Rationale: External beam radiotherapy (EBRT) is an important cornerstone in treatment to reduce locoregional relapse in high(-intermediate) risk endometrial cancer (EC) patients. MR-guided adaptive radiotherapy (MGART) allows for more accurate delivery of radiation beams to the patients by visualizing and correcting for interfraction motion, with subsequent reduction of safety margins around the radiotherapy plan. Objective: The MASTEC study will investigate the safety of hypofractionated MR-guided adaptive radiotherapy in five fractions for elective pelvic nodal and vaginal vault irradiation in endometrial cancer.

Study design: Phase II multicentre intervention study Study population: Patients with endometrial cancer that receive adjuvant EBRT to the pelvic nodal areas and vaginal vault (with or without vaginal brachytherapy).

Intervention (if applicable): MR-guided adaptive radiotherapy (MGART) with reduced PTV margins, including 5 times 6Gy to the vaginal vault and pelvic nodal areas, 2 times a week .

Main study parameters/endpoints: The primary endpoint is acute gastrointestinal and genitourinary toxicity, scored by the Common Terminology Criteria Adverse Events version 6.0. Secondary endpoints are late gastrointestinal and genitourinary toxicity, quality of life and disease free survival; disease-specific survival; overall survival and metastasis-free survival.

Inclusion criteria

In order to be eligible to participate in this study, a participant must meet all of the following criteria:

• Women aged 18 years or older with histologically proven carcinoma of the endometrium, treated with curative hysterectomy with or without bilateral salpingo-oophorectomy. All histologies may be included (endometrioid, clear cell carcinoma, de-/undifferentiated carcinoma, carcinosarcoma, other non-endometrioid)

• Candidate for adjuvant EBRT as decided by multidisciplinary tumour board, amongst others:

  • FIGO IB grade 3 endometrioid carcinoma
  • FIGO stage II endometrioid carcinoma
  • FIGO stage IIIA-C1 endometrioid carcinoma
  • FIGO IB-II clear cell carcinoma
  • FIGO IA-III carcinosarcioma

• Patients receiving adjuvant systemic therapy are eligible when these therapies will not be administered concurrently

• Patients receiving vaginal vault brachytherapy are eligible

• Capable of giving informed consent

Exclusion criteria

A potential participant who meets any of the following criteria will be excluded from participation in this study:

• Contra-indications for MRI according to the guidelines of the local department of Radiology, inability to lay on a treatment table for 45-60 minutes or severe claustrofobia
• Presence of para-aortic lymph node metastases (FIGO stage IIIC2)
• Prior pelvic radiotherapy
• WHO performance score > 2