PV QA 3 - Poster Viewing Q&A 3
Purpose/Objective(s):To assess and correct the intra-fraction prostate motion in prostate Stereotactic Body Radiation Therapy (SBRT) using an in-house real-time position monitoring system.
Materials/Methods:An online x-ray image based position monitoring system, SeedTracker, was developed to monitor the position of prostate radiopaque fiducial markers. The developed system works in conjunction with the kV imaging system available on some linear accelerators. The monoscopic x-ray images acquired during the treatment delivery are used to monitor the prostate position. The SeedTracker system is currently being used at Southwestern Sydney Local Health District, Australia to monitor the prostate position of the patients treated within PROMETHEUS trial (UTN: U1111-1167-2997) for SBRT boost treatment. The dual arc VMAT plans generated by a planning system were used for the treatment. To date the system has been successfully used for the real-time position verification of 50 patients (100 treatment fractions) on the trial. A gating event criteria of 3mm positional deviation for >5s was used and where the position deviation exceeded the tolerance criteria the treatment was stopped, and the patient was re-positioned using the variable angle stereoscopic imaging available in SeedTracker. In the treatment fractions where gating events occurred, the difference in dose delivered between the treatment with and without real-time monitoring and position corrections was assessed by introducing the observed position deviations, at the respective time of delivery, in treatment plans and recalculating the dose.
Results:The mean (σ) treatment time and dose delivery time of individual SBRT fractions were 32.4(12.6) mins and 5.5(2.3) mins respectively. In 14% of treatment fractions the position deviations were observed at the start of treatment delivery just before turning on the treatment beam and 79% of time this occurred at the first treatment fraction. A continuous drift in prostate position during the treatment delivery, which resulted in a gating event, was observed in 12% of the treatment fractions and 42% of these events are multiple occurrences in the same fraction. In 21% of the treatment fractions a slow continuous drift occurred, but was less than the threshold deviation. Where these deviations occurred during the delivery of the first treatment arc, the position correction was performed before the start of the second treatment arc. A transient excursion of the prostate position occurred in 5% of the treatment fractions. The retrospective dose reconstruction study showed that the prostate D98 would have decreased by a maximum 12% compared to the planned D98 in some cases if real-time position monitoring had not been performed and position corrections were not under taken.
Conclusion:The real-time position monitoring of prostate SBRT was successfully achieved using an in-house system and the developed system has been shown to improve the accuracy of treatment delivery.
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