Synthetic computed tomography generation for abdominal adaptive radiotherapy using low-field magnetic resonance imaging.

Background and Purpose: Magnetic Resonance guided Radiotherapy (MRgRT) still needs the acquisition of Computed Tomography (CT) images and co-registration between CT and Magnetic Resonance Imaging (MRI). The generation of synthetic CT (sCT) images from the MR data can overcome this limitation. In this study we aim to propose a Deep Learning (DL) based approach for sCT image generation for abdominal Radiotherapy using low field MR images. Materials and methods: CT and MR images were collected from 76 patients treated on abdominal sites. U-Net and conditional Generative Adversarial Network (cGAN) architectures were used to generate sCT images. Additionally, sCT images composed of only six bulk densities were generated with the aim of having a Simplified sCT.Radiotherapy plans calculated using the generated images were compared to the original plan in terms of gamma pass rate and Dose Volume Histogram (DVH) parameters. Results: sCT images were generated in 2 s and 2.5 s with U-Net and cGAN architectures respectively.Gamma pass rates for 2%/2mm and 3%/3mm criteria were 91% and 95% respectively. Dose differences within 1% for DVH parameters on the target volume and organs at risk were obtained. Conclusion: U-Net and cGAN architectures are able to generate abdominal sCT images fast and accurately from low field MRI.

Is MRI-Linac helpful in SABR treatments for liver cancer?

Objective: To determine the MRI-Linac added value over conventional image-guided radiation therapy (IGRT) in liver tumors Stereotactic ablative radiation therapy (SABR). Materials and methods: We retrospectively compared the Planning Target Volumes (PTVs), the spared healthy liver parenchyma volumes, the Treatment Planning System (TPS) and machine performances, and the patients' outcomes when using either a conventional accelerator (Versa HD®, Elekta, Utrecht, NL) with Cone Beam CT as the IGRT tool or an MR-Linac system (MRIdian®, ViewRay, CA). Results: From November 2014 to February 2020, 59 patients received a SABR treatment (45 and 19 patients in the Linac and MR-Linac group, respectively) for 64 primary or secondary liver tumors. The mean tumor size was superior in the MR-Linac group (37,91cc vs. 20.86cc). PTV margins led to a median 74%- and 60% increase in target volume in Linac-based and MRI-Linac-based treatments, respectively. Liver tumor boundaries were visible in 0% and 72% of the cases when using CBCT and MRI as IGRT tools, respectively. The mean prescribed dose was similar in the two patient groups. Local tumor control was 76.6%, whereas 23.4% of patients experienced local progression (24.4% and 21.1% of patients treated on the conventional Linac and the MRIdian system, respectively). SABR was well tolerated in both groups, and margins reduction and the use of gating prevented ulcerous disease occurrence. Conclusion: The use of MRI as IGRT allows for the reduction of the amount of healthy liver parenchyma irradiated without any decrease of the tumor control rate, which would be helpful for dose escalation or subsequent liver tumor irradiation if needed.

A novel lung SBRT treatment planning: Inverse VMAT plan with leaf motion limitation to ensure the irradiation reproducibility of a moving target.

This study exposed the implementation of a novel technique (VMATLSL) for the planning of moving targets in lung stereotactic body radiation therapy (SBRT). This new technique has been compared to static conformal radiotherapy (3D-CRT), volumetric-modulated arc therapy (VMAT) and dynamic conformal arc (DCA). The rationale of this study was to lower geometric complexity (54.9% lower than full VMAT) and hence ensure the reproducibility of the treatment delivery by reducing the risk for interplay errors induced by respiratory motion. Dosimetry metrics were studied with a cohort of 30 patients. Our results showed that leaf speed limitation provided conformal number (CN) close to the VMAT (median CN of VMATLSL is 0.78 vs 0.82 for full VMAT) and were a significant improvement on 3D-CRT and DCA with segment-weight optimized (respectively 0.55 and 0.57). This novel technique is an alternative to VMAT or DCA for lung SBRT treatments, combining independence from the patient's breathing pattern, from the size and amplitude of the lesion, free from interplay effect and with dosimetry metrics close to the best that could be achieve with full VMAT.

Adaptive Magnetic Resonance-Guided External Beam Radiation Therapy for Consolidation in Recurrent Cervical Cancer.

