Ultrasound image fusion for external beam radiotherapy for prostate cancer

Purpose : To determine whether real-time ultrasound imaging and targeting system for the treatment of prostate cancer was feasible. The initial phase of this project included a study to develop and determined (a) software for the fusion of ultrasound images to standard x-rays obtained during simulation, and (b) the potential reduction in field size with real time imaging. Methods and Materials : During 13 patient simulations a transrectal ultrasound image was obtained. Orthogonal x-ray films were acquired with the rectal probe in place. Both the x-ray and ultrasound images were digitized and a fusion image was created of the prostate position in relation to the probe, bladder, and rectum. The two-dimensional area of the rectum, bladder, and prostate was determined in the lateral projection. Potential conformal blocks were designed for the lateral portals in a four-field treatment technique. Results : The transrectal ultrasound probe enabled real-time prostrate imaging. The lateral field size can be reduced to 6.08 × 5.68 cm 2 ± 0.62 × 0.48 cm 2 from the standard 8 × 8 cm 2 field. The posterior rectal wall was physically displaced out of the lateral field. The area of the rectum included in the lateral field in 1.75 cm 2 ± 0.85 cm 2. Conclusion : The prostate position can be determined with certainty on a regular basis with transrectal ultrasonography. The amount of normal tissue in the high dose volume can be reduced. This approach may reduce acute and chronic morbidity and allow further dose escalation.