Introduction/Background The aim of this study was to assess the outcomes of a large series of patients treated conservatively for stage II or III serous borderline tumors of the ovary (SBOTs) with a long-term follow-up.

Methodology Patients with SBOTs and peritoneal implants, treated in or referred to our institution, were retrospectively reviewed. Outcomes of patients treated conservatively (preservation of the uterus and at least a part of one ovary) to promote subsequent fertility were specifically analyzed.

Results Between 1971 and 2017, 212 patients were identified and followed-up. Among these patients, 65 underwent conservative treatment; eight patients had invasive implants. Among patients treated conservatively, 38 (58%) patients recurred. Twenty-eight recurrences were observed under the form of borderline tumor on the spared ovary and/or noninvasive implants, but eight patients had a recurrence under the form of invasive disease. Compared with radical surgery, the use of conservative treatment (p < 0.0001) was a prognostic factor on disease-free survival (DFS), but without an impact on overall survival (OS). Nevertheless, three deaths occurred. Twenty-four pregnancies (13 spontaneous) were observed in 20 patients (29 patients wanted to become pregnant).

Conclusion In this series collecting the largest number of patients undergoing conservative surgery for stage II/III SBOTs, spontaneous pregnancies can be achieved after conservative treatment of advanced-stage disease, but the recurrence rate is high and three deaths were observed. These patients were spared their fertility but with a high rate of recurrence. Uncertainties regarding the safety of conservative treatment should be exposed to these patients.