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Measurement of Prostate Motion Trajectories During Prostate SBRT Boost Treatments with a Rectafix

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K Legge

K Legge1*, D Nguyen2 , J Ng2 , L Wilton3 , J Booth4 , P Keall2 , J Martin3 , D J O'Connor1 , P Greer1,3 , (1) University of Newcastle, Australia (2) University of Sydney, Australia (3) Calvary Mater Newcastle, Australia (4) Royal North Shore Hospital, Australia.


SU-G-JeP4-10 (Sunday, July 31, 2016) 5:30 PM - 6:00 PM Room: ePoster Theater

Purpose:To determine prostate motion during SBRT boost treatments with a Rectafix rectal sparing device in place using kV imaging during treatment.
Methods:Patients each had three gold fiducial markers inserted into the prostate and received two VMAT boost fractions of 9.5-10 Gy under the PROMETHEUS clinical trial protocol with a Rectafix rectal retractor in place. Two-dimensional kilovoltage images of fiducial markers were acquired continuously during delivery. Three patients were treated on a Varian Clinac iX linear accelerator (6X, 600 MU/min), where kV images were acquired at 5 Hz during treatment. Seven patients were treated on a Varian Truebeam linear accelerator (10XFFF, 2400 MU/min) where kV images were acquired every 3 seconds. Images were processed off-line using the Kilovoltage Intrafraction Monitoring (KIM) software after treatment. KIM determines prostate position in three dimensions from 2D kV projections using a probability density model and a pre-treatment kV arc. The 3D displacement of the prostate was quantified as a function of time throughout each fraction.
Results:From all fractions analyzed, it was found that the prostate had moved less than 1 mm in any direction from its initial position 84.6% of the time. The prostate was between 1 and 2 mm from its initial position 14.2% of the time, between 2 and 3 mm of its initial position 0.8% of the time and was greater than 3 mm from its initial position only 0.4% of the time.
Conclusion:The amount of prostate motion observed during prostate SBRT boost treatments with a Rectafix device in place was minimal and lower than that observed in non-Rectafix studies. The Rectafix device reduces rectal dose as well as immobilizing the prostate.

Funding Support, Disclosures, and Conflict of Interest: Kimberley Legge is the recipient of an Australian Postgraduate Award.

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