Accuracy of automatic structure propagation for daily magnetic resonance image-guided head and neck radiotherapy.

BACKGROUND AND PURPOSE: Deformable image registration (DIR) and contour propagation are used in daily online adaptation for hybrid MRI linac (MRL) treatments. The accuracy of the propagated contours may vary depending on the chosen workflow (WF), affecting the amount of required manual corrections. This study investigated the impact of three different WFs of contour propagations produced by a clinical treatment planning system for a high-field MRL on head and neck cancer patients. METHODS: Seventeen patients referred for curative radiotherapy for oropharyngeal cancer underwent standard CT-based dose planning and MR scans in the treatment position for planning (pMR), and at the 10th (MR10), 20th (MR20) and 30th (MR30) fraction (±2). The primary tumour, a metastatic lymph node and 8 organs at risk were manually delineated on each set of T2 weighted images. Delineations were repeated one month later on the pMR by the same observer to determine the intra-observer variation (IOV). Three WFs were used to deform images in the treatment planning system for the high-field MRL: In WF1, only the planning image and contours were used as a reference for DIR and propagation to MR10,20,30. The most recently acquired image set prior to the daily images was deformed and uncorrected (WF2) versus manually corrected (WF3) structures propagated to the session image. Dice similarity coefficient (DSC), mean surface distance (MSD) and Hausdorff distance (HD) were calculated for each structure in each model. RESULTS: Population median DSC, MSD and HD for WF1 and WF3 were similar and slightly better than for WF2. WF3 provided higher accuracy than WF1 for structures that are likely to shrink. All DIR workflows were less accurate than the IOV. CONCLUSIONS: WF1 and WF3 provide higher accuracy in structure propagation than WF2. Manual revision and correction of propagated structures are required for all evaluated workflows.

End-to-end validation of the geometric dose delivery performance of MR linac adaptive radiotherapy.

The clinical introduction of hybrid magnetic resonance (MR) guided radiotherapy (RT) delivery systems has led to the need to validate the end-to-end dose delivery performance on such machines. In the current study, an MR visible phantom was developed and used to test the spatial deviation between planned and delivered dose at two 1.5 T MR linear accelerator (MR linac) systems, including pre-treatment imaging, dose planning, online imaging, image registration, plan adaptation, and dose delivery. The phantom consisted of 3D printed plastic and MR visible silicone rubber. It was designed to minimise air gaps close to the radiochromic film used as a dosimeter. Furthermore, the phantom was designed to allow submillimetre, reproducible positioning of the film in the phantom. At both MR linac systems, 54 complete adaptive, MR guided RT workflow sessions were performed. To test the dose delivery performance of the MR linac systems in various adaptive RT (ART) scenarios, the sessions comprised a range of systematic positional shifts of the phantom and imaging or plan adaptation conditions. In each workflow session, the positional translation between the film and the adaptive planned dose was determined. The results showed that the accuracy of the MR linac systems was between 0.1 and 0.9 mm depending on direction. The highest mean deviance observed was in the posterior-anterior direction, and the direction of the error was consistent between centres. The precision of the systems was related to whether the workflow utilized the internal image registration algorithm of the MR linac. Workflows using the internal registration algorithm led to a worse precision (0.2-0.7 mm) compared to workflows where the algorithm was decoupled (0.2 mm). In summary, the spatial deviation between planned and delivered dose of MR-guided ART at the two MR linac systems was well below 1 mm and thus acceptable for clinical use.

Multicenter evaluation of MRI-based radiomic features: A phantom study.

