SU‐E‐J‐141: Comparison of Dose Calculation On Automatically Generated MRBased ED Maps and Corresponding Patient CT for Clinical Prostate EBRT Plans

Abstract

Purpose: To analyze the effect of computing radiation dose on automatically generated MR‐based simulated CT images compared to true patient CTs. Methods: Six prostate cancer patients received a regular planning CT for RT planning as well as a conventional 3D fast‐field dual‐echo scan on a Philips 3.0T Achieva, adding approximately 2 min of scan time to the clinical protocol. Simulated CTs (simCT) where synthesized by assigning known average CT values to the tissue classes air, water, fat, cortical and cancellous bone. For this, Dixon reconstruction of the nearly out‐of‐phase (echo 1) and in‐phase images (echo 2) allowed for water and fat classification. Model based bone segmentation was performed on a combination of the DIXON images. A subsequent automatic threshold divides into cortical and cancellous bone. For validation, the simCT was registered to the true CT and clinical treatment plans were re‐computed on the simCT in pinnacle3. To differentiate effects related to the 5 tissue classes and changes in the patient anatomy not compensated by rigid registration, we also calculate the dose on a stratified CT, where HU values are sorted in to the same 5 tissue classes as the simCT. Results: Dose and volume parameters on PTV and risk organs as used for the clinical approval were compared. All deviations are below 1.1%, except the anal sphincter mean dose, which is at most 2.2%, but well below clinical acceptance threshold. Average deviations are below 0.4% for PTV and risk organs and 1.3% for the anal sphincter. The deviations of the stratifiedCT are in the same range as for the simCT. All plans would have passed clinical acceptance thresholds on the simulated CT images. Conclusion: This study demonstrated the clinical usability of MR based dose calculation with the presented Dixon acquisition and subsequent fully automatic image processing. N. Schadewaldt, H. Schulz, M. Helle and S. Renisch are employed by Phlips Technologie Innovative Techonologies, a subsidiary of Royal Philips NV.

Dosimetric and workflow evaluation of first commercial synthetic CT software for clinical use in pelvis

“…The commercial MRCAT package for syn-CT generation uses a single mDIXON MRI sequence and proprietary algorithm to generate electron density information (figure 2) (Helle et al 2014, Schadewaldt et al 2014, Köhler et al 2015). MRCAT is based on a dual-echo 3D Cartesian mDIXON acquisition that generates fat only, water only, in-phase and out-of-phase sequences.…”

Feasibility of MR-only proton dose calculations for prostate cancer radiotherapy using a commercial pseudo-CT generation method

“…In the following we will briefly outline the main ideas of the approach. MRCAT is a bulk assignment pCT generation method based on sequential intensityand model-based segmentation techniques (Schadewaldt et al 2014). In particular, MRCAT segments the images from the dedicated MR sequence (see section 2.1) into 5 classes, and assigns each class the pseudo-HU values specified in table 1 (middle column).…”

Quantification of confounding factors in MRI-based dose calculations as applied to prostate IMRT

“…257, Philips Healthcare, Vantaa, Finland). MRCAT is a bulk assignment pCT generation method based on sequential intensity-and model-based segmentation techniques (Schadewaldt et al 2014). In particular, MRCAT segments images from a dedicated MR sequence (see section 2.1) into five classes, and assigns to each class the pseudo-HU specified in table 1.…”

“…To estimate tissue attenuation properties from MRI, several methods have been proposed in the last few decades (Lee et al 2003, Chen et al 2004, Schmidt and Payne 2015, Prior et al 2016. More recently, MRI vendors have launched solutions as well (Schadewaldt et al 2014, Siversson et al 2015. The proposed methods can be categorised into three classes (Prior et al 2016): (1) atlas-based, (2) voxel-based or (3) bulk assignment-based techniques.…”