Purpose: Adaptive magnetic resonance (MR)-guided brachytherapy takes an important place as consolidation within the care of cervical malignancies, but may be impracticable in some unusual cases. This work aimed to present the case of adaptive MR-guided external beam radiation therapy (aMRgRT) used as a boost in a recurrence of cervical cancer. Methods and Materials: We report on a case of a parametrial recurrence in a 31-year-old patient who already underwent a trachelectomy as treatment for her primary growth. After concomitant radio-chemotherapy, a brachytherapy boost was performed. Because of its position in relation to the left uterine artery after trachelectomy, impeding interstitial catheters set up, the relapse was insufficiently covered. With the aim to refine the coverage of target volumes, aMRgRT treatment was undertaken to allow for achievement of the dosimetric goals. Results: In clinical circumstances where the brachytherapy step was hindered, aMRgRT presents many advantages. First, daily native MR-imaging outperforms usual x-ray imaging in the pelvis, refining repositioning. Second, its specific workflow allows for the performance of adaptive treatment, with consideration of both the inter- and intrafraction motions of organs at risk and target volumes. Conclusion: In nonfeasible brachytherapy situations, aMRgRT could be a satisfying substitute. Nevertheless, brachytherapy remains the standard of care as a boost in locally advanced cervical cancer.

[Start of activity with the MRIdian® system: The first 200 patients treated at the Institut Paoli-Calmettes]. / Retour d'expérience clinique à propos des 200 premiers patients traités au MRIdian® à l'Institut Paoli-Calmettes.

INTRODUCTION: Several centers have recently been equipped with MRI-guided radiotherapy systems, including the Paoli-Calmettes Institute which was the first French center to start this activity. We report in this article our early experience. METHODS: Data related to patients treated on the MRIdian® (Viewray®) were prospectively collected. Procedures concerning the implementation of the system and internal organizational issues were summarized. RESULTS: Between February 2019 and March 2020, 201 patients were treated: 40% of treatments were normofractionated (n=70) and 60% used hypofractionation (n=105). The reported monthly occupancy rate at one, six and twelve months was 30%, 62%, and 90%. The distribution of normofractionated treatments was dominated by prostatic (29%) and pancreatic (26%) cancers, followed by abdomino-pelvic irradiations for gynecological cancers (12%) or lymph node diseases (12%) and boosts for rectal or vaginal cancers (11%). Regarding treatments with moderate hypofractionation (dose by fraction between 3 and 5Gy), they corresponded mainly to integrated boost for abdomino-pelvic lymph nodes (38%), while the stereotaxic treatments primarily concerned hepatic lesions (15%), bones (30%). DISCUSSION: The MRIdian® was initially used widely in our service corresponding to a learning curve for MRI guidance. This new tool for image-guided radiotherapy helped us to secure our practice providing solutions for both inter and intra-fraction movements making it possible to reduce the additional margin in order to better protect the organs at risk. The main technical difference with conventional accelerators is the possibility of performing adaptive radiotherapy in real time, the start of which was more gradual.

Toward a comparison and an optimization of CT protocols using new metrics of dose and image quality part I: prediction of human observers using a model observer for detection and discrimination tasks in low-dose CT images in various scanning conditions.

In the context of reducing the patient dose coming from CT scanner examinations without penalizing the diagnosis, the assessment of both patient dose and image quality (IQ) with relevant metrics is crucial. The present study represents the first stage in a larger work, aiming to compare and optimize CT protocols using dose and IQ new metrics. We proposed here to evaluate the capacity of the Non-PreWhitening matched filter with an eye (NPWE) model observer to be a robust and accurate estimation of IQ. We focused our work on two types of clinical tasks: a low contrast detection task and a discrimination task. We designed a torso-shaped phantom, including Plastic Water®slabs with cylindrical inserts of different diameters, sections and compositions. We led a human observer study with 13 human observers on images acquired in multiple irradiation and reconstruction scanning conditions (voltage, pitch, slice thickness, noise level of the reconstruction algorithm, energy level in dual-energy mode and dose), to evaluate the behavior of the model observer compared to the human responses faced to changing conditions. The model observer presented the same trends as the human observers with generally better results. We rescaled the NPWE model on the human responses by scanning conditions (kVp, pitch, slice thickness) to obtain the best agreement between both observer types, estimated using the Bland-Altman method. The impact of some scanning parameters was estimated using the correct answer rate given by the rescaled NPWE model, for both tasks and each insert size. In particular, the comparison between the dual-energy mode at 74 keV and the single-energy mode at 120 kVp showed that, if the 120 kVp voltage provided better results for the smallest insert at the lower doses for both tasks, their responses were equivalent in many cases.