91ÌÒÉ«

91ÌÒÉ« Lunchtime Slide Club – April 2026 Round-Up

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The 91ÌÒÉ« lunchtime slide club returned in April with a series of instructive cytology cases highlighting diagnostic pitfalls and the critical importance of clinical context in interpretation.

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Urine Cytology After BCG Therapy – A Diagnostic Trap

A voided urine sample from a 69-year-old man with previous high-grade urothelial carcinoma showed markedly atypical cells with high nuclear-to-cytoplasmic ratios, hyperchromasia, and irregular nuclear membranes.

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However, further clinical review revealed the sample had been taken three weeks after intravesical BCG therapy. These cytological changes were therefore considered treatment-related rather than diagnostic of recurrent malignancy.

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Key learning point:

Urine cytology should be deferred until at least six weeks post-BCG to avoid false interpretation due to therapy-induced atypia.

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BK Virus Reactivation Mimicking Malignancy

A urine sample from a 65-year-old man presenting with haematuria showed atypical cells raising concern for high-grade urothelial carcinoma.

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Closer assessment identified two cell populations, including cells with enlarged nuclei and others with viral-type changes. SV40 immunostaining confirmed BK virus infection, supporting a diagnosis of viral reactivation rather than malignancy.

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Key learning point:

BK virus infection can closely mimic carcinoma in urine cytology—immunohistochemistry is essential in equivocal cases.

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Biliary Brushings in Primary Sclerosing Cholangitis

Brushings from a biliary stricture in a patient with primary sclerosing cholangitis showed mild nuclear enlargement but finely dispersed chromatin, smooth nuclear contours, and no significant atypia.

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The findings were reported as reactive changes, consistent with the clinical context.

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Key learning point:

In PSC-associated strictures, reactive atypia can mimic malignancy, but preservation of nuclear features supports a benign interpretation.

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Inflammatory Stricture – Avoiding Overcalling

A second biliary brushing case demonstrated acute and chronic inflammation with epithelial atypia, but without features of cholangiocarcinoma such as marked anisonucleosis or irregular nuclear membranes.

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This was also reported as negative for malignancy.

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Key learning point:

Inflammatory changes alone should not lead to overdiagnosis—morphological thresholds for malignancy must be strictly applied.

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Recurrent Ameloblastoma Mimicking Basaloid Neoplasm

An FNA from a subcutaneous neck nodule showed a highly cellular basaloid tumour with stromal material, initially raising concern for adenoid cystic carcinoma.

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Clinical history revealed recurrent ameloblastoma, and the cytological findings were interpreted in that context as recurrence/metastasis.

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Key learning point:

Rare tumours can closely mimic more common entities—clinical history is often decisive in avoiding misclassification.

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Overall Takeaways

Across all cases, a consistent theme emerged:

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- Clinical context is essential for accurate cytological interpretation

- Timing of sampling (e.g. post-treatment) can profoundly affect findings

- Ancillary tests (e.g. immunostains) are critical in difficult cases

- Reactive and inflammatory changes remain a major diagnostic pitfall

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The next 91ÌÒÉ« slide club will take place in late May, and members are encouraged to contribute cases for discussion.

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