Focal salvage treatment for radiorecurrent prostate cancer: A magnetic resonance-guided stereotactic body radiotherapy versus high-dose-rate brachytherapy planning study

Background and Purpose: Magnetic resonance imaging (MRI)-guided focal salvage high-dose-rate brachytherapy (FS-HDR-BT) is one of the treatment options for radiorecurrent localized prostate cancer. However, due to the invasive nature of the treatment, not all patients are eligible. Magnetic resonance linear accelerator (MR-Linac) systems open up new treatment possibilities and could potentially replace FS-HDR-BT treatment. We conducted a planning study to investigate the feasibility of delivering a single 19 Gy dose to the recurrent lesion using a 1.5 Tesla MR-Linac system. Materials and Methods: Thirty patients who underwent FS-HDR-BT were included. The clinical target volume (CTV) encompassed the visible lesion plus a 5 mm margin. Treatment plans were created for a 1.5 Tesla MR-Linac system using a 1 mm planning target volume (PTV) margin. A dose of 19 Gy was prescribed to ≥ 95% of the PTV. In case this target could not be reached, i.e. when organs-at-risk (OAR) constraints were violated, a dose of ≥ 17 Gy to ≥ 90% of the PTV was accepted. MR-Linac plans were compared to clinical FS-HDR-BT plans. Results: Target dose coverage was achieved in 14/30 (47%) FS-HDR-BT plans and 17/30 (57%) MR-Linac plans, with comparable median D95% and D90%. In FS-HDR-BT plans, a larger volume reached ≥ 150% of the prescribed dose. Urethra D10%, rectum D1cm3, and rectum D2cm3 were lower in the FS-HDR-BT plans, while bladder dose was comparable for both modalities. Conclusion: Single fraction treatment of recurrent prostate cancer lesions may be feasible using stereotactic body radiotherapy (SBRT) on a MR-Linac system

Ecological Role of Submarine Canyons and Need for Canyon Conservation: A Review

Submarine canyons are major geomorphic features of continental margins around the world. Several recent multidisciplinary projects focused on the study of canyons have considerably increased our understanding of their ecological role, the goods, and services they provide to human populations, and the impacts that human activities have on their overall ecological condition. Pressures from human activities include fishing, dumping of land-based mine tailings, and oil and gas extraction. Moreover, hydrodynamic processes of canyons enhance the down-canyon transport of litter. The effects of climate change may modify the intensity of currents. This potential hydrographic change is predicted to impact the structure and functioning of canyon communities as well as affect nutrient supply to the deep-ocean ecosystem. This review not only identifies the ecological status of canyons, and current and future issues for canyon conservation, but also highlights the need for a better understanding of anthropogenic impacts on canyon ecosystems and proposes other research required to inform management measures to protect canyon ecosystemsVersión del edito

Technical developments for real-time MRI-guided HDR brachytherapy

Magnetic resonance imaging (MRI) may be an ideal imaging modality for real-time guidance and verification of high dose rate (HDR) brachytherapy. MRI provides superior tissue contrast and is currently applied for manual catheter reconstruction and treatment planning. However, techniques for real-time treatment verification are not clinically available. MRI may be an ideal imaging modality for real-time guidance and verification of the treatment, as it allows visualization of the anatomy as well as visualization and detection of interventional devices. We have proposed an MRI-guided HDR brachytherapy workflow in which we apply MR-based source localization for reconstruction of the source dwell positions and for real-time treatment verification. In this scenario, the patient is positioned inside the MRI scanner during the full treatment. For this purpose, the impact of the magnetic field on the dose distribution was investigated. This impact was shown to be negligible, allowing the patient to be positioned inside the MRI scanner during irradiation. Furthermore, a prototype MR-conditional afterloader was developed and tested. Simultaneous functioning of the MR-conditional afterloader and a 1.5 T MRI system was demonstrated, allowing the acquisition of MR images while simultaneously using the afterloader. Additionally, a method for localization of the HDR brachytherapy source and a titanium needle was proposed. By simulating the MRI artifacts induced by the object, followed by template matching between the simulated artifact and the MRI artifact, the object position could be determined. This leads to an MRI-guided HDR brachytherapy workflow where we apply automatic reconstruction of the source dwell positions and real-time treatment verification

Development and Testing of a Magnetic Resonance (MR) Conditional Afterloader for Source Tracking in Magnetic Resonance Imaging-Guided High-Dose-Rate (HDR) Brachytherapy

