• Wan-Yu Lin, Keh-Bin Wang, Shih-Chung Tsai, and Shung-Shung Sun.
• Department of Nuclear Medicine, Taichung Veterans General Hospital, Taichung, Taiwan. wylin@vghtc.gov.tw
• Clin Nucl Med. 2009 Sep 1; 34 (9): 560-3.

Background: Bladder irrigation and retrograde filling technique has been used to reduce urinary F-18 FDG (FDG) activity for better image interpretation in patients with pelvic tumors. Despite the zealous use of this technique, FDG accumulation in the urinary bladder has been reported and might cause false-positive or false-negative results. In this study, we analyzed the pattern and estimated the incidence of the unexpected accumulation of urinary FDG activity after bladder irrigation and retrograde filling with sterile saline.Methods: We reviewed 53 patients with pelvic malignancies who underwent FDG-PET scans. All of them had bladder irrigation and retrograde filling with sterile saline via triple lumen Foley Catheters. If there was high FDG activity, either focal or diffuse, in the urinary bladder on the standard 1-hour image, bladder irrigation and retrograde filling were repeated and the delayed 3-hour images were acquired. If delayed 3-hour scanning was not possible due to time restraint or the patient's inability to cooperate, pelvic images were immediately acquired while the patient changed to the prone position.Results: On the 1-hour images, 43 of the 53 (81.1%) patients showed low urinary FDG activities and satisfactory image quality for interpretation. However, 10 (18.9%) patients had high FDG activity in the urinary bladder, which could interfere with pelvic image interpretation. Of these 10 patients, 3 (5.7%) patients had diffuse FDG activity in the urinary bladder, which could obliterate a nearby FDG-avid lesion. All of the diffuse urinary FDG activity was cleared with repeated bladder irrigation and retrograde filling. The remaining 7 (13.2%) patients had focal FDG-accumulation in the bladder, which might cause false-positive result. Five of them had repeated bladder irrigation and retrograde filling and all of the focal FDG-accumulations disappeared on the delayed images. Bladder irrigation and retrograde filling could not be performed in 2 patients and pelvic images were acquired after the patients changed to the prone position. The urinary FDG accumulation changed from the posterior portion of the bladder in the supine position to the anterior portion of the bladder in the prone position.Conclusion: Unexpected FDG activity in the urinary bladder, either focal or diffuse, may occur after bladder irrigation and retrograde filling and interfere with pelvic image interpretation. Great caution is required to avoid misdiagnosis: focal FDG accumulation may mimic tumor uptake of FDG whereas diffuse FDG activity may obliterate a FDG-avid pelvic lesion. Repeated bladder irrigation and retrograde filling, and prone-position imaging are useful techniques to ascertain the nature of the FDG accumulation.

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