Program Information
Real-Time Monitoring of the Prostate Bed Using Ultrasound
A Smith1*, K Stephans2 , M Kolar2 , O Mian2 , R Tendulkar2 , P Xia2 , A Godley2 , (1) Cleveland State University, Cleveland, Ohio, (2) Cleveland Clinic, Cleveland, OH
Presentations
SU-I-GPD-U-4 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall
Purpose: To measure real-time intra-fractional motion of the prostate bed volume of prostatectomy patients with ultrasound during radiation treatment and to determine margin guidelines for such radiation therapy.
Methods: The Elekta Clarity system was used to monitor in real-time a defined prostate bed position in 3 patients for a total of 76 successful tracking sessions with an average monitoring time of 4 minutes. This volume was defined as the space lining the neck of the bladder to the rectum. The raw displacement versus time data was smoothed with a moving average window of 2 seconds, to remove quickly resolved jumps in the data. Time points were taken from the corrected data when the bed volume initially left a 1, 2 and 3 mm margin. A Kaplan-Meier (KM) estimate was derived to predict the probability that a certain fraction would remain within a given margin at a specific time into tracking.
Results: The maximum shift for any fraction was found to be 5.87 mm, with the average maximum shift over all fractions and patients being 1.9±1.4 mm. At the average treatment time of 4 minutes, the probability of remaining entirely within 1, 2 or 3 mm is 0.16, 0.63 and 0.83, respectively. For all fractions, the average percent of time beyond a 3 mm margin was 0.9%, excluding fractions that remained within the margin, this increases to 15.2%. Only 58% of fractions had the maximum shift occur within the final two-tenths of treatment time, indicating that a post-treatment CBCT may not catch the full extent of intra-fraction motion.
Conclusion: Motion generally increased with treatment time, with the maxima located towards the end of treatment. A 3 mm margin is reasonable for prostate bed treatment < 4 minutes. If smaller margins are desired, real-time tracking would be necessary to gate the treatment.
Contact Email: