A criterion for the reliable use of MRI-only radiotherapy

Abstract

BackgroundMRI-only radiotherapy will eliminate the systematic registration errors introduced when transferring MRI information to the CT. However, challenges concerning the missing information on electron density, necessary for dose calculation and patient setup on bony anatomy are introduced. This study presents a possible statistical approach to evaluate, if deviations based on MRI-only radiotherapy as compared to the CT based radiotherapy are acceptable.Methods18 head-and-neck, 21 prostate, 10 vesica and 8 pelvic patients were included in the study. Data from each patient contained a CT and a T2-weighted MRI scan, a structure set and a clinically approved CT based treatment plan, which was re-calculated with identical parameters on the density corrected MRI scans. A statistical analysis including a 95% confidence interval was performed in clinically relevant DVH points.ResultsThe mean differences in the investigated DVH points were in the order of 1.5% for the PTV and up to 4.2% for organs at risk. In addition, a proposed criterion of 2% dose difference in the PTV coverage for 95% of the patients was fulfilled for all diagnostic groups for a bulk segmented MRI in the DVH points, Dmedian and D2%, while only head-and-neck and prostate further fulfilled the criterion in D98%.ConclusionHere, we suggested a method for establishing a reliable use of MRI-only radiotherapy. A population-based study comparing CT based dose calculations with those obtained on a suggested segmentation of MRI should be initiated and acceptable deviations in clinically relevant DVH points should be established. Such a population-based approach could form a part of the clinical commissioning of MRI-only radiotherapy.

Evaluation of a multi-atlas CT synthesis approach for MRI-only radiotherapy treatment planning

“…For both the target and the OARs, both sCT-based dose distributions differed from the corresponding CT-based dose distribution, on average, no more than 1% of the original dose. These results are comparable to those already presented in the literature [8], [9], [11], [19], [23], [24]. Mean percentages of passing points within the external contour of 98–100% and 94–97% were achieved for both methods and cancer sites for the 3D local Ɣ-analyses with constraints of 3%_3 mm and 2%_2 mm, respectively.…”

“…For prostate patients, bone (1.22 g/cm 3 ) and for H&N patients, bone (1.53 g/cm 3 ) and air (0.001 g/cm 3 ) tissue classes were defined. Physical densities were defined according to the literature [8], [9], [28]. Bone delineation was performed manually as no efficient threshold exists for bone segmentation using traditional MR sequences.…”

“…However, as no fundamental relationship between MR image intensities and electron density values exists [6], an accurate method to derive CT equivalent information from MR data (referred to as synthetic CT) is required to perform dose calculations. To assess the feasibility of MR-based treatment planning, the first experiments consisted of assigning single bulk densities to tissue classes (such as bone, air and soft-tissue) delineated either from a CT image [7], [8], [9] or manually from an MR image [10], and then comparing the resulting synthetic CT-based plan to the original CT-based plan. For both H&N and prostate target volumes, dosimetric errors were reported to be 1–2% different from the CT-based dose calculation [8], [9], [10].…”

“…To assess the feasibility of MR-based treatment planning, the first experiments consisted of assigning single bulk densities to tissue classes (such as bone, air and soft-tissue) delineated either from a CT image [7], [8], [9] or manually from an MR image [10], and then comparing the resulting synthetic CT-based plan to the original CT-based plan. For both H&N and prostate target volumes, dosimetric errors were reported to be 1–2% different from the CT-based dose calculation [8], [9], [10]. Korhonen et al [11] then explored the possibility of assigning subject-specific density values to the bone class by manually segmenting an MR image and converting the MRI intensity values to HUs using a second-order polynomial model.…”

Patch‐based generation of a pseudo CT from conventional MRI sequences for MRI‐only radiotherapy of the brain

“…37 Although a study with more patients is still needed, we note that for PTV 1, all methods fulfilled this criterion. This speaks in favor of a reliable use of MRI-only for targets positioned away from cavities.…”