Objectives The goal of this study was to evaluate the depth of myometrial invasion as a predictor of distant recurrence in patients with node-negative stage IB endometrioid endometrial cancer.

Methods A retrospective multicenter study, including surgically staged endometrial cancer patients at Mayo Clinic, Rochester (MN, USA) between January 1999 and December 2017, and Fondazione Policlinico Universitario A. Gemelli (Rome, Italy) between March 2002 and March 2017, was conducted. Patients without lymph node assessment were excluded. The follow-up was restricted to the first 5 years following surgery. Recurrence-free survival was estimated using the Kaplan-Meier method. Cox proportional hazards models were fit to evaluate the association of clinical and pathologic characteristics with the risk of recurrence.

Results Of 386 patients, the mean (SD) depth of myometrial invasion was 70.4 (13.2)%. We identified 51 recurrences (14 isolated vaginal, 37 non-vaginal); the median follow-up of the remaining patients was 4.5 (IQR 2.3–7.0) years. At univariate analysis, the risk of non-vaginal recurrence increased by 64% (95% CI 1.28 to 2.12) for every 10-unit increase in the depth of myometrial invasion. International Federation of Gynecology and Obstetrics (FIGO) grade and myometrial invasion were independent predictors of non-vaginal recurrence. The 5-year non-vaginal recurrence-free survival was 95.2% (95% CI 92.0% to 98.6%), 84.0% (95% CI 76.6% to 92.1%), and 67.1% (95% CI 54.2% to 83.0%) for subsets of patients with myometrial invasion <71% (n=207), myometrial invasion ≥71% and grade 1–2 (n=132), and myometrial invasion ≥71% and grade 3 (n=47), respectively. A total of 256 (66.3%) patients received either vaginal brachytherapy only or no adjuvant therapy. Patients who received adjuvant chemotherapy, regardless of receipt of external beam radiotherapy or vaginal brachytherapy, had an approximately 70% lower risk of any recurrence (HR adjusted for age, grade, myometrial invasion 0.31, 95% CI 0.12 to 0.85) and of non-vaginal recurrence (adjusted HR 0.32, 95% CI 0.10 to 0.99).

Conclusion The invasion of the outer third of the myometrium and histologic grade were found to be independent predictors of distant recurrence among patients with endometrioid, node-negative stage IB endometrial cancer. Future studies should investigate if systemic adjuvant therapy for patients with myometrial invasion of the outer third would improve outcomes.