Recent developments in urologic oncology: positron emission tomography molecular imaging

Breathing-induced Errors in Quantification and Description of Dominant Intra-Prostatic Lesions (Dils) in PET Images: A Simulation Study by Means of The 4D NCAT Phantom

BACKGROUND

Respiratory movement and the motion range of the diaphragm can affect the quality and quantity of prostate images.

OBJECTIVE

This study aimed to investigate the magnitude of respiratory-induced errors to determine Dominant Intra- prostatic Lesions (DILs) in positron emission tomography (PET) images.

MATERIAL AND METHODS

In this simulation study, we employed the 4D NURBS-based cardiac-torso (4D-NCAT) phantom with a realistic breathing model to simulate the respiratory cycles of a patient to assess the displacement, volume, maximum standardized uptake value (SUV_max), mean standardized uptake value (SUV_mean), signal to noise ratio (SNR), and the contrast of DILs in frames within the respiratory cycle.

RESULTS

Respiration in a diaphragm motion resulted in the maximum superior-inferior displacement of 3.9 and 6.1 mm, and the diaphragm motion amplitudes of 20 and 35 mm. In a no-motion image, the volume measurement of DILs had the smallest percentage of errors. Compared with the no-motion method, the percentages of errors in the average method in 20 and 35 mm- diaphragm motion were 25% and 105%, respectively. The motion effect was significantly reduced in terms of the values of SUV_max and SUV_mean in comparison with the values of SUV_max and SUV_mean in no- motion images. The contrast values in respiratory cycle frames were at a range of 3.3-19.2 mm and 6.5-46 for diaphragm movements' amplitudes of 20 and 35 mm.

CONCLUSION

The respiratory movement errors in quantification and delineation of DILs were highly dependent on the range of motion, while the average method was not suitable to precisely delineate DILs in PET/CT in the dose-painting technique.

Evaluation of Dose-Painting in the Dominant Intraprostatic Lesions by Radiobiological Parameters using 68Ga- PSMA PET/CT

BACKGROUND

Patients diagnosed with dominant intraprostatic lesions (DIL) may need radiation doses over than 80 Gy. Dose-painting by contours (DPC) is a useful technique which helps the patients. Dose-painting approach need to be evaluated.

OBJECTIVE

To evaluate the DCP technique in the case of boosting the DILs by radiobiological parameters, tumor control probability (TCP), and normal tissue complication probability (NTCP) via PET/CT images traced by ⁶⁸Ga-PSMA.

MATERIAL AND METHODS

In this analytical study, ⁶⁸Ga-PSMA PET/CT images were obtained from patients with DILs that were delineated using the Fuzzy c-mean (FCM) algorithm and thresholding methods. The protocol of therapy included two phases; at the first phase (ph1), a total dose of 72 Gy in 36 fractions were delivered to the planning target volume (PTV1); the seconds phase consisted of the application of variable doses to the PTV2. Moreover, two concepts were also considered to calculate the TCP using the Zaider-Minerbo model.

RESULTS

The lowest volume in DILs belonged to the DIL1 extracted by the FCM method. According to dose-volume parameters of the rectum and bladder, by the increase in the PTV dose higher than 92 Gy, the amounts of rectum and bladder doses are increased. There was no difference between the TCPs of DILs at doses higher than 86 Gy and 100 Gy for ordinary and high clone density, respectively.

CONCLUSION

Consequently, our dose-painting approach for DILs, extracted by the FCM method via PET/CT images, can reduce the total dose for prostate radiation with 100% tumor control and less normal tissue complications.

18F-fluorodeoxyglucose positron emission tomography/ computed tomography in the diagnosis of suspected paraneoplastic syndromes: A retrospective analysis

