Objectives To study recurrence rates in Danish high-risk stage I endometrial cancers not given radiotherapy in accordance with the decision of the Danish Gynecological Cancer Group.

Methods This prospective national cohort study includes all 4707 endometrial carcinomas diagnosed from 2005 to 2012. Of these, 623 patients had grade 3 endometroid adenocarcinoma with >50% myometrial invasion or serous/clear/undifferentiated carcinoma (with any depth of invasion). In 305 patients with high-risk stage I on final pathology, 14.1% received adjuvant external beam radiotherapy and 9.6% adjuvant chemotherapy. No patients received brachytherapy. 5-year Kaplan-Meier survival estimates and actuarial recurrence rates were calculated, and adjusted Cox regression analysis used for comparison. Recurrence rates were compared with historical Danish population data (DEMCA 98–99).

Results For non-irradiated patients, 5-year overall survival, cancer-specific survival, and progression-free survival rates in high-risk stage I patients were 65%, 78%, and 73%, respectively. For non-irradiated patients, isolated local recurrences were uncommon (vaginal 3.1%, pelvic 0.4%). Death was mainly due to a high occurrence of non-local recurrences, with 8.8% experiencing a first recurrence in the abdominal cavity (outside the field where radiation traditionally have been given) and 13.0% a distant metastasis outside the abdominal cavity. Grade 3 tumors with >50% myometrial invasion seem to be characterized by a different pattern of recurrences, with significantly more isolated vaginal recurrences (7.9% vs 2.2%) and fewer total number of abdominal recurrences (7.9% vs 15.3%) as compared with unfavorable tumor types.

Conclusion Isolated vaginal and pelvic recurrences were rare (3–5%) in patients with a final pathologic diagnosis of high-risk stage I endometrial cancer even after the Danish Gynecological Cancer Group decided to omit all types of postoperative radiotherapy and introduce lymph node staging.