Purpose: To evaluate inter- and intrafraction organ motion with an ultrasound-based prostate localization system (BAT) for patients treated with intensity-modulated radiotherapy for prostate cancer.
Patients and methods: After set-up to external skin marks, 260/219 ultrasound-based alignments were performed before/after irradiation in 32 consecutive patients. Image quality was classified as good, satisfactory or poor. Patient- and imaging-related parameters were analyzed to identify predictors for poor image quality. Shifts in relation to the treatment planning computed tomography (CT) were recorded before/after irradiation in the superior-inferior (SI), anterior-posterior (AP) and right-left (RL) directions to determine inter-/intrafraction prostate motion.
Results: The thickness of tissue anterior to the bladder and bladder volume during the ultrasound localization as well as an inferior prostate position relative to public symphysis (determined in treatment planning CT) were found to be independent predictors of a poor image quality. Interfraction shifts (mean+/-standard deviation: -0.2+/-4.8 [SI], 2.4+/-6.6 [AP] and 1.9+/-4.6 [RL]) varied much stronger than intrafraction shifts (0.0+/-2.0 [SI], 0.6+/-2.2 [AP] and 0.2+/-1.9 [RL]). A larger pressure of the ultrasound probe (determined as a larger reduction of the distance abdominal skin to prostate between the planning CT and the ultrasound) was applied in case of poor image quality, associated with larger systematic posterior prostate displacements.
Conclusion: Intrafraction prostate shifts are considerably smaller in comparison to interfraction shifts. Bladder filling and a small pressure on the ultrasound probe are crucial to achieve an adequate image quality without systematic prostate displacements.