Residency · Residency · Urology

Nuclear Medicine in Urology

Introduction

Nuclear medicine plays a crucial role in urologic evaluation by providing functional information that complements the anatomical details obtained from CT and MRI. Techniques such as renal scintigraphy, bone scanning, and positron emission tomography (PET) are essential for assessing differential renal function, urinary obstruction, vesicoureteral reflux, bone metastases, and oncologic staging. A thorough understanding of the radiopharmaceuticals used, imaging protocols, and clinical interpretation of nuclear studies is a fundamental skill for urology residents.

Renal Scintigraphy

Radiopharmaceuticals

Several radiopharmaceuticals are employed in renal scintigraphy, each with distinct clearance mechanisms and clinical applications. Tc-99m MAG3 (mercaptoacetyltriglycine) is primarily cleared by tubular secretion and provides excellent imaging even in patients with impaired renal function. It is the preferred agent for diuretic renography and evaluating obstruction due to its extraction efficiency of approximately 60%. In contrast, Tc-99m DTPA (diethylenetriaminepentaacetic acid) is exclusively cleared by glomerular filtration, allowing estimation of glomerular filtration rate (GFR). However, it has a lower extraction efficiency of about 20%, resulting in inferior image quality in cases of renal insufficiency. Tc-99m DMSA (dimercaptosuccinic acid) binds to proximal tubular cells and remains localized in the renal cortex, making it the gold standard for cortical imaging and detecting renal scarring. It is primarily used in pediatric populations to evaluate pyelonephritis-related scarring and to localize ectopic kidneys.

Diuretic Renography (Lasix Renogram)

Diuretic renography is indicated to differentiate obstructive from non-obstructive hydronephrosis and to assess differential renal function. The protocol involves intravenous injection of Tc-99m MAG3 with dynamic image acquisition, followed by administration of furosemide (Lasix) at a standardized time point. The commonly used protocols include the F+20 protocol, where furosemide is given 20 minutes after the tracer; the F-15 protocol, with furosemide administered 15 minutes before the tracer in well-hydrated patients to achieve well-tempered diuresis; and the F0 protocol, where furosemide and tracer are given simultaneously.

Interpretation of the renogram focuses on the half-time (T1/2) of tracer washout. A T1/2 less than 10 minutes indicates a non-obstructed system, between 10 and 20 minutes is indeterminate, and greater than 20 minutes suggests obstruction. Differential renal function is also assessed, with a normal split being 45-55%; a contribution below 40% is considered significantly reduced. It is important to recognize pitfalls such as dehydration, poor renal function, massive hydronephrosis causing a reservoir effect, and patient non-compliance, all of which can lead to false-positive diagnoses of obstruction.

<image>Diuretic renogram tracings showing three patterns: normal rapid washout after furosemide (T1/2 < 10 min), obstructed pattern with progressive tracer accumulation (T1/2 > 20 min), and indeterminate pattern, with corresponding sequential scintigraphic images of each kidney</image>

DMSA Renal Cortical Scintigraphy

DMSA renal cortical scintigraphy is primarily used to detect renal cortical scarring, assess differential renal function in children, and localize ectopic kidneys. Static images are obtained 2 to 4 hours after tracer injection. Photopenic defects on these images indicate areas of cortical scarring or acute pyelonephritis. The sensitivity of DMSA scintigraphy for detecting renal scarring due to vesicoureteral reflux exceeds 90%, making it superior to ultrasound for scar detection in pediatric urinary tract infection evaluations.

Bone Scintigraphy

Technique

Bone scintigraphy utilizes Tc-99m MDP (methylene diphosphonate), which accumulates in regions of increased osteoblastic activity. Whole-body images are typically acquired 2 to 4 hours after injection. The addition of SPECT/CT enhances lesion localization and characterization, improving diagnostic accuracy.

Urologic Applications

In urology, bone scintigraphy is primarily used for staging prostate cancer, particularly when prostate-specific antigen (PSA) levels exceed 20 ng/mL, Gleason scores are 8 or higher, clinical stage is T3 or T4, or when patients experience bone pain. For renal cell carcinoma, bone scans are less sensitive compared to prostate cancer, and CT or PET/CT are preferred for detecting bone involvement. Bone scintigraphy is also employed in staging muscle-invasive bladder cancer. Interpretation requires careful correlation with cross-sectional imaging because increased uptake ("hot spots") can represent metastases, fractures, arthritis, or Paget disease. A notable phenomenon is the flare effect, a transient increase in uptake following initiation of hormonal therapy for prostate cancer, which should not be mistaken for disease progression.