Paraneoplastic syndromes are a rare clinical presentation of tumor thought to affect 0.01% of patients with cancer. Paraneoplastic syndromes present a diagnostic challenge as a wide variety of signs and symptoms may appear. This study examines the use of ¹⁸F-fluorodeoxyglucose positron emission tomography/computed tomography (¹⁸F-FDG PET/CT) as a diagnostic imaging tool for detecting tumor in suspected paraneoplastic syndrome cases. This single-center retrospective study included patients with suspected paraneoplastic syndrome who underwent whole-body ¹⁸F-FDG PET/CT scan between December 2005 and December 2016. Associated clinical data were gathered via electronic chart review. Patient records were reviewed for age, sex, clinical signs and symptoms, ancillary diagnostic procedures, date of diagnosis, and follow-up time. Ninety-nine patients met inclusion criteria for this study. Mean follow-up period was 1.8 years. Cancer prevalence was 12.1%. The ¹⁸F-FDG PET/CT results are as follows: 10 true positives, 5 false positives, 82 true negatives, and 2 false negatives. The diagnostic values are as follows: sensitivity 83.3%, specificity 94.3%, positive predictive value 66.7%, and negative predictive value (NPV) 97.6%. The high NPV in our study supports the effectiveness of ¹⁸F-FDG PET/CT to rule out tumor in suspected paraneoplastic syndrome. Future research aims to analyze which patients with suspected paraneoplastic syndrome would benefit most from ¹⁸F-FDG PET/CT.

Role of contrast-enhanced 18F-FDG PET/CT imaging in the diagnosis and staging of renal tumors

PURPOSE

The objectives of this prospective study are to compare intravenous contrast-enhanced (CE) fluorine-18-fluorodeoxyglucose (F-FDG) PET/computed tomography (CE F-FDG PET/CT) with conventional methods (CT/MRI) and to evaluate the relationship of maximum standardized uptake value (SUVmax) with Fuhrman grade in patients with renal tumors.

PATIENTS AND METHODS

A total of 62 patients [35 males and 27 females; mean age 55.8±12.7 (range: 27-81) years] were enrolled in the study. CE F-FDG PET/CT scanning included whole-body (early) and abdominal imaging (late) 1 and 2 h after intravenous F-FDG administration, respectively. SUVmax was calculated for primary tumors. CE F-FDG PET/CT and CT/MRI findings were compared with respect to primary tumors and staging.

RESULTS

The sensitivity of CE F-FDG PET/CT in primary tumor detection was 98%, which was very close to that of CT/MRI (100%). CE F-FDG PET/CT resulted in correct staging in 84% of the cases, compared with 68% of the cases with conventional methods (52 vs. 42 patients). SUVmax values of early PET for the primary tumors were significantly correlated with the Fuhrman grades (P<0.001). CE F-FDG PET/CT enabled the detection of synchronous tumors in four patients, one of which was incorrectly diagnosed as having metastasis by CT. Distant metastases were detected in 16 patients with CE F-FDG PET/CT and in 13 patients with routine conventional methods.

CONCLUSION

CE F-FDG PET/CT showed similar results compared with CT/MRI in the detection of primary tumors, but it was superior to conventional methods in the detection of metastasis and staging. Given the highly significant correlation between SUVmax values and the Fuhrman grading, CE F-FDG PET/CT may play a significant role in the evaluation of patient prognosis.

Multi-field-of-view SPECT is superior to whole-body scanning for assessing metastatic bone disease in patients with prostate cancer

Aim

The aim of this study was to compare the diagnostic performance of whole-body bone scintigraphy (WBS) and multi-field-of-view single photon emission tomography (multi-FOV SPECT) with 99mTc-oxidronate (99mTc-HDP) in patients with prostate cancer (PCa).

Methods

In a prospective study, WBS and SPECT acquisitions were performed in194 patients with histologically confirmed PCa and serum prostate-specific antigen (PSA) levels above 10 ng/mL. Scans obtained using the two modalities were interpreted separately. Clinical and biochemical follow-up, radiological studies and biopsies served as benchmarks for the assessments. The impact of PSA level on WBS and SPECT results was also evaluated.

Results

The patient-based sensitivity, specificity, accuracy, PPV and NPV values of SPECT examinations were higher than those of WBS, especially in patients with serum PSA levels ≶40 ng/mL.

Conclusion

Multi-FOV SPECT proved to be more sensitive and specific than WBS in detecting bone metastases in PCa patients.

PET/CT Imaging in Oncology: Exceptions That Prove the Rule

¹⁸F-FDG PET/CT is a diagnostic three-dimensional non-invasive device, routinely employed in neurology, cardiology, and oncology, and which contributes to patient care giving functional informations about glucose metabolism. In particular, staging, restaging, follow-up and response to treatment of tumors are the most common applications in oncologic field. Many neoplasms show increased glucose metabolism and consequent ¹⁸F-FDG uptake. Nevertheless, some relative differentiated cancers, such as clear cell carcinoma of the kidney and bronchioloalveolar adenocarcinoma, show tipically faintly/no uptake resulting in a consequent negative PET/CT scan. This case report represents an extreme case in which three relative well-differentiated cancer forms, all characterized by low glucose metabolism, affect the same patient at the same time while ¹⁸F-FDG PET/CT scan is negative.

