Markus Zimmermann

Reply To: Safety and Efficacy of Y-90 Radioembolization After Prior Major Hepatic Resection: Dosimetric Consideration

We appreciate the comments put forward in response to our article, ‘‘Safety and Efficacy of Y-90 Radioembolization After Prior Major Hepatic Resection’’ [1]. As the authors of this letter accurately point out, certain characteristics of our patient cohort, such as the relatively large average remnant liver volume and hypervascular tumor entities in a majority of patients, likely had a positive influence on the safety outcome in our study. However, we do not think these are unusual patient characteristics. From our personal experience at a large academic hepatobiliary center, a majority of patients develop significant hypertrophy (almost up to near-normal volumes) of the liver remnant after hepatectomy during midto long-term follow-up (several months to years). Additionally, our results are in line with those of a previously published study by Kessler et al. [2] albeit their patient cohort included patients treated with less extensive liver surgery. That being said, we do acknowledge the fact that our limited sample size limits the ability to draw definitive conclusions about the applicability of these results to the general population and that there remains a risk of overdosing in patients with less hypertrophy of the remnant liver—although we did not observe such a case in our study. We absolutely agree with the call for a more personalized dosing method for Y-90 RE, since the correlation of BSA with the actual liver volume has been questioned in several studies. The other currently accepted method is the partition model. While this model considers the actual liver mass and a mean absorbed dose in a volume of interest, it is severely limited by the fact that it can only be used in patients with a discrete tumor mass in the liver. In patients with a diffusely infiltrating tumor or multiple metastases, both not uncommon scenarios in patients undergoing Y-90 RE, this model cannot be applied since the target volume cannot be clearly defined. Because of these reasons and the lack of other alternatives, the BSA-based dosing method remains the probably most commonly used dosing method and further investigations are clearly needed to develop alternative, more personalized dosing methods.

Endovascular placement of an extraluminal arterial bypass graft - in vitro feasibility study

Abstract Purpose: A novel approach for arterial bypass grafting using exclusively endovascular techniques was established in-vitro in a phantom model. Material and methods: The experimental setting consisted of a gel-wax phantom with two embedded parallel fluid-filled silicon tubes simulating the superficial femoral vessels. Through an 8-French sheath, a re-entry catheter (OUTBACK®, Cordis) was placed in the simulated artery and used to puncture the vascular wall. Then a 0.014-inch guide wire was advanced into the extravascular space. With the curved needle of the re-entry catheter, the guide wire was steered on a course parallel to the vessel wall in the extravascular space for 5-10 cm. At the desired reentry site, the re-entry catheter was used to puncture the vascular wall again in order to regain access to the endovascular space. Once the tip of the guide wire had safely been placed in the vascular lumen, a self-expandable stent graft (VIABAHN® GORE®) was deployed to complete the extraluminal bypass. Results: Endovascular placement of an extraluminal bypass was successfully achieved in 20 attempts. The mean duration of the procedure amounted to 14:58 (minutes: seconds) (SD ± 3:56). Conclusions: This in-vitro study suggests that endovascular placement of an extraluminal arterial bypass graft is technically feasible.

Numerical Investigation of the Unsteady Interaction Within a Close-Coupled Centrifugal Compressor Used in an Aero Engine

The present work forms part of a research project of the Institute of Jet Propulsion and Turbomachinery at the RWTH Aachen University in collaboration with GE Aviation. The subject is the detailed numerical analysis of the unsteady flow field focusing on the interaction between the impeller and the passage diffuser of a close-coupled transsonic centrifugal compressor used in an aero engine.The centrifugal compressor investigated is characterized by a close-coupled impeller and passage diffuser with a radial gap of only 3.6%. The close coupling tends to provide a high aerodynamic efficiency but simultaneously cause a high unsteady interaction between the impeller and the diffuser. These unsteady effects can have a significant impact on the performance of both components [1,2] and present a challenge to state-of-the-art numerical methods. With increasing compressor efficiency, the more important it is to have a understanding of the detailed unsteady flow physics. Experimental data was obtained from a state-of-the art centrifugal compressor test rig located at the Institute of Jet Propulsion [3]. Steady and unsteady pressure measurements within the impeller and diffuser are used to gain detailed information on the temporal, time-averaged and spectral pressure distributions within the stage to validate the CFD.The work presented shows the unsteady phenomena caused by the interaction as well as the location and propagation of these phenomena within the centrifugal stage. Within the impeller, the exducer is in first order excited by the BPF of the diffuser, whereas in the diffuser both the BPF, as well as the PPF, are present up until the end of the pipe-diffuser. Significant effects on the integral component performance could only be identified for the impeller. Special focus is paid to evaluate the diffuser upstream pressure field, since this is the major source of unsteadiness within the impeller. The performance of the rotor decreases due to the unsteady interaction. This effect is traced back to the unsteady tip-clearance flow, in which the time-averaged mass transport decreases, whereas the specific entropy production increases in a nonlinear way. Within the diffuser, local effects counteracting with respect to the integral performance are found. In front of the throat, there is less decay in total pressure, as a result of tangentially expanding pressure waves. Within the passage an decrease in flow uniformity in the unsteady flow is identified as the reason for the lower diffusion up until the throat and higher losses within the downstream diffuser passage.Copyright