Prostate On Page 335 Immobilization: A planning CT scan is obtained in the treatment position. Patients are treated supine rather than prone as this has been shown to produce less prostate motion, reduce doses to normal organs at risk and is more comfortable for the patient. A bladder filling protocol should be used to maintain a constant bladder filling 'comfortably full'. Patients are asked to empty the bladder and drink 200 mL of water 20 to 40 minutes before the scan and before treatment each day. Bladder scanning during treatment can give helpful feedback to the patient and identify a personalized comfortable volume for the patient. A completely full bladder has been shown to displace small bowel away from the treated volume, but it also leads to greater variation in prostate position and is not recommended. The rectum should be empty for treatment, as a full rectum also leads to greater variation in prostate position. Patients should be advised on a low residue diet, and if they have a full rectum at the time of planning a CT scan, they should receive further dietary advice before repeating the scan and some patients may require daily micro enemas. An immobilization system using a head pad combined with individually adjustable knee and ankle supports provides a high degree of accuracy without the need for further pelvic immobilization (Figure 28.4). Spacer Devices: Biodegradable substances (polyethylene glycol hydrogel, hyaluronic acid and human collagen) can be inserted into the space between the rectum and prostate to temporarily increase the distance between them to reduce the amount of radiation delivered to the rectum. An RCT of a hydrogel spacer has shown that the spacer is safe and reduces the rectal dose, reduces toxicity and improves the patient's quality of life. NICE has recommended that the evidence supports the use of biodegradable spacer devices. The spacer is inserted trans-perineally under general anesthetic two weeks prior to radiotherapy planning. At the time of the spacer insertion, fiducial markers can also be inserted into the prostate. Some spacers are not visible on CT and an MRI scan at the time of planning is needed to co-register with the planning CT for target volume and normal organ definition. CT Scanning: With the patient immobilized in the treatment position following bladder and rectal preparation protocols, a radiotherapy planning CT scan is performed. Skin reference tattoos are placed anteriorly on the midline of the symphysis pubis and laterally over the hips and aligned with lasers to prevent lateral rotation. Radio-opaque markers are placed on the skin to locate the tattoos on the CT scans. The CT scan is taken with 3-5 mm slices from the mid sacroiliac joint to 1 cm below the anus/ischium to include the prostate, seminal vesicles, rectum and bladder, and is extended superiorly to L3 if the pelvic lymph nodes are to be treated. No oral or rectal contrast is used, but intravenous contrast is used to aid delineation of the pelvic lymph nodes. At the time of the planning CT scan, the size of the rectum and bladder should be assessed. If the bladder is empty or the rectum is >4 cm in AP diameter at the level of the prostate base, the scan should be repeated after implementing the bladder and rectal protocols until the desired parameters are met. Inappropriately large rectal volumes have been shown to reduce local control rates. CT data are then transferred to a radiotherapy planning computer for outlining and target volume definition. To improve target definition, MRI scans of the pelvis and PET CT scan images can be fused with CT and incorporated into radiotherapy planning protocols. Outlining studies have shown that the size of the prostate is overestimated on CT compared with MRI, which defines the apex of the prostate better and is associated with less image degradation with fiducial markers. Solutions to overcome the geometrical distortion and shift artifact seen with MRI are currently being investigated. CT-MRI image registration (Figure 28.5) is useful for defining the contour of the prostate, especially at the apex and for identifying hydrogel spacers.