• ECOG performance status 0-1 or KPS ≥ 70%

• Age ≥ 18 years

• Endometrioid adenocarcinoma histologic diagnosis on endometrial biopsy or dilatation and curettage

• No evidence of extrauterine disease, or suspicious pelvic lymph nodes, or distant metastases, or cervical invasion on pre-operative conventional imaging studies (Pelvic +/- Abdomen CT or MRI or sonogram, or body PET scan) and physical examination (uterine confined by exam and imaging )

• Suitable candidate for surgery

• Planned surgical treatment including hysterectomy in combination with SLN biopsy and a bilateral salpingo-oophorectomy

• No history of second primary cancer (invasive or in situ) within the past 5 years, not including non-melanoma skin cancer

• Approved and signed informed consent

• No history of neoadjuvant chemotherapy or radiotherapy for endometrial cancer

• No history of prior pelvic or abdominal radiotherapy

∙ Study Cohort (n=182)

∙ A patient will be enrolled in the study cohort if all the following criteria are met:

• At surgery, the patient must undergo:

‣ Hysterectomy

⁃ Bilateral salpingo-oophorectomy, unless already previously performed

⁃ Bilateral pelvic SLN mapping (bilateral sentinel nodes are negative for malignancy)

• On the final pathologic report, the patient must have a diagnosis of:

‣ Stage I intermediate-risk endometrial endometrioid cancer (Grade 1 or Grade 2 with ≥ 50% myometrial invasion or Grade 3 with \<50% myometrial invasion, including non-invasive disease)

⁃ Negative pelvic peritoneal cytology

• Adjuvant treatment as recommended by the multidisciplinary team must be as follows:

‣ No adjuvant treatment, or

⁃ Intravaginal radiation only