PURPOSE: For the purpose of magnetic resonance imaging (MRI)-guided high-dose-rate (HDR) brachytherapy, a prototype magnetic resonance (MR) conditional afterloader was developed. This study demonstrates the development and testing of the prototype, while operating simultaneously with MRI. In combination with an MR-based method for HDR source localization, this development enables treatment verification of HDR brachytherapy. Additionally, this allows a direct reconstruction of the source dwell positions after catheter insertion (when using a dummy source) and introduction of a clinical workflow where the patient remains in the same position during dwell position reconstruction, treatment planning and irradiation. METHODS AND MATERIALS: A prototype MR conditional afterloader was developed by providing radiofrequency (RF) shielding and a plastic source cable containing a dummy source. Simultaneous functioning of the afterloader and MRI acquisition was tested in an experimental setting where the afterloader was placed next to the scanner and programmed to send the source to predefined positions within a phantom, while acquiring MR images. The HDR source positions were determined using MR artifact simulation and matching of the MR images to the simulated artifact. Additionally, the impact of the presence and use of the afterloader on the MRI performance was investigated by assessment of RF interference, signal-to-noise ratio (SNR), and B0 field homogeneity. RESULTS: The experiments demonstrated that the prototype MR conditional afterloader and the MRI scanner fully functioned while operating simultaneously, without influencing the other system. The step sizes between the source positions obtained from the MR images corresponded with the afterloader settings. Besides, the MRI performance tests demonstrated no deterioration due to the presence or functioning of the afterloader next to the scanner. CONCLUSIONS: This research has demonstrated the feasibility of simultaneous MR acquisition and employment of an MR conditional afterloader. This development enables real-time HDR source localization for treatment verification of MRI-guided HDR brachytherapy using an MR conditional afterloader

Development and Testing of a Magnetic Resonance (MR) Conditional Afterloader for Source Tracking in Magnetic Resonance Imaging-Guided High-Dose-Rate (HDR) Brachytherapy

PURPOSE: For the purpose of magnetic resonance imaging (MRI)-guided high-dose-rate (HDR) brachytherapy, a prototype magnetic resonance (MR) conditional afterloader was developed. This study demonstrates the development and testing of the prototype, while operating simultaneously with MRI. In combination with an MR-based method for HDR source localization, this development enables treatment verification of HDR brachytherapy. Additionally, this allows a direct reconstruction of the source dwell positions after catheter insertion (when using a dummy source) and introduction of a clinical workflow where the patient remains in the same position during dwell position reconstruction, treatment planning and irradiation. METHODS AND MATERIALS: A prototype MR conditional afterloader was developed by providing radiofrequency (RF) shielding and a plastic source cable containing a dummy source. Simultaneous functioning of the afterloader and MRI acquisition was tested in an experimental setting where the afterloader was placed next to the scanner and programmed to send the source to predefined positions within a phantom, while acquiring MR images. The HDR source positions were determined using MR artifact simulation and matching of the MR images to the simulated artifact. Additionally, the impact of the presence and use of the afterloader on the MRI performance was investigated by assessment of RF interference, signal-to-noise ratio (SNR), and B0 field homogeneity. RESULTS: The experiments demonstrated that the prototype MR conditional afterloader and the MRI scanner fully functioned while operating simultaneously, without influencing the other system. The step sizes between the source positions obtained from the MR images corresponded with the afterloader settings. Besides, the MRI performance tests demonstrated no deterioration due to the presence or functioning of the afterloader next to the scanner. CONCLUSIONS: This research has demonstrated the feasibility of simultaneous MR acquisition and employment of an MR conditional afterloader. This development enables real-time HDR source localization for treatment verification of MRI-guided HDR brachytherapy using an MR conditional afterloader

MR-based source localization for MR-guided HDR brachytherapy

For the purpose of MR-guided high-dose-rate (HDR) brachytherapy, a method for real-time localization of an HDR brachytherapy source was developed, which requires high spatial and temporal resolutions. MR-based localization of an HDR source serves two main aims. First, it enables real-time treatment verification by determination of the HDR source positions during treatment. Second, when using a dummy source, MR-based source localization provides an automatic detection of the source dwell positions after catheter insertion, allowing elimination of the catheter reconstruction procedure. Localization of the HDR source was conducted by simulation of the MR artifacts, followed by a phase correlation localization algorithm, applied to the MR images and the simulated images, to determine the position of the HDR source in the MR images. To increase the temporal resolution of the MR acquisition, the spatial resolution was decreased, and a subpixel localization operation was introduced. Furthermore, parallel imaging (sensitivity encoding) was applied to further decrease the MR scan time. The localization method was validated by a comparison with CT, and the accuracy and precision were investigated. The results demonstrated that the described method could be used to determine the HDR source position with a high accuracy (0.4-0.6 mm) and a high precision (≤0.1 mm), at high temporal resolutions (0.15-1.2 s per slice). This would enable real-time treatment verification as well as an automatic detection of the source dwell positions

