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Real-Time 4D Ultrasound Prostate Gland Motion Tracking During Radiotherapy Fraction Delivery

R Sandhu

R Sandhu*, O Marina, J Wloch, S Martin, D Krauss, D Yan, D Ionascu, William Beaumont Hospital, Royal Oak, MI

MO-F-144-2 Monday 4:30PM - 6:00PM Room: 144

Purpose: The intra-fraction variability of target position during the prostate cancer radiotherapy may cause dose discrepancy between planned and delivered dose, especially with longer hypo-fractionated treatments. We report our clinical experience with real-time 4D ultrasound imaging (4D-US) to monitor intrafraction prostate motion.

Methods: Three prostate patients were treated on an IRB-approved protocol delivering 51 Gy in 10 fractions using single arc volumetric modulated arc therapy (VMAT). Each patient had three gold markers implanted and had simultaneous CT and 4D-US simulation, followed by an MRI scan. Target and normal organs were delineated on MR images. During setup simultaneous cone-beam CT (CBCT) and continuous 4D-US were acquired, and during VMAT delivery (about 2 min) 4D-US was acquired. The prostate 4D-US position was compared to the CBCT average position, and movement during treatment was characterized.

Results: The median (range) of mean intra-fraction prostatic motion in the right-left(RL), anterior-posterior(AP) and superior-inferior(SI) directions were 0.1 mm (-1.6 to 0.8 mm), 0 mm (-1.8 to 1.3 mm), and -0.1 mm (-2.2 to 1.4 mm), with respective median (range) of standard deviation were 0.2 mm (0 to 0.8 mm), 0.2 mm (0 to 1.2 mm), and 0.2 mm (0 to 0.7mm). There were 9/27 fractions with shifts >=2 mm in any direction, with an average duration of 23% of treatment time, with a single fraction having a shift greater than 3mm. The discrepancy between 4D-US and CBCT shifts were 0.6±1.6 mm, -0.2±1.4 mm and -0.4±0.7 mm in the RL, AP and SI directions. There was one instance of flatulence during treatment setup where vertical shifts >=3 mm (up to 6.1 mm) persisted for 108 sec.

Conclusion: Real-time imaging is essential for tracking hypo-fractionated prostate motion to reduce dosimetric uncertainty. 4D ultrasound imaging during treatment improves accuracy of dose delivery, and may allow a reduction of treatment margins.

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