Preliminary Study of Detecting Urothelial Malignancy with FDG PET in Taiwanese ESRD Patients

Discussion

Unlike the effectiveness of FDG PET in detecting distant and recurrent urothelial malignancies, results for the evaluation of locoregional disease are poor (13). Some procedures have been proposed to improve the efficacy of FDG PET. Kosuda et al. (14) used retrograde saline irrigation of the urinary bladder to remove ¹⁸F-FDG radioactivity. Leisure et al. (15) and Vesselle et al. (16) utilized diuretics, intravenous saline infusion and bladder catheters to eliminate artifacts from the urinary system. However, the need for transuretheral catheterization was invasive and uncomfortable and the effectiveness was still variable. Kamel et al. (17) used furosemide forced diuresis and parenteral hydration and significantly improved the sensitivity of FDG PET for detecting ureteral and bladder malignancies. However, limited improvement was observed when the lesions were in the renal regions.

Several new PET tracers were developed (9, 18) to address the great obstacle of physiological urinary FDG radioactivity in the detection of malignancies in the urinary system and limited current available solutions. ¹¹C-Methionine PET was reported to be superior to ¹⁸F-FDG PET (9). ¹¹C-Choline was also tried due to its low level of urinary excretion. Picchio et al. (19) and Gofrit et al. (20) found better results with ¹¹C-choline PET and PET/CT than CT alone in the detection of nodal metastases. However, the relatively short physical half-life and inconvenience/limited availability of ¹¹C still make ¹¹C-methionine and ¹¹C-choline less clinically practical.

In contrast, owing to the characteristic oliguria or auria of patients with ESRD, we assumed that FDG PET of urothelial malignancies would possibly not be interfered with by the usual abundant physiological excretory urinary radioactivity of FDG. Our findings revealed little difference of sensitivity between FDG PET and CT. Nevertheless, excretory urinary FDG radioactivity was not universally absent even though these patients had ESRD. However, a true lesion could be intuitively interpreted if any focalized FDG radioactivity in the urinary system higher than the background radioactivity in the routine whole-body images of those patients who did not have excretory urinary radioactivity was found (only one false-positive finding that was in reality inflammation but appeared as a true-positive lesion on FDG PET occurred). CT was somewhat more complicated because of its various manifestations in urothelial malignancies.

In addition, a tendency towards a positive correlation between the sensitivity of detecting urothelial malignancies by FDG PET and the histological malignant grade or tumor stage was found. This finding may reflect the fact that the single most prognostic factor of urothelial malignancies is the tumor stage and that is also positively correlated with the histological malignant grade. Therefore, FDG PET might potentially be able to play a role in prognosis in addition to its ability to detect urothelial malignancies.

There were 9 false-negative FDG PET findings, including 3 lesions smaller than 5 mm maximum length which were beyond the image resolution of PET, 2 lesions in the urinary bladder with the presence of excretory urinary radioactivity and a small perirenal lymph node metastasis obscured by the radioactivity of an adjacent huge renal pelvic tumor (50 mm maximum length). Another lesion in the urinary bladder without excretory urinary radioactivity which measured 5 mm maximum length, might still have been influenced partially by the volume effect. The reason for the other two false-negative findings was unknown. There were two false-positive findings, one was an inflammatory lesion and the other was focal persistent radioactivity in the urinary bladder where there was excretory urinary FDG radioactivity from the graft kidney. It seemed that although diuretics were applied, there was still limited improvement in the interpretation of the urinary bladder finding if excretory urinary radioactivity was still present.

Overall, the present study, albeit a small series, showed a possible feasible supplementary method using FDG PET to evaluate urothelial malignancies of ESRD patients who have significantly higher risk than the general population and whose urothelial malignancies are usually synchronous and multifocal when they manifest suspicious symptoms (5). The possibility for FDG PET to play a potential prognostic role was also revealed. However, FDG PET should still serve as a supplement to the current standard invasive studies because a negative FDG PET scan cannot obviate the need for invasive studies for histological proof, especially when the lesions may be too small to be detected by FDG PET. On the other hand, any focalized FDG radioactivity in the urinary system without excretory urinary radioactivity in a uremic patient should prompt further investigation of the highly possible urothelial malignacy.