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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.

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 treatment plan no longer represented the implant geometry at treatment, and so, plan adjustment was performed for improved comparison.

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.

Dose metrics were generated from dose grids calculated from measured dwell positions as inputs. Good agreement was seen for metrics less sensitive to small uncertainties. For the above example, the PTV D90% and V100% exhibited differences of 3% and <1%. When measurement uncertainties due to implant shift are accounted for, measured dose metrics (table 1) show good agreement with the plan, except for urethra D10%, where systematic uncertainties that contribute to individual dwell positions to be measured cranial of their true location, producing artefacts of higher urethra dose.

Conclusion Our source tracking system provides valuable information for routine clinical treatment verification. Pre-treatment imaging provides confidence the implant position is correct and tracked source dwells confirm treatment is delivered as expected. Post treatment dosimetric analysis provide valuable metrics that represent actual delivered patient dose. References: 1: Smith, R. L., et al (2017) (in press) An integrated system for clinical treatment verification of HDR prostate brachytherapy combining source tracking with pretreatment imaging. Brachytherapy .