Subject(s)
Dose Fractionation, Radiation , Fractures, Compression/prevention & control , Radiosurgery/methods , Radiotherapy Planning, Computer-Assisted/methods , Spinal Fractures/prevention & control , Spinal Neoplasms/radiotherapy , Dose-Response Relationship, Radiation , Humans , Neoplasm Metastasis , Retrospective Studies , Risk Factors , Spinal Neoplasms/complications , Spinal Neoplasms/pathology , Tumor Burden/radiation effectsABSTRACT
The purpose of this study is to investigate the attenuation characteristics of a novel radiofrequency (RF) coil, which is the first coil that is solely dedicated to MR guided radiotherapy with a 1.5 T MR-linac. Additionally, we investigated the impact of the treatment beam on the MRI performance of this RF coil. First, the attenuation characteristics of the RF coil were characterized. Second, we investigated the impact of the treatment beam on the MRI performance of the RF coil. We additionally demonstrated the ability of the anterior coil to attenuate returning electrons and thereby reducing the dose to the skin at the distal side of the treatment beam. Intensity modulated radiation therapy simulation of a clinically viable treatment plan for spinal bone metastasis shows a decrease of the dose to the planned tumor volume of 1.8% as a result of the MR coil around the patient. Ionization chamber and film measurements show that the anterior and posterior coil attenuate the beam homogeneously by 0.4% and 2.2%, respectively. The impact of the radiation resulted in a slight drop of the time-course signal-to-noise ratio and was dependent on imaging parameters. However, we could not observe any image artifacts resulting from this irradiation in any situation. In conclusion, the investigated MR-coil can be utilized for treatments with the 1.5 T-linac system. However, there is still room for improvement when considering both the dosimetric and imaging performance of the coil.
Subject(s)
Bone Neoplasms/radiotherapy , Image Processing, Computer-Assisted/methods , Magnetic Resonance Imaging/methods , Particle Accelerators/instrumentation , Phantoms, Imaging , Radiotherapy, Image-Guided/methods , Spinal Neoplasms/radiotherapy , Bone Neoplasms/diagnostic imaging , Bone Neoplasms/secondary , Humans , Signal-To-Noise Ratio , Spinal Neoplasms/diagnostic imaging , Spinal Neoplasms/pathologyABSTRACT
The purpose of this study was to investigate what pseudo-CT (pCT) strategy is sufficient for online MRI based treatment plan adaptation of a stereotactic treatment for spinal bone metastases. For this purpose, the dosimetric accuracy of five increasingly complex pCT strategies was evaluated using the planning CT data of 20 patients suffering from spinal metastases. For each pCT, a treatment plan was developed and simulated on both the pCT and the original CT data of the patient. The two resulting dose distributions were compared using gamma analysis of 2%/2 mm. In this paper, a Gamma Pass Rate (GPR) of ⩾ 95% within the Target Volume (TV) was considered clinically acceptable. We additionally demonstrated in this paper the automatic generation of each investigated pCT strategy with the use of dedicated MRI data complemented with pre-treatment CT data of a patient in treatment position. The dosimetric accuracy of a pCT increases when additional bulk densities are utilized for a pCT. However, the dosimetric accuracy of even the most complex 'bulk density' pCT strategy used in this study had an average GPR of only 78% within the TV. However, if information on the heterogeneous electron density distribution within the affected vertebral body was available, a clinically acceptable 99% mean GPR was observed. All pCTs could successfully be generated using the MRI data in combination with the CT data of a patient in treatment position. The results presented in this study show that a simple 'bulk density' pseudo-CT strategy is not feasible for online MRI based treatment plan adaptation for spinal bone metastases. However, a clinically acceptable result is generated if the information on the heterogeneous electron density (ED) distribution within the affected vertebral bone is available. Therefore, any pCT strategy for this tumor site should include a method which can estimate the heterogeneous ED of the affected vertebral bone.
Subject(s)
Magnetic Resonance Imaging , Radiometry , Radiotherapy Planning, Computer-Assisted/methods , Spinal Neoplasms/radiotherapy , Tomography, X-Ray Computed , Case-Control Studies , Feasibility Studies , Humans , Spinal Neoplasms/secondaryABSTRACT
The purpose of this paper is to evaluate the impact of a flexible radiofrequency coil on the treatment delivery of an online MR-guided radiotherapy treatment. For this study, we used a Synergy MR body coil (Philips, Best) in combination with the current MRL prototype of the UMC Utrecht. The compatibility of the coil is evaluated in two steps. First, we evaluated the dosimetric impact of the MR coil on both a simple and a complex irradiation strategy for treating spinal bone metastases. This tumor site will likely be chosen for the first in-man treatments with the UMC Utrecht MRL system. Second, we investigated the impact of the treatment beam on the MRI performance of the body coil. In case a single posterior-anterior rectangular field was applied, dose to the target volume was underestimated up to 2.2% as a result of beam attenuation in the MR coil. This underestimation however, decreased to 1% when a stereotactic treatment strategy was employed. The presence of the MR coil in or near the distal site of the treatment beam decreased the exit dose when a magnetic field was present. The MRI performance of the coil was unaffected as the result of the radiation. It is feasible to use the Synergy MR body coil for an online MR-guided radiotherapy treatment without any modification to the coil or attenuation correction methods in the planning stage. The effect of the MRI coil on the dose delivery is minimal and there is no effect of the treatment beam on the SNR of the acquired MRI data.