Comparison of treatment plans for a high-field MRI-linac and a conventional linac for esophageal cancer.

PURPOSE: To compare radiotherapy treatments plans in esophageal cancer calculated for a high-field magnetic resonance imaging (MRI)-linac with plans for a conventional linac. MATERIALS AND METHODS: Ten patients with esophageal squamous cell carcinomas were re-planned retrospectively using the research version of Monaco (V 5.19.03, Elekta AB, Stockholm, Sweden). Intensity modulated radiotherapy (IMRT) plans with a nine-field step-and-shoot technique and two-arc volumetric modulated arc therapy (VMAT) plans were created for the Elekta MRI-linac and a conventional linac, respectively. The prescribed dose was 60 Gy to the primary tumor (PTV60) and 50 Gy to elective volumes (PTV50). Plans were optimized for optimal coverage of the 60 Gy volume and compared using dose-volume histogram parameters. RESULTS: All calculated treatment plans met predefined criteria for target volume coverage and organs at risk dose both for MRI-linac and conventional linac. Plans for the MRI-linac had a lower number of segments and monitor units. No significant differences between both plans were seen in terms of V20Gy of the lungs and V40Gy of the heart with slightly higher mean doses to the heart (14.0 Gy vs. 12.5 Gy) and lungs (12.8 Gy vs. 12.2 Gy). CONCLUSION: Applying conventional target volume and margin concepts as well as dose-fractionation prescription reveals clinically acceptable dose distributions using hybrid MRI-linac in its current configuration compared to standard IMRT/VMAT. This represents an important prerequisite for future studies to investigate the clinical benefit of MRI-guided radiotherapy exploiting the conceptional advantages such as reduced margins, plan adaptation and biological individualization and hypofractionation.

Differentiation of bladder cancer with water flow elastography (WaFE).

Cancer affects the mechanical properties of tissue. Therefore, elastography techniques can be used to differentiate cancerous from healthy tissue. Due to probe size and restricted handling, most elastography techniques are not applicable in minimally invasive surgery (MIS). Established techniques such as endoscopic ultrasound elastography measure under undefined boundary conditions, making the determination of quantitative mechanical properties challenging. Water flow elastography (WaFE) has recently been introduced for application in MIS. Here, we present an improved WaFE measurement method in which the probe attaches itself to the sample with a small suction pressure. This leads to defined boundary conditions, allowing for a quantitative determination of the Young's modulus of tissue. To facilitate fast measurements, we developed a correction model for the hydrodynamic resistance and the fluid inertia of the tubing. We used WaFE for ex vivo measurements on human bladders and found a significantly larger Young's modulus for cancerous vs. healthy tissue. We determined the optimal classification threshold for the Young's modulus to be 8 kPa and found that WaFE can differentiate between cancerous and healthy tissue with a sensitivity of 0.96 and a specificity of 1. Our results underline that WaFE can be a helpful differentiating tool in MIS.

Water flow elastography - A promising tool to measure tissue stiffness during minimally invasive surgery.

Mechanical properties are important markers for pathological processes in tissue. Elastography techniques are therefore becoming more and more useful for diagnostics. In minimally invasive surgery (MIS), however, the probe size is limited and the handling is restricted, thereby excluding the application of most established elastography techniques. In this paper we introduce water flow elastography (WaFE) as a new technique that benefits from a small and inexpensive probe. This probe flows pressurized water against the sample surface to locally indent it. The volume of the indentation is measured with a flow meter. We use finite element simulations to find the relation between the indentation volume, the water pressure, and the Young's modulus of the sample. We used WaFE to measure the Young's modulus of silicone samples and porcine organs, finding agreement within 10% to measurements with a commercial material testing machine. Our results show that WaFE is a promising technique for providing local elastography in MIS.