Detecting primary bladder cancer using delayed ¹⁸F-2-fluoro-2-deoxy-D-glucose-positron emission tomography/computed tomography imaging after forced diuresis

INTRODUCTION

High-uptake of ¹⁸F-2-fluoro-2-deoxy-D-glucose (FDG) in urothelial cancer of the bladder was already reported in 1991.[1] Nowadays, FDG-positron emission tomography combined with computed tomography (FDG-PET/CT) is an established standard for pre-operative staging and detecting metastatic lesions of bladder cancer.[234] The visualization of primary bladder tumors with FDG-PET/CT, however, has remained problematic. FDG is excreted into the urine, causing high-FDG activity in the bladder. Standard FDG-PET/CT protocols are therefore, not useful for imaging or response evaluation of bladder tumors.

Elimination of accumulated FDG from the urinary system is essential to overcome this limitation of FDG-PET/CT. Several strategies have been proposed to overcome this limitation.[5678910] The most promising strategy seems to be delayed pelvic FDG-PET/CT imaging after adequate hydration and voiding. A limited number of small studies have assessed the role of delayed PET/CT in the detection of bladder cancer. However, studies lacked standardization of the technique and the majority of patients had recurrent bladder cancer after previous cystectomy.

The purpose of this study was to evaluate the use of delayed pelvic FDG-PET/CT imaging, according to a standardized protocol including pre-hydration and forced diuresis, for the detection of primary bladder cancer.

MATERIALS AND METHODS

Patients

All patients with primary bladder cancer referred to our hospital from June 2011 to January 2012 for whole-body FDG-PET/CT were retrospectively included. All had previously undergone transurethral resection (TUR) of the tumor for confirmation of the diagnosis. The patients were referred to the bladder cancer out-patient clinic of our hospital for staging and definitive treatment (radical cystectomy, brachytherapy, neoadjuvant chemotherapy, irradiation or a combination of therapies). Baseline characteristics of the patients are summarized in Table 1.

Table 1 Baseline patient characteristics

RESULTS

The procedure was completed successfully in 37/38 patients. In one patient the procedure failed: At standard imaging the bladder did not contain urine. This patient had a large cT4 bladder tumor which almost covered the entire bladder and obstructed the ureteral ostia causing severe bilateral hydroureteronephrosis. A percutaneous nephrostomy and a transurethral catheter were placed. We excluded this patient from our accuracy analysis. Delayed images after furosemide and oral hydration showed reduction of urinary bladder activity to (near) background levels in 17 of 37 remaining cases (46%). At delayed imaging there was a significantly lower mean bladder SUV_max (7.5 ± 4.8 vs. 17.3 ± 11.0, P < 0.001) and mean bladder SUV_mean (5.9 ± 4.8 vs. 15.2 ± 4.0, P = 0.001) than at standard imaging. Standard FDG-PET/CT detected hypermetabolic bladder lesions in 15 patients (40.5%). Delayed FDG-PET/CT detected hypermetabolic bladder lesions in 30 patients (81.1%).

Accuracy for the presence of tumor

The reference standard revealed a bladder tumor in 26/37 cases. The remaining 11 patients were considered to be pathologically or clinically free of primary bladder tumor (total removal of tumor by TUR). First, we calculated the diagnostic values with indeterminate lesions considered positive. Table 2 displays estimates of sensitivity, specificity and predictive values of standard and delayed FDG-PET/CT. Delayed imaging reduced the number of false negative results (14 vs. 3), but yielded 4 additional false positive findings (3 vs. 7). The difference in proportion of correct diagnoses tended towards significance (P = 0.059). Figure 1 shows a good reduction of FDG-activity in the bladder at delayed images, and a true positive result. We repeated the analysis after redefining indeterminate lesions as being no tumor [Table 3]. In this setting, delayed imaging reduced the number of false negative results much more (20 vs. 4), but yielded 2 additional false positive findings (1 vs. 3). The proportion of correct diagnosis was significantly higher with delayed imaging than that with standard imaging (P = 0.001). The proportion of correct diagnosis was higher with indeterminate lesions considered negative, but this difference did not reach statistical significance (P = 0.240).

