BAC April Slide Club

BAC Lunchtime Slide Club – April 2026 Round-Up

The BAC 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.

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.

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.

Key learning point:

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

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.

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.

Key learning point:

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

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.

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

Key learning point:

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

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.

This was also reported as negative for malignancy.

Key learning point:

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

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.

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

Key learning point:

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

Overall Takeaways

Across all cases, a consistent theme emerged:

- 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

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