A rather liberal translation, but in this case it fits (it would sound very elaborate to say "determination of diagnostic entity", but this could also fit).

Die Bestimmung somatischer genetischer Alterationen im Tumorgewebe kann zur Entitätsbestimmung (differentialdiagnostische Eingrenzung einer malignen Erkrankung) beitragen.

siehe
https://www.helios-gesundheit.de/ambulant/berlin-zehlendorf-...



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Note added at 2 Stunden (2020-01-17 15:32:12 GMT)
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sowie hier:

Bei Weichteiltumoren gelingt mit der Kontrastmittelapplikation die Entitätsbestimmung sicherer, insbesondere die Abgrenzung gegenüber den häufigen Ganglien.
in
https://www.thieme-connect.de/products/ejournals/pdf/10.1055...

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Note added at 2 Stunden (2020-01-17 15:34:44 GMT)
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Entität

Englisch: entity
Definition

Als Entität bezeichnet man in der Medizin einen Betrachtungsgegenstand, der für sich genommen eine eigene Einheit bzw. Ganzheit darstellt.

So ist etwa eine Krankheit eine gut definierbare pathologische Entität.

Low confidence level due to lack of context. This answer is only about the use of "Entitäten" in the context of cancer diseases.

Immunohistochemistry can be used to demonstrate the presence or absence of various substances in or on tumour cells. Breast cancer, for example, may be HER2-positive or HER2-negative, oestrogen receptor-positive or negative, progesterone receptor-positive or negative, etc. This knowledge affects the therapeutic choices for the cancer patient.

A given tumour disease (such as breast cancer) is therefore considered to comprise a whole bunch of "Entitäten" which all require different therapeutic approaches. A well-known "Entität" of breast cancer is, for example, the so-called "triple-negative breast cancer" which has neither HER2 nor oestrogen receptors nor progesterone receptors.

I am fairly sure that "identification of the entity", as suggested by Veronika, would be understood in this context, but it does not seem to be the most common expression in English. Based on a web search, most uses of "tumour entity" obviously are from non-native authors.

Less specific terms like "tumour type" or "tumour characteristics" may also be used, but they often include more general information, such as tumour stage, tumour histology, and others.

hth