Passage: The lungs are a common place for the spread of tumors from other parts of the body. Secondary cancers are classified by the site of origin; for example, breast cancer that has been spread to the lung is called metastatic breast cancer. Metastases often have a characteristic round appearance on chest radiograph.Primary lung cancers also most commonly metastasize to the brain, bones, liver, and adrenal glands. Immunostaining of a biopsy usually helps determine the original source. The presence of Napsin-A, TTF-1, CK7, and CK20 help confirm the subtype of lung carcinoma. SCLC that originates from neuroendocrine cells may express CD56, neural cell adhesion molecule, synaptophysin, or chromogranin.

Question: What is used to help determine the original source?
Immunostaining of a biopsy