Table 2 Sensitivity and specificity of standard and delayed FDG-PET/CT for detecting residual bladder cancer after transurethral resection (dichotomized scored discrete categories; category transurethral resection 1 [no tumor: FDG-PET/CT negative] versus category 2-3 [uncertain tumor or definite tumor: FDG-PET/CT positive]), compared to reference standard. There was a statistically significant difference in sensitivity between standard and delayed imaging (P<0.001)

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Figure 1 An 83-year-old male with a solitary cT2N0M0 G3 UCC of the bl adder. ¹⁸F-2-fluoro-2-deoxy-D-glucose-positron emission tomography combined with computed tomography (FDG-PET/CT) and FDG-PET (from left to right) images before furosemide, (a) show high FDG activity in the bladder standardized uptake values (SUV_max = 22.2). Delayed pelvic images after intravenous furosemide and oral hydration, (b) show good tracer washout in the bladder (SUV_max = 5.0). It is clearly possible to identify uptake in the right bladder wall, which can be delineated with 50% isocontour (SUV_max = 16.1). CE-CT (c) showed focal wall thickening, corresponding to the intense uptake area on FDG-PET/CT. The tumor was also visualized by routine cystoscopy. The patient was treated with brachytherapy preceded by external radiotherapy because of a solitary cT2N0M0 G3 UCC of the bladder

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Table 3 Estimates of sensitivity, specificity, and predictive values of standard and delayed FDG-PET/CT for residual bladder cancer after transurethral resection detecting bladder tumors (dichotomized scored discrete categories; category 1-2 (no tumor or uncertain tumor: FDG-PET/CT negative) versus category 3 (definite tumor: FDG-PET/CT positive), compared to reference standard. There was again a statistically significant difference in the sensitivity of standard and delayed imaging (P<0.001)

Indeterminate and inaccurate findings

Eight of 15 visible lesions (53.3%) at standard imaging were indeterminate. After furosemide and oral hydration, five of these indeterminate lesions were clearly visible as bladder tumors, of which four were confirmed by the reference standard and one turned out to be false positive. The three other indeterminate lesions remained indeterminate. Additionally, two other indeterminate lesions became visible at delayed images in patients in whom no bladder tumor was visible at standard images. According to the reference standard, four of the five indeterminate lesions at delayed imaging were negative, one was positive.

The proportion of inaccurate findings at delayed imaging was 10/37. Delayed pelvic FDG-PET/CT yielded seven false positive and three false negative results. False positive results were seen for several reasons or a combination of reasons. In 4/7 false positive results, FDG-PET/CT was performed < 6 weeks after TUR. Histopathology (cystoprostatectomy, TUR or biopsy) showed inflammatory changes in these patients, a well-known cause of increased FDG-uptake [Figure 2]. In three patients, a layer of FDG active urine was present in (a) a TUR lesion, (b) a diverticulum of the bladder wall or (c) in the dorsal surface [Figure 3]. In these cases, FDG-active urine was not sufficiently eliminated, mimicking a tumor. Six of the seven false positive results occurred in patients with non-muscle invasive bladder cancer. False negative results were mainly caused by insufficient reduction of FDG-activity in the bladder. When redefining indeterminate lesions as being no tumor, the proportion of inaccurate findings was 7/37.

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Figure 2 A 59-year-old male with a muscle invasive squamous cell carcinoma in the left dorsal bladder wall, who had a transurethral resection (TUR) and a second TUR. The last TUR was performed 4 weeks prior to ¹⁸F-2-fluoro-2-deoxy-D-glucose-positron emission tomography combined with computed tomography (FDG-PET/CT) imaging. Standard FDG-PET/CT and FDG-PET (from left to right) images, (a) show high FDG activity in the bladder (SUV_max = 18.7). At delayed pelvic images, (b) FDG-activity in the bladder is reduced to near background levels (SUV_max = 3.2). Suspect FDG accumulation in the dorsocaudal wall of the bladder can be identified (SUV_max = 8.0). CE-CT, (c) showed focal wall thickening, corresponding to the intense uptake area on FDG-PET/CT. At cystoscopy a widespread area of necrosis was seen, but no active tumor. A cystoprostatectomy was performed. Histopathology of the specimen revealed no residual tumor. Extensive inflammatory changes were found in the area with FDG accumulation. The false-positive area of metabolic activity may be due to inflammation after recent TUR

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Figure 3 A 64-year-old male who underwent TUR of a papillary T1 tumor in a bladder diverticulum near the ureteral left orifice. Standard FDG-PET/CT and FDG-PET (from left to right) images, (a) show high FDG activity in the bladder (SUV_max = 21.8). Delayed pelvic images, (b) show a reduction of FDG-activity (SUV_max = 9.7), though not to background levels. Indeterminate FDG accumulation in the dorsal bladder wall and in the diverticula can be identified (SUV_max = 86.5). The differential diagnosis includes a (residual) bladder tumor, or a layer of FDG active urine. Contrast enhanced-CT, (c) and follow-up revealed no signs of tumor and/or recurrence. The false-positive area of metabolic activity might be due to insufficient elimination of FDG active urine, mimicking a tumor

