Evaluation of intrafraction prostate motion tracking using the Clarity Autoscan system for safety margin validation

Background: The purpose of the study was to monitor intrafraction prostate motion in real-time using transperineal 4D ultrasound (TPUS) and analyze trajectories to validate clinical safety margins.

Methods: 401 trajectories of US monitoring sessions were retrospectively evaluated for 14 patients treated for prostate cancer. The Elekta Clarity Autoscan system was used for intrafraction monitoring along the 3 directions: superior-inferior (SI), left-right (LR) and anterior-posterior (AP).

Results: The intrafraction monitoring resulted in a mean prostate displacement of (-0.06 ± 0.49) mm, (-0.09 ± 0.61) mm and (-0.01 ± 0.78) mm in the SI, LR and AP directions, respectively. Even though large deviations up to 8 mm were detected, the frequency of occurrence was less than 0.1%. The prostate moved within ±2 mm in 99%, 98.1%, and 96.6% of the treatment time in the SI, LR and AP directions, respectively. During 100 s of monitoring, the median displacement increased from 0.2 mm to 0.8 mm and the maximum displacements increased from 5.2 mm to 7.8 mm. The majority of displacement values (99%) were within the clinical safety margins which ensures a good target coverage.

Conclusions: The largest variation of intrafraction prostate displacement was observed along the AP direction. Throughout most of the treatment time, the prostate moved within a few millimeters. The extent of prostate displacement increased for longer monitoring times. During most of the tracking time, the prostate position was within the clinically safety margins.