Pharmacokinetics, metabolism, biodistribution, radiation dosimetry, and toxicology of (18)F-fluoroacetate ((18)F-FACE) in non-human primates

INTRODUCTION

To facilitate the clinical translation of (18)F-fluoroacetate ((18)F-FACE), the pharmacokinetics, biodistribution, radiolabeled metabolites, radiation dosimetry, and pharmacological safety of diagnostic doses of (18)F-FACE were determined in non-human primates.

METHODS

(18)F-FACE was synthesized using a custom-built automated synthesis module. Six rhesus monkeys (three of each sex) were injected intravenously with (18)F-FACE (165.4 ± 28.5 MBq), followed by dynamic positron emission tomography (PET) imaging of the thoracoabdominal area during 0-30 min post-injection and static whole-body PET imaging at 40, 100, and 170 min. Serial blood samples and a urine sample were obtained from each animal to determine the time course of (18)F-FACE and its radiolabeled metabolites. Electrocardiograms and hematology analyses were obtained to evaluate the acute and delayed toxicity of diagnostic dosages of (18)F-FACE. The time-integrated activity coefficients for individual source organs and the whole body after administration of (18)F-FACE were obtained using quantitative analyses of dynamic and static PET images and were extrapolated to humans.

RESULTS

The blood clearance of (18)F-FACE exhibited bi-exponential kinetics with half-times of 4 and 250 min for the fast and slow phases, respectively. A rapid accumulation of (18)F-FACE-derived radioactivity was observed in the liver and kidneys, followed by clearance of the radioactivity into the intestine and the urinary bladder. Radio-HPLC analyses of blood and urine samples demonstrated that (18)F-fluoride was the only detectable radiolabeled metabolite at the level of less than 9% of total radioactivity in blood at 180 min after the (18)F-FACE injection. The uptake of free (18)F-fluoride in the bones was insignificant during the course of the imaging studies. No significant changes in ECG, CBC, liver enzymes, or renal function were observed. The estimated effective dose for an adult human is 3.90-7.81 mSv from the administration of 185-370 MBq of (18)F-FACE.

CONCLUSIONS

The effective dose and individual organ radiation absorbed doses from administration of a diagnostic dosage of (18)F-FACE are acceptable. From a pharmacologic perspective, diagnostic dosages of (18)F-FACE are non-toxic in primates and, therefore, could be safely administered to human patients for PET imaging.

Imaging techniques in the management of chronic kidney disease: current developments and future perspectives

The measurement of both renal function and structure is critical in clinical nephrology to detect, stage, and monitor chronic kidney disease (CKD). Current imaging modalities especially ultrasound (US), computed tomography, and magnetic resonance imaging (MRI) provide adequate information on structural changes but little on functional impairment in CKD. Although not yet considered first-line procedures for evaluating patients with renal disease, new US and MR imaging techniques may permit the assessment of renal function in the near future. Combined with established imaging techniques, contrast-enhanced US, dynamic contrast-enhanced MRI, blood oxygen level dependency MRI, or diffusion-weighted imaging may provide rapid, accurate, simultaneous, and noninvasive imaging of the structure of kidneys, macrovascular and microvascular renal perfusion, oxygenation, and glomerular filtration rate. Recent developments in molecular imaging indicate that pathophysiological pathways of renal diseases such as apoptosis, coagulation, fibrosis, and ischemia will be visualized at the tissue level. These major advances in imaging and developments in hardware and software could enable comprehensive imaging of renal structure and function in four dimensions (three dimensions plus time), and imaging is expected to play an increasing role in the management of CKD.

Course of size and density of metastatic renal cell carcinoma lesions in the early follow-up of molecular targeted therapy

OBJECTIVE

Imaging-based monitoring of molecular therapies in oncology remains a challenge. Molecular therapies might have more pronounced effects on lesion density than on lesion size. We analyzed changes in lesion diameter and density in patients with metastasized renal cell cancer (mRCC) in the early follow-up of targeted therapy and compared size-based measurements according to Response Evaluation Criteria in Solid Tumors (RECIST) with size- and density-based response evaluations according to the Choi criteria.

