MO‐F‐144‐02: Real‐Time 4D Ultrasound Prostate Gland Motion Tracking During Radiotherapy Fraction Delivery

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.