Abstract
BackgroundThe aim of this study was to compare the plan quality of magnetic-resonance image-based intensity modulated radiation therapy (MRI-based-IMRT) with the MRIdian Linac system to that of volumetric modulated arc therapy (VMAT) with the TrueBeam STx system for lung stereotactic ablative radiotherapy (SABR).MethodsA total of 22 patients with tumors located in the lower lobe were retrospectively selected for the study. For each patient, both the MRI-based-IMRT and VMAT plans were generated using an identical CT image set and identical structures with the exception of the planning target volume (PTV). The PTVs of the MRI-based-IMRT were generated by adding an isotropic margin of 3 mm from the gross tumor volume, whereas those of VMAT were generated by adding an isotropic margin of 5 mm from the internal target volume. For both the MRI-based-IMRT and VMAT, the prescription doses to the PTVs were 60 Gy in four fractions.ResultsThe average PTV volume of the MRI-based-IMRT was approximately 4-times smaller than that of VMAT (p < 0.001). The maximum dose to the bronchi for the MRI-based-IMRT was smaller than that for the VMAT (20.4 Gy versus 24.2 Gy, p < 0.001). In addition, V40Gy of the rib for the MRI-based-IMRT was smaller than that for the VMAT (1.8 cm3 versus 7.7 cm3, p = 0.008). However, the maximum doses to the skin and spinal cord for the MRI-based-IMRT (33.0 Gy and 14.5 Gy, respectively) were larger than those for the VMAT (27.8 Gy and 11.0 Gy, respectively) showing p values of less than 0.02. For the ipsilateral lung, the mean dose, V20Gy, V10Gy, and V5Gy for the MRI-based-IMRT were smaller than those for the VMAT (all with p < 0.05). For the contralateral lung, V5Gy, V10Gy, D1500cc, and D1000cc for the MRI-based-IMRT were larger than those for the VMAT (all with p < 0.05). The mean dose and V50% of the whole body for the MRI-based-IMRT were smaller than those for the VMAT (0.9 Gy versus 1.2 Gy, and 78.7 cm3 versus 103.5 cm3, respectively, all at p < 0.001).ConclusionsThe MRI-based-IMRT using the MRIdian Linac system could reduce doses to bronchi, rib, ipsilateral lung, and whole body compared to VMAT for lung SABR when the tumor was located in the lower lobe.
Highlights
The aim of this study was to compare the plan quality of magnetic-resonance image-based intensity modulated radiation therapy (MRI-based-IMRT) with the MRIdian Linac system to that of volumetric modulated arc therapy (VMAT) with the TrueBeam STx system for lung stereotactic ablative radiotherapy (SABR)
The planning target volume (PTV) sizes of VMAT, including margins compensating for the respiratory motion of the lung tumors, were much larger than those of magnetic resonance images (MRI)-based IMRT, which did not include the margins for respiratory motion
Despite the smaller PTV sizes of magnetic resonance imageguided radiation therapy (MR-IGRT) than those of conventional internal target volume (ITV)-based radiotherapy, a previous study demonstrated that the lung SABR plan quality of the ViewRay system was not better than that of VMAT owing to the poor characteristics of Co-60 beams and the large multi-leaf collimator (MLC) width of the ViewRay system [2]
Summary
The aim of this study was to compare the plan quality of magnetic-resonance image-based intensity modulated radiation therapy (MRI-based-IMRT) with the MRIdian Linac system to that of volumetric modulated arc therapy (VMAT) with the TrueBeam STx system for lung stereotactic ablative radiotherapy (SABR). Since the release of the MRIdian® Linac system (ViewRay Inc., Oakwood Village, OH), linear accelerators with a magnetic resonance imaging system (MR-linac) have become clinically available. The MRIdian Linac system can generate 6 MV flattening filter free (FFF) beams with a maximum dose rate of 6 Gy/min using an S-band standing wave linear accelerator (linac) [1]. ViewRay® system (ViewRay Inc., Oakwood Village, OH), which is a previous model for magnetic resonance imageguided radiation therapy (MR-IGRT) that uses Co-60 radioisotopes to generate treatment beams [2, 3]. The MRIdian Linac system is equipped with a linac, 3D volumetric and 2D planar magnetic resonance images (MRI) can be acquired using a 0.35 T magnetic field, similar with the ViewRay system [1]. In addition to the successful integration of the linac and MR imaging systems, a notable feature of the MRIdian Linac system is a double-
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