Abstract
BackgroundBecause of superior soft tissue contrast, the use of magnetic resonance imaging (MRI) as a complement to computed tomography (CT) in the target definition procedure for radiotherapy is increasing. To keep the workflow simple and cost effective and to reduce patient dose, it is natural to strive for a treatment planning procedure based entirely on MRI. In the present study, we investigate the dose calculation accuracy for different treatment regions when using bulk density assignments on MRI data and compare it to treatment planning that uses CT data.MethodsMR and CT data were collected retrospectively for 40 patients with prostate, lung, head and neck, or brain cancers. Comparisons were made between calculations on CT data with and without inhomogeneity corrections and on MRI or CT data with bulk density assignments. The bulk densities were assigned using manual segmentation of tissue, bone, lung, and air cavities.ResultsThe deviations between calculations on CT data with inhomogeneity correction and on bulk density assigned MR data were small. The maximum difference in the number of monitor units required to reach the prescribed dose was 1.6%. This result also includes effects of possible geometrical distortions.ConclusionsThe dose calculation accuracy at the investigated treatment sites is not significantly compromised when using MRI data when adequate bulk density assignments are made. With respect to treatment planning, MRI can replace CT in all steps of the treatment workflow, reducing the radiation exposure to the patient, removing any systematic registration errors that may occur when combining MR and CT, and decreasing time and cost for the extra CT investigation.
Highlights
Because of superior soft tissue contrast, the use of magnetic resonance imaging (MRI) as a complement to computed tomography (CT) in the target definition procedure for radiotherapy is increasing
The dose volume histograms (DVHs) for the target from the CT calculation was compared with the DVH for the bulk densities recommended by the ICRU for bone and lung and with the exact same treatment plan, i.e., the same beam setup and number of monitor units (MUs) per beam. In this part of the study, we investigated what impact the bulk density approach had on the DVH shape and assessed the sensitivity of the DVH to the bulk density assignment
Evaluation of DVH The shapes of the target volume DVHs were fairly insensitive to the bulk density assignment Figure 2, figure 3, figure 4, figure 5 and table 2 show that the density values recommended in ICRU 46[25] provide a clinically acceptable agreement between bulk density DVH and DVH based on the CT study
Summary
Because of superior soft tissue contrast, the use of magnetic resonance imaging (MRI) as a complement to computed tomography (CT) in the target definition procedure for radiotherapy is increasing. We investigate the dose calculation accuracy for different treatment regions when using bulk density assignments on MRI data and compare it to treatment planning that uses CT data. Computed tomography (CT) has been the basis for treatment planning since the introduction of 3D conformal radiotherapy because of its availability, high geometrical accuracy, and direct connection to electron density used in dose calculations. A recent report notes that MR different imaging sessions. This implies that the registration result can significantly depend on the surrounding tissues and in itself introduce significant uncertainty [14,15]. The geometrical distortions and the lack of electron density information are the main obstacles associated with using MR images when developing treatment plans
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