ABSTRACT
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 analyze the dosimetric effects on off-center tumour treatment plan resulting from the MR-Linac-based isocenter position radiotherapy plan.Methods:The cases of 19 patients who were treated in Sun Yat-sen University Cancer Center in 2020 were collected in this study. Two different IMRT plans were designed for each patient with off-center tumor both for group A with planned isocenter position as IMRT and group B with planed target center position as geometric center. The conformity index and homogeneity index of target, the dose normal tissue and the number of MU were compared between two plans.Results:The two IMRT plans met clinical dosimetric requirements. No statistical differences were found both in homogeneity index and conformity index ( P>0.05). Also there was no differences found in doses to normal tissues. However, the MU number (1 149±903, t=2.804, P=0.012) in group A was higher than that in group B (970±652). Conclusions:It is feasible to perform MR-Linac-based off-center treatment plan.
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Objective:To investigate the impacts of electron streaming effect (ESE) on out-of-field dose distribution in 1.5 T MRI-guided radiotherapy.Methods:Firstly, the Monaco v5.40.1 (Elekta AB, Stockholm, Sweden) treatment planning system (TPS) was implemented to investigate the ESE in a square field (5 cm × 5 cm) at the entry and exit sides of a special homogeneous water phantom. Afterward, a retrospective investigation was conducted into one laryngeal cancer case and one breast cancer case who had been treated on a conventional linear particle accelerator (linac). Then doses were recalculated in the Monaco system using a Unity machine model. Meanwhile, the out-of-field skin dose enhancement induced by ESE was investigated.Results:ESE-induced dose variations were observed at both the entry and exit sides of the phantom surface in the presence of a magnetic field, with the ESE on the exit side notably stronger than that on the entry side. For the laryngeal cancer case, the ESE was not notable and had insignificant impacts on the out-of-field skin dose. In contrast, ESE-induced in-air high-dose region outside the body stretched to the chin area for the breast cancer case. This led to the skin dose escalation of the chin at D1 cm 3 454.6 cGy. After the application of 1 cm bolus, the corresponding skin dose of the chin D1 cm 3 reduced to as low as 113.6 cGy, which is almost equivalent to that in the absence of a magnetic field ( D1 cm 3=92.5 cGy). Conclusions:The ESE in a magnetic field can alter out-of-field dose and lead to local dose enhancement along the electron path. Although the ESE had insignificant impacts on the out-of-field dose of the laryngeal cancer case, it reached the chin area of the breast cancer case. ESE can be effectively shielded by adding protective bolus.
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Objective:To investigate the feasibilityof the adaptive radiotherapy using high-field MR-Linac systems for head neck cancers and perform the evaluation of target coverage and dose criteria.Methods:This study investigated 128 treatment plans of six patients who were treated on 1.5T MR-Linacsystems in Sun Yat-sen University Cancer Center in 2019, compared the differences in target coverage and dose criteria between the dose accumulation in the adaptive radiotherapy using MR-Linac systems and the reference plans, and evaluated the target coverage and dose criteria of each fraction of adaptive plan based on daily MRI anatomy.Results:There was no significant change in the target coverage and dose criteria for each treatment fraction(<1%). However, the change of lens dose was significant (maximum 98%). In addition, the result showed that there was no significant difference in target coverage and dose criteria between the dose accumulation in adaptive radiotherapy using MR-Linac systems and reference plans.In contrast, the average dose to lens was increased by 31.7%.Conclusions:It is feasible to perform adaptive radiotherapy using 1.5T MR-Linacsystems for head neck cancers according tothe evaluation of target coverage and dose criteria. Additionally, since the actual dose tolens was quite different from the reference plan, the lens exposure should be considered in clinical practice.