This study aims to understand whether endometriosis lesions - and tumors that can arise in association with endometriosis - occur more often on one side of the pelvis than the other.

During surgery, doctors will record the exact location of each lesion to see if there is a consistent right- or left-sided pattern.

Understanding these patterns may help improve knowledge about how endometriosis develops (pathogenesis), and thus improve its treatment.

Inclusion Criteria:

• age ≥18 years at the time of histological diagnosis of endometriosis
• patients undergoing surgery with endometriotic lesions or patients with endometriotic lesions associated with endometrioid or clear cell ovarian cancer (Stage I-II) or borderline seromucinous ovarian cancer
• first operation for endometriosis in the reference center (if recurrence consider only if both operations were performed at the reference center).

Exclusion Criteria:

• age <18 years
• patients affected by Stage I-II endometrioid or clear cell carcinoma or seromucinous borderline ovarian cancer not associated with endometriosis or in a more advanced stage of disease
• women with an ultrasound diagnosis of endometriosis who have not undergone surgery
• previous operations for endometriosis performed at another center

All patients with histological diagnosis of endometriosis and endometriosis associated with endometrioid carcinoma, clear cell carcinoma and seromucinous borderline tumor operated at the Fondazione IRCCS Ca' Granda Ospedale Maggiore Policlinico and at the centers participating in the study from 01/2013 to 12/2023 will be recruited.

Data relating to histological diagnosis will be collected from medical records.

In particular, in addition to tumours associated with endometriosis, data will be collected from patients with a histological diagnosis of:

1. ovarian endometriosis
2. intestinal endometriosis
3. ureteral endometriosis
4. sciatic nerve endometriosis
5. inguinal-round ligament endometriosis
6. uterosacral ligament endometriosis
7. peritoneal endometriosis
8. diaphragmatic endometriosis
9. pleural and pulmonary endometriosis
10. umbilical endometriosis (associated with right hemidiaphragm and/or left hemidiaphragm)