It would depend upon your topic. If writing about a particular tumor (one person's tumor), you might say, "Malignancy was a cause of the tumor. Malignancy caused a tumor.
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kpripperHello, I have a problem with articles.Yes, but I think you also have a problem with word meaning.
kpripperMalignancy as a cause of hydrothoraxThis is OK just as you wrote it (above).