IMPACT OF ULTRASOUND AND COMPUTED-TOMOGRAPHY PROSTATE VOLUME REGISTRATION ON EVALUATION OF PERMANENT PROSTATE IMPLANTS

Purpose: Ultrasound (US)-guided permanent prostate implants typically use US prostate volumes to plan the implant procedure and CT prostate volumes for 3D dosimetric evaluation of the implant. Such a protocol r equires that CT and US prostate volumes be registered. We have studied the impact of prostate volume registration on postimplant dosimetry f or patients with low-grade prostate cancer treated with combined US an d fluoroscopic-guided permanent implants. Methods and Materials: A US image set was obtained with the patient in the lithotomy position to d elineate the prostate volume that was subsequently used for treatment planning. Each plan was customized and optimized to ensure complete co verage of the US prostate volume. After implant, a CT scan was obtaine d for postimplant dosimetry with the patient lying supine. Sources wer e localized on CT by interactively creating orthogonal images of small cubes, whose dimensions were slightly larger than the source, to assu re unique identification of each seed. Ultrasound and CT 3D surfaces w ere registered using either (a) the rectal surface and base of the pro state, or (b) the Foley balloon and urethra as the alignment reference . A dose distribution was assigned to the US prostate volume based on the CT source distribution, and the dose-volume histogram (DVH) was ca lculated. Result: Prostate volumes drawn from US images differ from th ose drawn from CT images with the CT volumes being typically larger th an the US volumes. Urethral registration of the prostate volume based on aligning the prostatic urethra generates a dose distribution that b est follows the preimplant plan and is geometrically the preferable ch oice for dosimetry. Conclusion: The dose distribution and the DVH for the US prostate is sensitive to the mode of registration limiting the ability to determine if acceptable dose coverage has been achieved. (C ) 1997 Elsevier Science Inc.