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DISCUSSION

The present study evaluates the use of delayed pelvic FDG-PET/CT imaging, according to a standardized protocol including pre-hydration and forced diuresis, for the detection of primary bladder cancer. Bladder tumors are usually not visible using standard PET/CT due to FDG active urine in the bladder at the time of scanning. Several strategies have been proposed to improve PET/CT imaging of bladder tumors. First, catheter-assisted FDG-PET/CT imaging has been investigated as a possible solution,[1112] but this is an invasive procedure which does not yield optimal results. Moreover, it increases the exposure of workers to ionizing radiation. Protocols applying diuretics proved to reduce bladder urinary FDG-activity and increase bladder size,[131415] but could not decrease FDG-activity to background levels. Subsequently, delayed imaging protocols after furosemide administration were proposed as a less invasive alternative for catheter-assisted FDG-PET/CT by Anjos et al. who showed 100% sensitivity and specificity for the detection of recurrent bladder tumors.[5] These results were confirmed by two other retrospective studies.[610] Delayed pelvic FDG-PET/CT imaging is therefore considered a promising technique in detecting recurrent bladder tumors.

Accordingly, the present study demonstrates that delayed pelvic FDG-PET/CT imaging also has significantly better sensitivity and success of quantification than standard FDG-PET/CT imaging for the detection of primary bladder tumors. However, despite adhering to a standardized protocol, delayed pelvic imaging did not reduce bladder FDG activity to (near) background levels in more than half of the patients. Not all indeterminate lesions could be clarified with the help of delayed imaging: Indeterminate lesions remained present, albeit considerably fewer than reported at standard FDG-PET/CT images, but also additional false-positive lesions were diagnosed on delayed PET/CT.

As illustrated in our series, multiple factors may cause inaccurate results. Homogeneous FDG-active urine in the bladder can hamper tumor recognition, resulting in false negatives. On the other hand, retention of concentrated urine in a diverticulum can cause false positive results. Incorrect or indeterminate results were also obtained for non-muscle invasive tumors after recent TUR. TUR can eradicate tumors completely, particularly non-muscle invasive tumors,[16] whereas inflammatory tissue reactions with enhanced metabolism can cause increased regional FDG-uptake, leading to avidly FDG-active false positive lesions. Since four of the five indeterminate visible lesions were inaccurate, the best accuracy was obtained when we considered indeterminate lesions as being no tumor.

Next to the difference in primary tumor and recurrent disease, other differences between the current study and the existing body of literature need to be discussed. Anjos et al. managed to avoid false positive results due to inflammatory reaction after biopsy or TUR by introducing a 3-month time interval between the resection and the FDG-PET/CT scan. When using FDG-PET/CT as a staging modality in potentially curable pre-treatment patients, a 3-month delay is not acceptable. Yang et al., on the other hand, did not use a standardized protocol: Delayed PET/CT imaging was performed 1-3 h after the administration of FDG. If the delayed pelvic images still showed high concentration of FDG, more additional images were performed. These methodological differences may explain why other studies showed more favorable results.

Limitations of the present study are mainly those inherent to its retrospective design. Another potential drawback is the lack of histopathological studies in a subset of patients. Follow-up imaging with CE-CT, MRI, and cystoscopy are not perfect in identifying bladder tumors, particularly following previous biopsy, TUR or intravesical chemotherapy. However, we considered a combination of these imaging modalities and clinical findings during at least 6 months follow-up, as the most appropriate “second-best” reference standard in the absence of histopathology.

The authors acknowledge that interpretation according to a 3-point scale including an “indeterminate” option is subjective and may lead to interobserver variability. In clinical practice, however, it is not always possible to make a clear distinction between positive and a negative lesions, since a SUV cut off value is not applicable in this context. Another strength of this study is the fact that the image sets are evaluated by two observers, blinded for patient data.

A strength of the present study was the standardized protocol used to perform delayed PET/CT imaging. Moreover, this study closely resembled to clinical practice. The setting of the current study was a bladder cancer outpatient clinic of a tertiary cancer hospital. Patients with proven bladder cancer were referred for further treatment. PET/CT images were obtained for staging bladder cancer–currently the only situation for which there is convincing evidence that PET/CT is of additional value.[234] When a PET/CT has to be performed, delayed pelvic imaging can be used as an attempt for baseline imaging of the situation. Whether delayed pelvic PET/CT is really of additional clinical value has yet to be substantiated by larger prospective cohorts.

In conclusion, these data suggest that delayed pelvic FDG-PET/CT imaging after forced detects more primary bladder tumors than standard FDG-PET/CT protocols. However, indeterminate bladder lesions on delayed PET/CT remain a problem and should be interpreted cautiously in order to avoid false positive results.