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Assessment of Intrafraction Motion Based On Intraprostatic Fiducials During VMAT Delivery

R Tolakanahalli1*, S D Davis1, (1) Miami Cancer Institute, Miami, FL


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: Use of fiducial markers during treatment for image-guided intrafraction motion detection is often used for hypofractionated prostate treatments with tight margins.

Methods: Clinical implementation for patients with intraprostatic fiducials receiving volumetric modulated arc therapy (VMAT) utilizes periodic on-board kV imaging with 40° intervals during treatment delivery and an 8 mm tolerance diameter for tracking fiducials. All patients were initially setup using surface imaging. Motion was corrected if a single fiducial was outside of tolerance. We retrospectively analyzed 15 patients and 75 CBCTs with associated 2D kV images to analyze the changes in anatomy leading to re-setup. Time elapsed between CBCT and treatment was recorded for each fraction, and initial shift and intrafraction shifts based on either kV image/MV-kV orthogonal pair or repeat CBCT were analyzed.

Results: Across 15 patients treated with a 5 fractions, the initial mean vertical, longitudinal, and lateral shifts and pitch, roll and yaw corrections was 0.3±0.5 cm, -0.4±0.9 cm, 0.0±0.4 cm, -0.2±1.4°, 0.0±1.0°, and -0.2±0.9°. In 48% of fractions, subsequent shifts were required based on intrafraction imaging. Analysis of the shifts showed the origin of intrafraction motion mainly due to three reasons: bladder filling after CBCT and during treatment, movement of gas through the rectum, lack of lateral shift information during the first kV intrafraction image, and patient hip flexion/relaxation. The mean time between CBCT acquisition and initial beam-on was 9.6±2.7 min. Intrafraction shifts tended to be smaller, with mean vertical, longitudinal, and lateral shifts of 0.1±0.2 cm, 0.1±0.3 cm, and 0.0±0.1 cm. The maximum intrafraction shift recorded was 0.7 cm.

Conclusion: Periodic kV imaging with automatic detection of motion during VMAT prostate treatments is commercially available, and can be successfully implemented to mitigate effects of intrafraction motion with careful attention to software settings.


Prostate Therapy, Image-guided Therapy


Not Applicable / None Entered.

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