Endometriosis affects approximately 10% of women of reproductive age [2]. OEs are found in 17–44% of all adult patients with endometriosis [7, 8], comprising 35% of all benign ovarian cysts [7, 8]. Meanwhile, the prevalence of OEs in children remains uncertain, as most ovarian cysts in pediatric patients are serous or mucinous cystadenomas or mature teratomas. It is especially rare for a child to develop an OE prior to menstruation. There were nine cases of endometriosis diagnosed before menstruation reported in literature, aged between 8 and 13 years, as shown in Table 1. In 1996, Reese et al. reported cases of endometriosis before menarche in two adolescents aged 12 and 13 [9, 10]. Marsh et al. reported a series of five premenarcheal girls with pelvic pain who were found to have visual lesions consistent with endometriosis on laparoscopic evaluation [3]. Two cases in this study were reported of recurrence after menarche, through a second laparoscopic procedure performed for worsening pelvic pain unresponsive to hormonal therapy received since the onset of menarche. Reports of OE before menstruation are limited, with only two cases noted. Gogacz et al. described an 11-year-old adolescent postoperatively diagnosed with left OE [1]. Uchida et al. reported a case of OE in a 12-year-old girl before menarche [11]. To our knowledge, this is the youngest report of an OE diagnosed in a patient prior to menstruation.

It is difficult to initially include an OE in the differential diagnosis of pediatric ovarian cysts, particularly before the onset of menstruation. All prior cases were postoperatively diagnosed through exploratory laparoscopy intended for diagnostic and therapeutic purposes. In individuals with OEs, including pediatric patients, surgical intervention alone is deemed insufficient owing to the potential persistence of microscopic residual disease, necessitating adjunctive medical suppression [6, 12]. In addition, evidence suggests that endometriosis may gradually progress in adolescents who do not adhere to medical therapy [1, 13]. Younger age has been reported as an independent risk factor for endometriosis recurrence after conservative surgical treatment of endometriosis in adults. [12, 14]. However, there are no reports regarding the long-term prognosis or preferred post-operative treatment of OEs diagnosed before menstruation.

Therapeutic intervention not only provides relief from clinical symptoms but also prevents the progression of endometriosis and future infertility. The first-line medical therapy for adolescents are low-dose oral contraceptive pills (OCPs) and non-steroidal anti-inflammatory drugs (NSAIDs) [1, 12]. OCPs suppress ovarian hormone production, inducing decidualization of endometrial tissue, and are known to be safe for long-term use [9]. Gonadotropin-releasing hormone (GnRH) agonists are not recommended for girls younger than 16 years of age due to concerns regarding their adverse effects on bone formation and density [6].

In the present case, we did not immediately begin hormone treatments, since the patient did not experience chronic pelvic pain, and remained asymptomatic after the surgery, as she had yet to reach menarche. However, given that endometriosis is an estrogen-dependent, chronic, progressive disease, the patient may develop symptoms upon the onset of menstruation in the future; therefore, we intend to continue regular follow-ups. Close monitoring, along with collaborative efforts between pediatric surgeons, paediatricians, and gynecologists, is essential to prevent disease recurrence or progression and potential future infertility.