• Cytokeratin
• small cell carcinoma
• pulmonary
• CK7
• CK20
• cancer
• immunocytochemistry
• lung cancer
• small round cell tumours

Metastatic small cell neuroendocrine carcinomas in the neck lymph nodes require further investigation including immunohistochemistry to find out the possible primary sites. The commonest immunoprofile for metastatic small cell carcinoma from a lung primary is positive staining with neuroendocrine markers CK7 and TTF-1 and no reaction to CK20. The differential cytokeratin staining is very useful in excluding other primary neuroendocrine tumours like Merkel cell carcinoma and small cell carcinoma of the salivary gland. Here we describe a unique example of cytokeratin switching by a pulmonary small cell carcinoma, a previously undescribed phenomenon creating diagnostic difficulty.

An elderly patient presented to the emergency department with an acute history of breathlessness and a lump in the right neck. Signs of superior vena cava obstruction were present. Fine needle aspiration of the lump was carried out, showing a cellular aspirate with a necrotic background. The cellularity comprised a diffuse population of atypical cells with high nucleus:cytoplasm ratio, scanty cytoplasm, clumped chromatin and nuclear moulding.

Immunohistochemistry showed that the atypical cells were positive with synaptophysin, chromogranin and CD56 (neuroendocrine markers). The tumour cells also showed paranuclear dot like positivity with MNF116 and CAM 5.2. CK20 was strongly positive while CK7 was negative (figure …