Benign and Inflammatory Urinary Tract Findings

Most urinary cytology specimens are benign, and recognising benign, reactive, and infectious findings is essential to avoid overcalling malignancy. Reactive urothelial changes, instrumentation effects, urolithiasis, treatment effects, and viral cytopathic changes — notably polyomavirus 'decoy cells' — are the principal benign mimics of high-grade urothelial carcinoma.

Definition

Benign and inflammatory urinary tract findings are non-neoplastic cellular changes in urinary specimens — reactive, infectious, mechanical, or treatment-related — that must be distinguished from urothelial carcinoma to prevent false-positive cytologic interpretation.

Scope

This entry covers the spectrum of non-neoplastic urinary cytology findings and the features that separate them from malignancy: reactive and inflammatory atypia, the effects of instrumentation and lithiasis, treatment-related changes, and viral cytopathic effects. It is an educational reference and offers no clinical diagnostic or treatment instruction.

Core questions

• What benign processes produce atypical-appearing cells in urine?
• How are polyomavirus decoy cells distinguished from high-grade carcinoma?
• Which features indicate a reactive rather than a malignant process?
• How do instrumentation and lithiasis alter urinary cytology?

Key concepts

• Reactive urothelial atypia
• Polyomavirus (BK) decoy cells
• Inflammatory and infectious changes
• Lithiasis (calculus) effect
• Instrumentation effect
• Treatment effect (intravesical therapy, radiation)
• Benign mimics of malignancy

Mechanisms

Reactive urothelial cells enlarge and may show nucleoli and mild nuclear changes in response to inflammation, calculi, or instrumentation, but they retain relatively smooth nuclear membranes and proportionate cytoplasm, keeping the nuclear-to-cytoplasmic ratio lower than in high-grade carcinoma. Polyomavirus (BK virus) infection produces 'decoy cells' with large, glassy, homogeneous intranuclear inclusions that can superficially mimic malignant nuclei; recognition of the ground-glass inclusion and smooth nuclear outline distinguishes them from carcinoma. Intravesical therapy and radiation can produce marked but benign cytologic atypia (fogazzi-2001; rosenthal-2016-negative).

Clinical relevance

Accurate recognition of benign findings prevents false-positive cytology and unnecessary downstream investigation; this descriptive role concerns diagnostic accuracy and does not constitute individualised clinical guidance. The content is educational only.

Evidence & guidelines

The Paris System's 'negative for high-grade urothelial carcinoma' category encompasses benign and reactive findings and provides criteria for separating them from malignancy; the polyomavirus decoy cell is a classically described benign mimic in this context (rosenthal-2016-negative; fogazzi-2001; kurtycz-2020).

History

The challenge of benign mimics has accompanied urinary cytology since its inception; the description of polyomavirus decoy cells clarified one important infectious pitfall, and standardised reporting later codified the benign category and its distinguishing criteria (fogazzi-2001; rosenthal-2016-negative).

Debates

How should reactive atypia be classified relative to the atypical category?

Distinguishing benign reactive atypia from a genuinely atypical urothelial cell finding is reproducibility-limited, and where to draw that boundary affects both false-positive and false-negative rates.

Key figures

• Dorothy Rosenthal
• Giovanni Fogazzi
• Eva Wojcik

Seminal works

• fogazzi-2001
• rosenthal-2016-negative

Frequently asked questions

What are decoy cells?

Decoy cells are urinary epithelial cells infected by polyomavirus (BK virus) that develop large, glassy intranuclear inclusions; they are benign but can mimic high-grade carcinoma, hence the name.

How are reactive changes told apart from cancer in urine?

Reactive cells typically keep smooth nuclear membranes and proportionate cytoplasm with a lower nuclear-to-cytoplasmic ratio, whereas high-grade carcinoma shows hyperchromatic, irregular nuclei with markedly increased nuclear-to-cytoplasmic ratio.