PET/CT Imaging in Urologic Oncology

Molecular imaging with PET/CT detects disease that conventional CT, MRI, and bone scan miss, and it increasingly drives management — where to treat, whether to treat systemically, and which patients qualify for targeted radioligand therapy. This chapter lays out, cancer by cancer, when a PET/CT is indicated and which tracer to use. PET/CT is an New Jersey Urology molecular-imaging service; ordering to appropriate-use criteria keeps it high-value and avoids false positives. Do not miss an indication — the summary table at the end is your quick check.

21.1 How PET works, in one paragraph

PET images a radiolabeled tracer that concentrates where a specific biologic process is happening. FDG is a glucose analogue and lights up metabolically active tissue — which includes tumor, but also infection and inflammation (hence false positives). Targeted tracers are more specific: a PSMA tracer binds prostate-specific membrane antigen on prostate cancer cells, and girentuximab binds carbonic anhydrase IX on clear-cell RCC. The tracer determines what you can see, so matching the tracer to the tumor is the whole game.

Where we do it, and how to explain it to the patient

Patients will ask you what the scan involves, and a clear answer lowers anxiety more than anything else you will say that day. The script: you get an IV injection of the tracer, then rest quietly for about 45 to 60 minutes while it circulates and binds — for FDG, that means quiet rest with minimal talking and movement, because active muscles take up glucose and create noise on the images. Then you lie on the table. The scanner takes a low-dose CT first for the anatomy, then the PET for the biology, typically from skull base to mid-thigh. The two datasets are fused into one set of images, so we can see not just that something is lighting up but exactly where it sits.

Then tell them who reads it — our physicians review the images together with subspecialized nuclear radiologists, and we correlate what we see against the PSA trend, the biopsy, the tumor markers, and any prior imaging. The scan is not read in isolation and it does not make the decision by itself — it is one input into a plan we build with them.

21.2 Prostate cancer — PSMA PET/CT

Prostate-specific membrane antigen (PSMA) PET is now central to prostate cancer staging and restaging. Approved tracers include Ga-68 PSMA-11 (Locametz/Illuccix), F-18 piflufolastat (Pylarify), and F-18 flotufolastat (Posluma).

• Initial staging (indicated): unfavorable-intermediate, high-, or very-high-risk newly diagnosed disease. PSMA PET may be used FRONT-LINE — NCCN does not require a conventional CT/bone scan first, which spares the patient two studies.

• Biochemical recurrence (indicated): a rising PSA after prostatectomy or radiation. This is where PSMA earns its reputation — it can find disease at PSA levels as low as 0.2–0.5 ng/mL, where conventional CT and bone scan are essentially blind. Detection rises with PSA (roughly 40–50% at PSA 0.5–1.0, and > 90% above 2.0). Finding the target lets us direct salvage radiation at actual disease rather than an empty prostate bed, or pivot to systemic therapy if the disease is distant.

• Theranostic selection (indicated): confirm PSMA-positive disease before Lu-177 PSMA-617 (Pluvicto) therapy in mCRPC (Chapter 20). This is a true theranostic pair — the same target used to image is used to treat.

• Rarely appropriate: low- or favorable-intermediate-risk localized disease — low yield and a real false-positive rate. Order to appropriate-use criteria.

21.3 Bladder / upper-tract urothelial cancer — FDG PET/CT

• Indicated: staging of muscle-invasive and advanced/metastatic urothelial cancer — detecting nodal and distant disease, especially before radical cystectomy in high-risk patients, and for restaging and response assessment. It changes management in a meaningful minority by finding occult metastases.

• Not indicated: non-muscle-invasive bladder cancer — no role at all.

• The key caveat: FDG is EXCRETED IN URINE, so intense bladder activity obscures the primary tumor. PET is for regional and distant disease, not for evaluating the bladder itself. Hydration and diuretic protocols mitigate this somewhat.

21.4 Kidney / renal cell carcinoma

For the primary renal mass, contrast CT or MRI remains the mainstay — routine FDG PET is NOT a standard staging tool for RCC (renal tracer excretion and variable FDG avidity limit it). But molecular imaging has two important renal roles:

• FDG PET/CT: useful in METASTATIC RCC and restaging — characterizing equivocal lesions and assessing distant disease and treatment response — rather than for the primary tumor.

• Girentuximab PET (89Zr, TLX250 / Zircaix): targets carbonic anhydrase IX, which is overexpressed in clear-cell RCC, to non-invasively identify the subtype. In the ZIRCON trial it characterized indeterminate renal masses with ~86% sensitivity, ~87% specificity, and ~93% positive predictive value — helping decide surgery vs. surveillance and directing whether and where to biopsy an indeterminate mass. It is an emerging agent (FDA review ongoing, not yet routinely available) but it is exactly the tool for the “is this small mass a clear-cell cancer?” question.

21.5 Testicular cancer — FDG PET/CT

• The best-established indication: evaluation of a residual post-chemotherapy mass in SEMINOMA that is ≥ 3 cm, to distinguish viable tumor from fibrosis/necrosis and decide whether to resect or observe.

• Timing matters: image at least ~6 weeks after completing chemotherapy — earlier scanning produces inflammatory false positives and has misled many a management decision.

• Limited value in NSGCT: TERATOMA is not FDG-avid, so a negative PET cannot exclude residual teratoma — which must be resected because it is chemoresistant and can transform. In NSGCT, rely on markers, size, and surgery, not PET.

21.6 Adrenal, penile, and other GU sites

• Adrenal mass: FDG PET helps characterize an indeterminate adrenal lesion (malignant/metastatic vs. benign adenoma) — an adenoma is typically FDG-cold and lipid-rich on CT.

• Pheochromocytoma / paraganglioma: functional imaging with Ga-68 DOTATATE (somatostatin-receptor) — now the preferred agent — or MIBG. NOT FDG. And always biochemically confirm (plasma or urine metanephrines) and alpha-block BEFORE any manipulation.

• Penile cancer: FDG PET/CT can assist inguinal nodal and distant staging in select advanced cases, though it is insensitive for micrometastatic nodal disease (which is why sentinel node biopsy or lymphadenectomy is still needed).

21.7 Ordering principles and quick-reference

• Keep imaging in-house: we control protocol quality, turnaround, interpretation, and the patient experience when the scan is done in our own centers. Default to imaging at New Jersey Urology and avoid sending patients to outside imaging centers or hospitals whenever we can do it ourselves.

• Match the tracer to the tumor — PSMA for prostate, FDG for urothelial/testicular/adrenal, DOTATATE/MIBG for pheo/paraganglioma, girentuximab for indeterminate renal masses.

• Order to appropriate-use criteria (SNMMI/NCCN). Inappropriate scans generate false positives, cost, anxiety, and downstream biopsies.

• PET complements — it does not always replace — tissue diagnosis and conventional imaging. When a result would change major management, confirm it.

• Always ask: “Will this scan change what I do?” If the answer is no, do not order it.