Can endometriosis be considered a form of cancer?
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 more here: Novel Therapies Targeting Endometriosis (PubMed, 2011). Other treatments mentioned are anti-inflammatory drugs, selective estrogen or progesterone modulators, statins and antiangiogenic agents. There is also a study Open Label Immunotherapy of Endometriosis running - it is briefly described on ClinicalTrials.gov.

Chemotherapy, which is effective in some cancers, is based on killing quickly-dividing cells, but cells in endometriosis do not divide so quickly, so you can't treat it by chemotherapy in the same way.