How would you approach a high grade neuroendocrine carcinoma found in a mediastinal lymph node with unknown primary?

If it showed the histopathological signs of a small cell lung cancer (TTF1, Synaptophysin, Chromogranin positive, high-grade), then yes. Assuming PET/CT and diagnostic CT chest are both negative for other sites of disease?

Could consider EBUS in attempt to identify additional potential additional si...

If this does not appear to be "Small Cell Lung Cancer", then one option may be to perform concurrent chest radiotherapy with chemotherapy using the "Locoregional, unresectable" pathway of the "Extrapulmonary Poorly Differentiated" Category of the "Neuroendocrine and Adrenal Tumors Guideline" as a st...