ESTRO 37 Abstract book

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ESTRO 37

phantom was rotated to vary the angle between the source and B0 (0º-90º). A 3D CT scan was made with voxel size: 0.34x0.34x0.40mm³, using O-MAR (Philips). The CT source position was considered as the gold standard. Post processing The HDR source position was determined by simulation of the MR artifact (complex data) and matching the MR images to the simulations in a phase correlation algorithm to find the translation between the two images[1], for the coronal and sagittal slices (see Fig.1). The MR images were registered (translation) to the CT data set. Next, the registered 2D positions were combined to the 3D MRI source position. Accuracy and precision The accuracy was calculated as the mean Euclidean distance between the source positions from MRI and CT (over 10 dynamics). The precision was analyzed as the standard deviation over the distances between the positions from MRI and CT (over 10 dynamics).

Conclusion This validation study demonstrated a high, subvoxel accuracy (0.4-0.6 mm) and a high precision (≤0.1 mm) at high temporal resolutions (0.15-1.2 s) for an MR-based HDR source localization method. This makes the method highly valuable for real-time treatment verification and detection of the source dwell positions. 1.Beld E et al 2016 Proc. Intl. Mag. Reson. Med. 24 #3585 OC-0170 Routine clinical treatment verification in HDR prostate brachytherapy with source tracking M.D. Hanlon 1 , R.L. Smith 1,2 , V. Panettieri 2 , J.L. Millar 2 , B. Matheson 2 , A. Haworth 1,3 , R.D. Franich 1 1 RMIT University, School of Science, Melbourne, Australia 2 The Alfred, Alfred Health Radiation Oncology, Melbourne, Australia 3 The University of Sydney, Faculty of Science, Sydney, Australia Purpose or Objective The correct delivery of HDR brachytherapy treatments could be confirmed by source tracking, determining the measured positions and times, and comparing them with the plan. We describe initial routine experience of our source tracking system 1 in which source tracking data is captured for each fraction, allowing for post treatment analysis. Material and Methods The source was tracked by a flat panel detector (FPD) under the patient that measures source exit radiation. Pre-treatment imaging enabled registration between implant and plan to measure implant displacement. Dwell positions and times were calculated from source auto- radiographs in the coronal plane for comparison with the plan. Differences were determined as 2D Euclidian distances between planned and measured dwells. Dosimetric comparisons were made from dose grids calculated with a TG-43 based dose calculation engine with measured dwell positions as input. Results Data for 10 patients was collected over 2 treatment fractions, with pre-treatment imaging determining a mean catheter displacement of 5.4mm (max. 10.6mm) for fraction 1 and -1.0mm (max. 4.4mm) for fraction 2. In cases where the mean implant shift is minimal, the tracked source positions show good agreement with the planed positions without requiring corrections. In one example, a mean dwell position difference of 1.9 mm (max. 4.9 mm) was observed across 280 tracked dwell positions. A subset of catheters is shown in figure 1. Pre- treatment imaging showed that 1.3 mm (mean) and 2.3 mm (max.) was attributable to catheter movements. Dwell times extracted from the tracking data allowed further confirmation of correct treatment delivery. For fractions with large measured implant displacements, the

Results The distances between the HDR source positions from MRI and CT are given in Table 2. This showed that the method has a mean accuracy of 0.4-0.6 mm, depending on the resolution, irrespective of the use of SENSE. The case most plausible for practical application (2 mm resolution and SENSE=2) resulted in a mean accuracy of 0.5 mm at a temporal resolution of 0.25 s per image. Besides, in all cases the precision was ≤0.1 mm, meaning there is almost no variation in repeated measurements.