According to Endometriosis: benign, malignant, or something in between? (PubMed, 2017), endometriosis is not considered cancerous because:
Does not appear as an expansille tumor mass
Is not destructive
The cells in the lesions are normal, like in the uteral endometrium
Is not lethal
Immunotherapy of endometriosis is in the experimental phase - you can read ...
It is important to note that cancer includes loss of the property of contact inhibition in cells. This leads to uncontrolled divisions in cancerous cells. Although endometriosis includes cells dividing at ectopic locations, they still possess the property of contact inhibition. So, endometriosis can't be called a form of cancer.
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