Program Information
In Vivo Validation of Ultrasound Intrafraction Motion Monitoring in Prostate Radiotherapy Patients
A Grimwood1*, H McNair2 , T O'Shea3 , S Mason4 , A Tree5 , E Harris6 , (1) Innstitute of Cancer Research, Sutton, Surrey, (2) Royal Marsden NHS Foundation Trust, Sutton, Surrey, (3) Royal Marsden NHS Foundation Trust, Sutton, Surrey, (4) ,,,(5) Institute of Cancer Research, Sutton, Surrey, (6) Royal Marsden Hospital, Surrey,
Presentations
SU-F-708-7 (Sunday, July 30, 2017) 2:05 PM - 3:00 PM Room: 708
Purpose: To validate ultrasound monitoring of prostate position during IMRT by comparison with prostate position derived from pre-treatment cone-beam CT (CBCT) and electronic portal images (EPIs).
Methods: Using the Elekta Clarity® system, the prostate was imaged transperineally during CBCT, couch shift and radiotherapy delivery. The locations of intraprostatic fiducial markers on CBCT and EPIs were used to derive the prostate position for comparison with prostate positions estimated by Clarity.Ultrasound monitoring data of 6 fractions across 5 patients was analysed. Three observers manually identified the locations of markers visible in EPIs and CBCT projections acquired at the same detector angle. Marker locations in the CBCT projections were normalised to the initial prostate position recorded by Clarity. CBCT marker locations were projected onto the EPI detector plane and shifted by an amount derived from the monitoring data at the time of EPI acquisition. Monitoring accuracy was defined as the difference between the Clarity-shifted CBCT and EPI marker locations.Only markers identified by all observers were considered. Sag effects were corrected for in both CBCT and EPIs, as were variations in source-detector distances between the two.
Results: From 138 EPID-CBCT projection image pairs 95 contained visible markers. Clarity-shifted markers were accurate to EPI marker positions within a 2.1 mm standard deviation horizontally (u-axis) and 1.4 mm vertically (v-axis). Mean u and v differences between Clarity and EPI marker positions were -1.3 mm and -0.5 mm respectively, with mean absolute errors of 1.7 mm and 1.2 mm. Observer standard deviation from mean marker locations was 0.3 mm with 0 mm mean difference.
Conclusion: This initial validation of Clarity monitoring of intrafraction prostate motion has shown Clarity to be in good agreement with estimates of prostate position derived from CBCT and EPI images in patients with fiducial markers.
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