The plural of Tumor is not, as one might have expected, Tumore, but Tumoren.
The Duden says
der Tumor; Genitiv: des Tumors, Plural: die Tumoren, umgangssprachlich auch: Tumore
So Tumore is just the colloquial plural and sounds more natural. I hear many German native speakers say Tumore, even physicians. That's why I'm curious about the origin of Tumoren.
What's the etymology of Tumoren as the plural form of Tumor?
I'm aware of this earlier question in German about Skript and Tumor, which is also tagged etymology but doesn't seem to ask about the history of that plural and the only answer doesn't touch upon it either, even saying they never heard Tumoren. They seem to be from Austria, so maybe there are regional differences. So it doesn't answer my question.