<image>Whole-body bone scintigraphy showing multiple foci of increased radiotracer uptake in the axial skeleton (spine, pelvis, ribs) consistent with widespread osseous metastases from prostate cancer, with annotations identifying key metastatic sites and a comparison to a normal bone scan</image>

PET/CT in Urologic Oncology

F-18 FDG PET/CT

F-18 fluorodeoxyglucose (FDG) PET/CT has limited utility in prostate cancer due to the low FDG avidity of well-differentiated tumors. However, it is useful in bladder cancer for staging muscle-invasive disease and detecting nodal and distant metastases. In renal cell carcinoma, FDG uptake is variable; PET/CT is more beneficial for assessing metastatic disease than characterizing the primary tumor. For testicular cancer, FDG PET/CT is valuable in surveillance of seminoma and non-seminoma, demonstrating high sensitivity for detecting residual viable tumor after chemotherapy.

PSMA PET/CT

Prostate-specific membrane antigen (PSMA) is overexpressed on prostate cancer cells, and FDA-approved agents such as Ga-68 PSMA-11 and F-18 DCFPyL (Pylarify) have revolutionized imaging in prostate cancer. PSMA PET/CT is indicated for initial staging of unfavorable intermediate-risk and high-risk prostate cancer, as well as for detecting biochemical recurrence at PSA levels as low as 0.2 to 0.5 ng/mL. It is superior to conventional imaging modalities like bone scan and CT for identifying nodal and distant metastases. The detection rate correlates with PSA levels, being approximately 40% at PSA less than 0.5 ng/mL, 60% at 0.5 to 1.0 ng/mL, and 75% at 1.0 to 2.0 ng/mL. False positives can occur due to uptake in ganglia, sarcoidosis, other malignancies, and benign uptake in celiac and other ganglia.

Axumin (F-18 Fluciclovine) PET/CT

Axumin is a synthetic amino acid analog that targets amino acid transport and is FDA-approved for evaluating biochemical recurrence of prostate cancer. However, it is gradually being supplanted by PSMA PET/CT due to the superior sensitivity and specificity of PSMA-targeted agents.

Radionuclide Cystography

Direct radionuclide cystography (DRC) involves instilling Tc-99m pertechnetate into the bladder via catheter. It is used primarily for detecting vesicoureteral reflux (VUR), especially for follow-up in patients with known reflux. Compared to fluoroscopic voiding cystourethrography (VCUG), DRC offers the advantage of a lower radiation dose and continuous monitoring. However, it provides poorer anatomic detail and cannot grade reflux as precisely as VCUG.

Key Clinical Pearls

MAG3 is preferred over DTPA for diuretic renography because it yields superior image quality, particularly in patients with impaired renal function. The half-time (T1/2) on diuretic renogram must always be interpreted within the clinical context, ensuring adequate hydration and renal function before diagnosing obstruction. DMSA scintigraphy remains the gold standard for detecting renal cortical scars in the pediatric population. PSMA PET/CT has fundamentally transformed prostate cancer staging and recurrence detection, offering greater sensitivity than conventional bone scans and CT. Finally, the bone scan flare phenomenon observed after initiating androgen deprivation therapy should not be mistaken for disease progression.

References

1. Taylor AT. Radionuclides in nephrourology, part 1: Radiopharmaceuticals, quality control, and quantitative indices. J Nucl Med. 2014;55(4):608-615.
2. Hofman MS, Lawrentschuk N, Francis RJ, et al. Prostate-specific membrane antigen PET-CT in patients with high-risk prostate cancer before curative-intent surgery or radiotherapy (proPSMA): a prospective, randomised, multicentre study. Lancet. 2020;395(10231):1208-1216.
3. Piepsz A, Colarinha P, Gordon I, et al. Guidelines for 99mTc-DMSA scintigraphy in children. Eur J Nucl Med. 2001;28(3):BP37-BP41.
4. Jadvar H. Prostate cancer: PET with 18F-FDG, 18F- or 11C-acetate, and 18F- or 11C-choline. J Nucl Med. 2011;52(1):81-89.

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