PATIENTS AND METHODS

A total of 22 patients treated with sorafenib (800 mg/d) were retrospectively analyzed. Relative changes (in %) in the greatest diameter and density of defined neoplastic "target lesions" were determined 8 weeks and 1 year after start of therapy in relation to a pretherapeutic baseline investigation. Data were analyzed according to RECIST (ver. 1.0), and results were compared with the response assessment based on Choi. Median survival was determined for all subgroups according to Choi or RECIST at the 8-week and 1-year follow-up.

RESULTS

Applying RECIST, 18 patients (82%) demonstrated stable disease (SD) 8 weeks after the start of targeted therapy, 3 patients (14%) partial response (PR), and 1 patient (4%) progressive disease (PD). Partial responders at 8 weeks had a median survival of 48 months. After 1 year, 59% of all patients still showed SD. Applying Choi, 15 patients (68%) showed PR 8 weeks after the start of therapy, 5 patients (23%) SD, and 2 patients (9%) PD. After 1 year, PR was still the predominant response group (64% of the patients). Partial responders after 8 weeks had a median survival of 18 months.

CONCLUSION

Choi defined more patients as partial responders at early stages of therapy than RECIST, but this was not an effective selection for patients with prolonged median survival. Evaluation of a larger patient cohort will further clarify the role of combined size- and density-based follow-up strategies in targeted therapy of mRCC.

Skeletal metastasis in renal cell carcinoma: current and future management options

Metastasis to the skeleton is common in advanced renal cancer and leads to debilitating skeletal complications including severe pain, increased fracture rate and spinal cord compression. The incidence of renal cell carcinoma is increasing by around 2% per year and recent advances in targeted anti-angiogenic therapy for advanced disease are expected to lead to longer survival times. The clinical management of metastatic bone disease in renal cell carcinoma therefore merits greater focus than hitherto. Bone metastases arising from renal cancer are highly osteolytic and particularly destructive. Fortunately, the continuing development of anti-resorptive drugs is revolutionising the medical management of metastatic bone disease across many tumour types and making a major impact on quality of life. The bisphosphonate zoledronic acid is now licensed for use in advanced renal cell carcinoma and appears to yield a greater benefit in terms of reduction in skeletal related events than in bone metastases arising from other tumour types. Drugs which are directed at specific targets in the bone metastasis pathway are in development, including denosumab, a fully human monoclonal antibody against receptor activator of nuclear factor kappa B ligand, which has recently been licensed in the United States for use in renal cell carcinoma, with European licensing expected soon. This review examines the increasing options for treatment of metastatic bone disease in renal cell carcinoma, with a focus on drug-based advances and progress in the development of existing and new biomarkers to support clinical management.

From cancer metabolism to new biomarkers and drug targets

Great interest is presently given to the analysis of metabolic changes that take place specifically in cancer cells. In this review we summarize the alterations in glycolysis, glutamine utilization, fatty acid synthesis and mitochondrial function that have been reported to occur in cancer cells and in human tumors. We then propose considering cancer as a system-level disease and argue how two hallmarks of cancer, enhanced cell proliferation and evasion from apoptosis, may be evaluated as system-level properties, and how this perspective is going to modify drug discovery. Given the relevance of the analysis of metabolism both for studies on the molecular basis of cancer cell phenotype and for clinical applications, the more relevant technologies for this purpose, from metabolome and metabolic flux analysis in cells by Nuclear Magnetic Resonance and Mass Spectrometry technologies to positron emission tomography on patients, are analyzed. The perspectives offered by specific changes in metabolism for a new drug discovery strategy for cancer are discussed and a survey of the industrial activity already going on in the field is reported.

Efficacy of eradicative radiotherapy for limited nodal metastases detected with choline PET scan in prostate cancer patients

AIMS AND BACKGROUND

In patients with recurrent prostate cancer, discriminating local or systemic recurrence is critical to decide second-line treatment. We investigated the capability of stereotactic body radiotherapy to treat limited nodal recurrences, detected using choline PET scan.

METHODS AND STUDY DESIGN

Seventy-one patients with biochemical failure were studied after prostate cancer treatment: prostatectomy (28), radiotherapy (15) or both (28). Following computed tomography and choline PET imaging, stereotactic body radiotherapy was delivered on pathological lymphatic areas by 6 MV Linac, using dynamic micromultileaf collimation and intensity-modulated arc therapy optimization. Sixty days post-treatment, choline PET/CT imaging was carried out.

