (＞ 5％).Conclusion In the course of clinical application,the relative differences between Dm and Dw for lung cancers MC calculations should be noted when considering the dose limitations of bone tissue.
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Objective To study the difgerence between the IAEA code of practice TRS-Z77 and TRS-398 in the determination of the absorbed dose to water for high-energy photon beams using several cylindrical chambers. Methods For 6 different types of cylindrical chambers,the calibration factors ND,W,Q0 in terms of absorbed dose to water were calculated from the air exposure calibration factors N,,and were compared with the ND,W,Q0 measured in European standard laboratory. Accurate measurements were performed in Varian 6 MV photon beam using 6 cylindrical chambers according to TRS-277 and TRS-398.The beam quality correction factors kQ.Q0 as well as the water absorbed doses were compared.Results For the set of chambers,the difference between ND,W,Q0 computed from Nx and ND.-.Q0 obtained in European standard laboratory was 0.13％ ～ 1.30％.The difference of beam quality correction factors for TRS-277 and TRS-398 was 0.09％ ～0.45％.The distinction of the water absorbed doses obtained according to the two different protocols was 0.27％ ～ 1.40％,and was primarily due to their different calibration factors.Conclusions The discrepancy in absorbed doses determined according to two protocols using different cylindrical chambers is clinically acceptable.However,TRS-398 allows a more convenient localization of chambers,provides a more simple formulation,and offers the reduced uncertainty in the dosimetry of radiotherapy beams.
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ObjectiveTo test a three-dimensional dose verification system,which reconstructing dose to anatomy based on modeling and online measurements ( RDBMOM ),and to evaluate the accuracy and feasibility of its application in clinical intensity-modulated radiotherapy (IMRT) quality assurance.Methods Phantom plans of regular and irregular fields were selected for the testing.All test plans were implemented and the dose distributions were measured using the thimble ion-chamber and two-dimensional ion-chamber array,the accuracy of RDBMOM were then evaluated by comparing the corresponding results.Two practical treated nasopharyngeal carcinoma IMRT plans were verified with RDBMOM and the clinic significancy were valued.ResultsCompared with measurements of the thimble ion-chamber,deviations of RDBMOM were within 1％ in all tested cases except small field of 3 cm ×3 cm.The largest deviation of reconstructed dose in IMRT cases was 2.12％.The dose profile reconstructed by RDBMOM coincided with the measurement using two-dimensional ion-chamber array.The γ rates (3％/3 mm) were 94.56％ - 100％.The RDBMOM verification of IMRT cases shown that the γ rate ＞ 99％ in total and ＞ 98％ in planning target volume,deviation in D95 ＜0.4％,but the largest deviations in mean dose of the parotids and lens were 2.97％ and 59.58％ respectively.ConclusionsAccuracy of the tested system satisfies the demand of IMRT dose verification.RDBMOM is able to provide information of volumetric dosimctry and anatomical location of dose error,which is benefit for evaluating the clinical value of verification results.