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June Slide Club Review

The latest BAC lunchtime Slide Club featured three interesting cytology cases that highlighted common diagnostic pitfalls and the value of correlating morphology with clinical history and ancillary testing.

Case 1: Pleomorphic Oncocytic Thyroid Lesion in a Patient with Metastatic Prostate Cancer

Anthony Maddox (West Hertfordshire Teaching Hospitals NHS Trust) presented a thyroid fine needle aspiration from a patient with known metastatic prostate cancer. The thyroid nodule had been detected incidentally on PET imaging and subsequent ultrasound classified it as a U3 lesion because of its internal vascularity.

Cytology showed a highly cellular oncocytic lesion with abundant granular cytoplasm and striking nuclear pleomorphism. Given the patient's history, the principal diagnostic question was whether this represented metastatic prostate carcinoma involving the thyroid.

Immunohistochemistry showed NKX3.1 and PSA negativity, excluding metastatic prostate carcinoma, while thyroglobulin and TTF-1 positivity confirmed thyroid origin. Despite marked pleomorphism, the lesion was classified as Thy3F (Bethesda follicular lesion of undetermined significance with oncocytic features).

Learning points

  • Marked pleomorphism alone should not result in over-calling malignancy in oncocytic thyroid lesions.
  • Clinical history may raise suspicion for metastasis, but immunohistochemistry is essential when morphology is equivocal.
  • Oncocytic lesions can show considerable atypia while remaining within the Thy3F category.

Case 2: Metastatic Meningioma Diagnosed by Mediastinal EUS

Leonie Wheeldon (Royal Cornwall Hospitals NHS Trust) presented an unusual mediastinal EUS case from a 69-year-old man with a previous history of stable meningioma of the brain who developed mediastinal lymphadenopathy.

Rapid on-site evaluation initially yielded scant material, but repeat passes produced a highly cellular specimen of bland ovoid cells with occasional nuclear pseudoinclusions. Additional tissue was obtained for immunohistochemistry.

The immunoprofile supported metastatic meningioma, an exceptionally rare event affecting fewer than 0.1% of meningioma patients.

Learning points

  • ROSE is primarily about assessing adequacy rather than making a definitive diagnosis.
  • When morphology is unusual, obtaining additional material is critical.
  • Be aware of pencil graphite contamination mimicking melanin pigment.

Case 3: Papillary Thyroid Carcinoma with Extensive Psammoma Bodies

The final case involved a 28-year-old woman with an incidental 3 cm thyroid nodule identified during CT pulmonary angiography. Ultrasound demonstrated a heterogeneous U3 lesion with internal vascularity.

Fine needle aspiration was highly cellular with papillary fragments, fibrovascular cores and abundant laminated psammoma bodies. The combination of psammoma bodies and intranuclear inclusions supported a diagnosis of papillary thyroid carcinoma.

Although diffuse sclerosing papillary carcinoma was considered because of the extensive calcification, imaging demonstrated a discrete nodule rather than diffuse thyroid involvement.

Learning points

  • The combination of psammoma bodies and intranuclear inclusions is highly predictive of papillary thyroid carcinoma.
  • Extensive calcification may mimic the diffuse sclerosing variant.
  • Correlation between imaging and cytology is essential.

Key Messages

  • Avoid over-interpreting atypia in oncocytic thyroid lesions.
  • Use immunohistochemistry to resolve difficult differentials.
  • At ROSE, ensure adequate material is obtained whenever morphology appears unusual.
  • Always interpret cytology alongside clinical history and imaging.