INTRODUCTION: This work describes the development of a novel radiomics phantom designed for magnetic resonance imaging (MRI) that can be used in a multicenter setting. The purpose of this study is to assess the stability and reproducibility of MRI-based radiomic features using this phantom across different MRI scanners. METHODS & MATERIALS: A set of phantoms were three-dimensional (3D) printed using MRI visible materials. One set of phantoms were imaged on seven MRI scanners and one was imaged on one MRI scanner. Radiomics analysis of the phantoms, which included first-order features, shape and texture features was performed. Intraclass correlation coefficient (ICC) was used to assess the stability of radiomic features across eight scanners and the reproducibility of two printed models on one scanner. Coefficient of variation (COV) was used to assess the reproducibility of radiomics measurements in the phantom on a single scanner. RESULTS: The phantom models provide sufficient signal-to-noise and contrast in all the tumor models permitting robust automatic segmentation. During a 12-month period of monitoring, the phantom material was stable with T1 and T2 of 150.7 ± 6.7 ms and 56.1 ± 3.9 ms, respectively. Of all the radiomic features computed, 34 of 69 had COV < 10%. Features from first-order statistics were the most robust in stability across the eight scanners with eight of 12 (67%) having high stability. About 29 of 50 (58%) texture features had high stability and no shape features had high stability features across the eight scanners. CONCLUSION: A novel MRI radiomics phantom has been developed to assess the reproducibility and stability of MRI-based radiomic features across multiple institutions. The variation in radiomic feature stability demonstrates the need for caution when interpreting these features for clinical studies.

First clinical experiences with a high field 1.5 T MR linac.

Purpose: A 1.5 T MR Linac (MRL) has recently become available. MRL treatment workflows (WF) include online plan adaptation based on daily MR images (MRI). This study reports initial clinical experiences after five months of use in terms of patient compliance, cases, WF timings, and dosimetric accuracy. Method and materials: Two different WF were used dependent on the clinical situation of the day; Adapt To Position WF (ATP) where the reference plan position is adjusted rigidly to match the position of the targets and the OARs, and Adapt To Shape WF (ATS), where a new plan is created to match the anatomy of the day, using deformable image registration. Both WFs included three 3D MRI scans for plan adaptation, verification before beam on, and validation during IMRT delivery. Patient compliance and WF timings were recorded. Accuracy in dose delivery was assessed using a cylindrical diode phantom. Results: Nineteen patients have completed their treatment receiving a total of 176 fractions. Cases vary from prostate treatments (60Gy/20F) to SBRT treatments of lymph nodes (45 Gy/3F) and castration by ovarian irradiation (15 Gy/3F). The median session time (patient in to patient out) for 127 ATPs was 26 (21-78) min, four fractions lasted more than 45 min due to additional plan adaptation. For the 49 ATSs a median time of 12 (1-24) min was used for contouring resulting in a total median session time of 42 (29-91) min. Three SBRT fractions lasted more than an hour. The time on the MRL couch was well tolerated by the patients. The median gamma pass rate (2 mm,2% global max) for the adapted plans was 99.2 (93.4-100)%, showing good agreement between planned and delivered dose. Conclusion: MRL treatments, including daily MRIs, plan adaptation, and accurate dose delivery, are possible within a clinically acceptable timeframe and well tolerated by the patients.

Magnetic resonance only workflow and validation of dose calculations for radiotherapy of prostate cancer.

BACKGROUND: Current state of the art radiotherapy planning of prostate cancer utilises magnetic resonance (MR) for soft tissue delineation and computed tomography (CT) to provide an electron density map for dose calculation. This dual scan workflow is prone to setup and registration error. This study evaluates the feasibility of an MR-only workflow and the validity of dose calculation from an MR derived pseudo CT. MATERIAL AND METHODS: Thirty prostate cancer patients were CT and MR scanned. Clinical treatment plans were generated on CT using a single 18 MV arc volumetric modulated arc therapy (VMAT) with a prescription of 78 Gy/39 fractions. Dose was recalculated on pseudo CT and assuming uniform water density. Pseudo CT and uniform density based dose calculations were compared to CT dose calculations by gamma analysis. One patient was treated with a plan based solely on MR and pseudo CT including daily image guided radiotherapy (IGRT) performed by manual match of implanted gold markers. RESULTS: A pseudo CT was generated for 29 of the 30 patients. Median gamma pass rates for 1%/1 mm passing criteria for dose calculated on pseudo CT when compared to CT were 100% for most evaluated structures. Dose calculated on uniform density also yielded high median pass rates, but with a higher occurrence of pass rates below 95%. Cases of pass rate below 95% on pseudo CT proved to originate from the presence of rectal air on CT, not represented by the pseudo CT. Treatment based on MR alone was successfully delivered to one patient, including manually performed daily IGRT. CONCLUSIONS: Median gamma pass rates were high for pseudo CT and proved superior to uniform density. Local differences in dose calculations were concluded not to have clinical relevance. Feasibility of the MR-only workflow was demonstrated through successful delivery of a treatment course planned based on MR alone.