Differential diagnosis:
(1) second malignancy
(2) incorrect diagnosis (either first or second report)
(3) two or more tumor types in the primary tumor
(4) mixed differentiation in a tumor (for example Grades II and III out of III), with metastasis or overgrowth of the more poorly differentiated component
(5) biphasic tumor (carcinosarcoma, etc.)
(6) dedifferentiation of a tumor
A second malignancy may arise in a person with:
(1) a genetic predisposition to cancer
(2) an environmental exposure such as radiation
(3) previous chemoradiation for the first tumor (usually several years later)
Some examples of different tumor types in a primary tumor:
(1) non-small cell carcinomas of the lung, with large cell undifferentiated, squamous cell carcinoma and adenocarcinoma
(2) small cell undifferentiated ("oat cell") carcinoma of the lung, mixed with a minor or major large cell component
(3) prostate adenocarcinoma, with small cell undifferentiated carcinoma component
If there are different histologic types or differentiation in the original tumor, then one component may become dominant:
(1) selection by cancer therapy of one component
(2) more undifferentiated
