• Rare, less than 100 well documented primary cases in mediastinum
• If metastatic, may have symptoms due to primary intrabronchial tumor (cough, hemoptysis, dyspnea) or esophageal tumor (dysphagia)

• Also called oat cell carcinoma, undifferentiated neuroendocrine carcinoma

• Occurs anywhere in mediastinum, usually metastatic from lung or esophagus

• May have paraneoplastic syndrome (Cushing’s syndrome, syndrome of inappropriate antidiuretic hormone)

• Almost always fatal, although median survival is 36 months

• 66 year old man (Med Oncol 2009;26:247)

• Fleshy, gray-white (similar to lymphoma), hemorrhagic, necrotic

• Centered in thymus (suggests primary) or nodal
• Clusters of small blue cells with minimal cytoplasm, hyperchromatic nuclei, no / minimal nucleoli, nuclear molding, frequent mitotic figures, frequent necrosis, crush artifact common, encrustation of basophilic nuclear material around intratumoral blood vessels (Azzopardi phenomenon)
• May be mixed with squamous cell carcinoma or adenocarcinoma

Small cell carcinoma

• Keratin (perinuclear globules), chromogranin A, synaptophysin, CD57/Leu7

• CD45

• Uniformly sized (80 to 250 nm) neurosecretory granules with a peripheral halo, usually in clusters, primitive junctional complexes

• Basaloid carcinoma: eosinophilic basement membrane material, associated with thymic cysts
• Lymphoma: keratin-, CD45+

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