MRI artifact simulation for clinically relevant MRI sequences for guidance of prostate HDR brachytherapy

For the purpose of magnetic resonance imaging (MRI) guidance of prostate high-dose-rate (HDR) brachytherapy, this paper presents a study on the potential of clinically relevant MRI sequences to facilitate tracking or localization of brachytherapy devices (HDR source/titanium needle), and which could simultaneously be used to visualize the anatomy. The tracking or localization involves simulation of the MRI artifact in combination with a template matching algorithm. Simulations of the MRI artifacts induced by an HDR brachytherapy source and a titanium needle were implemented for four types of sequences: spoiled gradient echo, spin echo, balanced steady-state free precession (bSSFP) and bSSFP with spectral attenuated inversion recovery (SPAIR) fat suppression. A phantom study was conducted in which mentioned sequences (in 2D) as well as the volumetric MRI sequences of the current clinical scan protocol were applied to obtain the induced MRI artifacts for an HDR source and a titanium needle. Localization of the objects was performed by a phase correlation based template matching algorithm. The simulated images demonstrated high correspondences with the acquired MR images, and allowed localization of the objects. A comparison between the object positions obtained for all applied MRI sequences showed deviations (from the average position) of 0.2-0.3 mm, proving that all MRI sequences were suitable for localization of the objects, irrespective of their 2D or volumetric nature. This study demonstrated that the MRI artifact induced by an HDR source or a titanium needle could be simulated for the four investigated types of MRI sequences (spoiled gradient echo, spin echo, bSSFP and bSSFP-SPAIR), valuable for real-time object localization in clinical practice. This leads to more flexibility in the choice of MRI sequences for guidance of HDR brachytherapy, as they are suitable for both object localization and anatomy visualization

Visualization of gold fiducial markers in the prostate using phase-cycled bSSFP imaging for MRI-only radiotherapy

In this work, we present a new method for visualization of fiducial markers (FMs) in the prostate for MRI-only radiotherapy with a positive contrast directly at the MR console. The method is based on high bandwidth phase-cycled balanced steady-state free precession (bSSFP) sequence, which is available on many clinical scanners, does not require any additional post-processing or software, and has a higher signal-to-noise (SNR) compared to conventional gradient-echo (GE) imaging. Complex phase-cycled bSSFP data is acquired with different RF phase increment settings such that the manifestation of the artifacts around FMs in the acquired complex images is different for each dynamic acquisition and depends on the RF phase increment used. First, we performed numerical simulations to investigate the complex-valued phase-cycled bSSFP signal in the presence of a gold FM, and to investigate the relation of the true physical location of the FM with the geometrical manifestation of the artifacts. Next, to validate the simulations, we performed phantoms and in vivo studies and compared the experimentally obtained artifacts with those predicted in simulations. The accuracy of the method was assessed by comparing the distances between the FM's centers and the center of mass of FMs system measured using phase-cycled bSSFP MR images and using reference CT (or MRI-only) images. The results show accurate (within 1 mm) matching of FMs localization between CT and MR images on five patients, proving the feasibility of in vivo FMs detection on MR images only. The FMs show a positive contrast with respect to the prostate background on real/imaginary phase-cycled bSSFP images, which was confirmed by simulations. The proposed method facilitates robust FMs visualization with positive contrast directly at the MR console, allowing RT technicians to obtain immediate feedback on the anticipated feasibility of accurate FMs localization while the patient is being scanned

Origin, Evolution, and Loss of Bacterial Small RNAs

Despite the central role of bacterial noncoding small RNAs (sRNAs) in posttranscriptional regulation, little is understood about their evolution. Here we compile what has been studied to date and trace a life cycle of sRNAs—from their mechanisms of emergence, through processes of change and frequent neofunctionalization, to their loss from bacterial lineages. Because they possess relatively unrestrictive structural requirements, we find that sRNA origins are varied, and include de novo emergence as well as formation from preexisting genetic elements via duplication events and horizontal gene transfer. The need for only partial complementarity to their mRNA targets facilitates apparent rapid change, which also contributes to significant challenges in tracing sRNAs across broad evolutionary distances. We document that recently emerged sRNAs in particular evolve quickly, mirroring dynamics observed in microRNAs, their functional analogs in eukaryotes. Mutations in mRNA-binding regions, transcriptional regulator or sigma factor binding sites, and protein-binding regions are all likely sources of shifting regulatory roles of sRNAs. Finally, using examples from the few evolutionary studies available, we examine cases of sRNA loss and describe how these may be the result of adaptive in addition to neutral processes. We highlight the need for more-comprehensive analyses of sRNA evolutionary patterns as a means to improve novel sRNA detection, enhance genome annotation, and deepen our understanding of regulatory networks in bacteria