RESULTS

Median follow-up was 29 months (range, 14.4-48). Choline PET detected recurrences in 39 of 71 patients. Median PSA velocity was 0.40 ng/ml/year in PET-negative patients and 2.88 ng/ml/year in PET-positive subjects (P < 0.05). Twenty-five patients with limited nodal recurrences, out of the 71 submitted to choline PET, received eradicative radiotherapy. Persistent regression was recorded in 13; early spread to bone was found in 2 cases; lymph node recurrences in 8, all in sites outside the irradiated areas; 2 patients were lost to follow-up. At the 3-year follow-up, overall survival, disease-free survival and local control rates were 92%, 17% and 90%, respectively. In patients with a complete regression, PSA fell from 5.65 to 1.40 ng/ml (median). PSA nadir value (median 1.06 ng/ml) was maintained for 5.6 months (median).

CONCLUSIONS

Stereotactic body radiotherapy was effective in disease eradication of limited nodal recurrences from prostate cancer, saving patients from, or at least postponing, systemic treatments.

Prostate cancer

Prostate cancer is biologically and clinically a heterogeneous disease and its imaging evaluation will need to be tailored to the specific phases of the disease in a patient-specific, risk-adapted manner. We first present a brief overview of the natural history of prostate cancer before discussing the role of various imaging tools, including opportunities and challenges, for different clinical phases of this common disease in men. We then review the preclinical and clinical evidence on the potential and emerging role of positron emission tomography with various radiotracers in the imaging evaluation of men with prostate cancer.

Functional imaging of renal cell carcinoma

The incidence of early and advanced-stage renal cell carcinoma (RCC) is increasing. Methods of diagnosing, staging and evaluating tumor burden that are more accurate and reliable than the currently available options are needed in order to identify RCC at a stage at which it is curable and to accurately determine the response to treatment. Functional imaging, particularly with combined PET-CT, might improve accuracy of detection and provide essential information that has been unavailable to date. This approach is against a background in which targeted therapies for metastatic RCC have entered clinical practice in the past few years, further highlighting the importance of accurate imaging for patient selection and for monitoring response to treatment. We outline the current clinical status of functional imaging in RCC using PET-CT, which allows simultaneous capture and co-registration of functional and anatomical data. New radiotracers and approaches-including radiolabeled monoclonal antibodies and imaging of tumor hypoxia-are touched on, and areas of future research discussed.

Positive correlation between serum liver enzyme levels and standard uptake values of liver on FDG-PET

PURPOSE

The aim of the study is to assess the relationship between serum liver enzyme level, hepatic virus infectious state, and standard uptake value of liver on 2-fluoro-2-deoxy-D-glucose (FDG)-positron emission tomography (PET).

MATERIAL AND METHODS

It is the retrospective review study from September 2005 to September 2007. A total of 354 healthy subjects referred from the Department of Community Medicine and Health Examination Center of our hospital for health screening were reviewed retrospectively and recruited for analysis. Whole-body FDG-PET, serum liver enzyme levels [aspartate aminotransferase (AST) or SGOT and alanine aminotransferase (ALT) or SGPT], and hepatic B (HBV)/hepatic C virus (HCV) infectious states by checking serum antibodies were performed in all subjects. The mean and maximal values of standard uptake values (SUV) of liver were calculated. The relationships between serum liver enzyme levels, hepatic virus infectious states, and standard uptake values of liver on FDG-PET were evaluated.

RESULT

There is statistically significant positive correlation between SGOT/SGPT and SUV of liver on FDG-PET. However, there are no significant differences in the SUV of liver on FDG-PET between HBV/HCV infection and nonhepatic virus infection subjects.

CONCLUSION

High FDG uptake in the liver because of high level of SGOT/SGPT may lower the diagnostic sensitivity of hepatic malignant or infectious lesions on FDG-PET. For avoiding false-negative findings, careful evaluation of liver on FDG-PET and correlation with other clinical manifestations should be recommended in patients with high level of SGOT/SGPT. Nevertheless, neither HBV/HCV infectious states significantly influences on the SUV of liver on FDG-PET.

