MRI and CT scans for treatment planning were performed 7 days after fiducial implantation to ensure adequate tissue fixation and to allow for resolution of procedural edema/inflammation (Poggi et al., 2003 (link); Pouliot et al., 2003 (link); Kupelian et al., 2005 (link)). Precautions were taken to minimize prostate motion during the planning scans and treatment. Specifically, starting 5 days prior to acquisition of the planning scans until the end of treatment, patients maintained a low fiber diet to reduce intestinal gas (Smitsmans et al., 2008 (link)). Fasts were also initiated 4 h before both acquisition of the planning scans and each treatment session to minimize rectal movement. Enemas were performed 2 h prior to acquisition of the planning scans and each treatment session to minimize rectal volume. All patients were imaged and treated in the supine treatment position with a knee cushion to maximize patient comfort and limit prostate motion in response to respiration (Malone et al., 2000 (link)).
Fused thin cut CT images (1.25 mm) and high-resolution MR images were used for treatment planning (Figures 3A–C). MRI imaging was employed to define the target volume (Roach et al., 1996 (link)) and to aid in accurate localization of the bladder neck, membranous urethra, and penile bulb (Mclaughlin et al., 2005 (link)). Intra-prostatic fiducials were employed to guide image co-registration and limit fusion errors (Parker et al., 2003 (link)). MR images were obtained on a 1.5-T scanner in a phased-array torso coil. The MRI was quickly (<1 h) followed by a CT scan to minimize anatomical changes in the rectum that may interfere with image fusion. Two MRI sequences were employed to maximize visualization of the fiducials (susceptibility-weighted gradient-echo images) and the soft tissues (axial high-resolution turbo T2-weighted spin-echo images). Treatment planning included the prostate as the gross target volume (GTV). The CTV included the prostate and the proximal seminal vesicles. The PTV equaled the CTV expanded by 3 mm posteriorly and 5 mm in all other dimensions. Treatment planning was generally completed within 1 week of imaging and subsequent treatment was initiated within a 1- to 2-week window.