Diagnosis of bone metastases in urological malignancies--an update

The spread of urological malignancies to bone is a poor prognostic factor. Early detection of metastatic bony lesions assists with tailoring patient management and potentially improving quality of life. Newer therapies such as zolderonic acid for prostate cancer have reinvigorated the importance of clinicians treating bony disease. An array of biochemical and imaging options are available and the order, sensitivity and cost of such investigations need to be understood to maximise clinical benefit. Furthermore, the ideal time to investigate for bony metastases has often been controversial. Although simple history and examination, serum calcium and alkaline phosphatase and plain radiography with bone scintigraphy remain at the forefront of diagnosing bony disease, evolving diagnostic modalities, such as positron emission tomography and newer bone markers need to be considered. The aim of this review is to clarify the role of various investigations and to give clinicians a current analysis of the timing of such investigations in the context of evolving diagnostic modalities and accepted guidelines for urological malignancy.

Positron emission tomography/computed tomography and radioimmunotherapy of prostate cancer

PURPOSE OF REVIEW

Traditional morphologically based imaging modalities are now being complemented by positron emission tomography (PET)/computed tomography (CT) in prostate cancer. Metastatic prostate cancer is an attractive target for radioimmunotherapy (RIT) as no effective therapies are available. This review highlights the most important achievements within the last year in PET/CT and RIT of prostate cancer.

RECENT FINDINGS

Conflicting results exist on the use of choline for detection of malignant disease in the prostate gland. The role of PET/CT in N-staging remains to be elucidated further. However, F-choline and C-choline PET/CT have been demonstrated to be useful for detection of recurrence. F-choline and F-fluoride PET/CT are useful for detection of bone metastases. Prostate tumor antigens may be used as targets for RIT. Prostate-specific membrane antigen is currently under focus of a number of diagnostic and therapeutic strategies. J591, a monoclonal antibody, which targets the extracellular domain of prostate-specific membrane antigen, shows promising results. HER2 receptors may also have a potential as target for PET/CT imaging and RIT of advanced prostate cancer.

SUMMARY

PET/CT in prostate cancer has proven to play a significant role, in particular for detection of prostate cancer recurrence and bone metastases. RIT of metastatic prostate cancer warrants further investigations.

Nanomedicine--challenge and perspectives

The application of nanotechnology concepts to medicine joins two large cross-disciplinary fields with an unprecedented societal and economical potential arising from the natural combination of specific achievements in the respective fields. The common basis evolves from the molecular-scale properties relevant to the two fields. Local probes and molecular imaging techniques allow surface and interface properties to be characterized on a nanometer scale at predefined locations, while chemical approaches offer the opportunity to elaborate and address surfaces, for example, for targeted drug delivery, enhanced biocompatibility, and neuroprosthetic purposes. However, concerns arise in this cross-disciplinary area about toxicological aspects and ethical implications. This Review gives an overview of selected recent developments and applications of nanomedicine.

Engineered humanized diabodies for microPET imaging of prostate stem cell antigen-expressing tumors

We have previously demonstrated preclinical in vivo targeting of prostate stem cell antigen (PSCA) using a humanized anti-PSCA 2B3 monoclonal antibody (mAb). However, humanization resulted in 5-fold loss of apparent affinity relative to the parental mAb (1 nM). In this study, diabodies (scFv dimers of 55 kDa) were generated from 2B3 including variants with different linker lengths as well as back-mutations to original murine residues to improve affinity. Parental 2B3 (p2B3) and back-mutated 2B3 (bm2B3) diabodies (Dbs) with five- or eight-amino acid linkers (p2B3-Db5, p2B3-Db8, bm2B3-Db5 and bm2B3-Db8) were evaluated for binding to PSCA by flow cytometry and affinities were determined by surface plasmon resonance. Back-mutation restored the affinity from 5.4 to 1.9 nM. Stability, evaluated by size exclusion, revealed that diabodies with eight-residue linkers existed as a mixture of dimeric and monomeric species at low concentrations (<or =1 mg/ml). Shortening the linker from eight to five residues improved dimer stability, notably in the bm2B3-Db8 compared with bm2B3-Db5. Both p2B3-Db8 and bm2B3-Db8 were radioiodinated with (124)I and evaluated by serial micro-positron emission tomography imaging in mice bearing LAPC-9 human prostate cancer xenografts. Localization in LAPC-9 xenografts was seen at 4 h, whereas at 20 h most of the activity had cleared from the tumor. Highest tumor-to-background contrast ratios and best images were obtained at 12 h. Although the higher affinity bm2B3-Db8 demonstrated improved tumor retention at later time points (20 h), it did not improve tumor targeting or imaging compared with p2B3-Db8 at 12 h.