Abdominal masses in the pediatric population most commonly derive from the ovaries . Ovarian masses, including both non-neoplastic lesions and neoplastic tumors, can occur in all age groups. The incidence, clinical presentation and histological distribution of such lesions in children and adolescents are distinct from those in adults and require a particularized therapeutic approach . Masses of the ovary range from simple functional cysts to malignant neoplasms. Underneath, pediatric non-neoplastic ovarian masses are presented. These represent benign tumor-like lesions that are not composed of neoplastic cells and include functional cysts, endometrioma, torsion, abscess and lymphangioma.
2. Functional Cysts
Functional cysts, also known as physiological cysts, occur during the normal menstrual cycle and include follicular and corpus luteum cysts. Follicular cysts form when follicles fail to rupture during ovulation. Such cysts may develop due to a lack of the physiological release of the ovum as a result of excessive follicle-stimulating hormone (FSH) stimulation or absence of midcycle surge of luteinizing hormone (LH). They appear smooth, thin-walled and unilocular with a diameter larger than 3 cm. Corpus luteum cysts, on the other hand, arise when the dissolution of the corpus luteum does not occur. Under physiological conditions and unless pregnancy occurs, the functional life span of the corpus luteum is 14 days, after which it spontaneously involutes to form the corpus albicans, whereas, in cases of fertilization, the corpus luteum undergoes dissolution at 14 weeks of gestation. Corpus luteum cysts appear simple or complex, thick-walled, may contain internal debris and usually grow to 3 cm. Functional cysts are typically asymptomatic and spontaneously resolve without intervention 
Functional cysts are frequent among all pediatric age groups because of different hormonal insults. Causative mechanisms include the presence of maternal and placental hormones in infancy, the release of gonadotropins by the developing pituitary gland in the prepubertal period and dysfunctional ovulation in adolescence. The challenge in the management of girls with ovarian cysts lies in identifying a small percentage of neoplastic lesions among all these functional cysts 
As far as the sonographic appearance of functional cysts is concerned, follicular cysts are usually thin-walled with anechoic contents and rarely exceed 8–10 cm in diameter. Hyperechoic enhancement of the posterior wall is observed due to the reflection of the ultrasound beam off this wall after travelling through the anechoic window created by the clear contents of the cyst. Corpus luteum cysts appear thick-walled and hyperechoic (Figure 1
). They typically exhibit peripheral circumferential blood flow, which is known as the “ring of fire sign”. The cyst contents typically exhibit a spider-web-like appearance due to limited internal hemorrhage, although they may also show different features, such as blood clots that resemble solid components. These clots have no blood flow in Doppler examination and present a typical jelly-like “wobbling” movement within the cyst when the ultrasound transducer is used to gently prod the ovary 
Figure 1. Ultrasonograms of two adolescents demonstrating (a) a thin-walled follicular cyst with posterior acoustic enhancement and (b) a thick-walled corpus luteum cyst.
Endometriomas are cystic lesions that arise from the disease process of endometriosis, which is defined as the presence of endometrial glands and stroma outside the uterus . They contain a thick, dark brown, endometrial fluid that results from the accumulation of degenerated menstrual blood products and are, therefore, often referred to as chocolate cysts . Endometriomas most commonly develop in the ovaries and can affect almost all ages, although they are typically diagnosed during the reproductive years . Clinical manifestations include pelvic pain or tenderness, dysmenorrhea, dyspareunia, painful urination, painful defecation, urinary frequency, nausea/vomiting, back pain, palpable mass during bimanual examination in large enough lesions and acute abdomen in case of rupture . Patients with any of these findings usually undergo an ultrasound examination, which, in case of ovarian endometrioma, classically reveals a simple, unilocular, avascular cyst with ground-glass appearance, namely low-level homogenous echoes that result from old hemorrhagic debris (Figure 2) . On magnetic resonance imaging (MRI), endometriomas typically appear T1 hyperintense without loss of signal after fat saturation and T2 hypointense. The low signal intensity on T2-weighted images, which is caused by the high concentration of protein and iron that result from cyclical hemorrhage, is referred to as the “shading sign” and, combined with high T1 signal intensity, is highly suggestive of endometrioma . Treatment options for ovarian endometrioma include hormonal medications and surgery .
Figure 2. Ultrasonogram of a 17-year-old patient showing a typical unilocular endometrioma with ground-glass echogenicity of the cyst fluid.
4. Ovarian Torsion
Ovarian torsion refers to the rotation of the ovary around the supporting ligaments, which can lead to obstructed lymphatic, then venous and finally arterial flow and infarction 
. Adnexal torsion is a term used to describe the twisting of either the ovary, the fallopian tube or both 
. Most of the times, both the ovary and a portion of the ipsilateral fallopian tube are affected, which is known as tubo-ovarian torsion, while isolated ovarian or isolated tubal torsion is rare 
. Fifteen percent of cases of ovarian torsion occur in the pediatric population 
. Although it can be observed in pediatric patients of any age, a bimodal age distribution has been described, with 16% of cases occurring in infants and 52% in the perimenarchal period 
Most pediatric patients with ovarian torsion have an underlying pathological condition in the involved adnexa that is present in 51–84% of cases. The underlying adnexal lesions are typically benign, with mature teratomas and follicular cysts most frequently detected, while malignant lesions associated with torsion are extremely rare in the pediatric population, possibly due to the presence of adhesions to the surrounding structures 
. In a population of 86 girls aged up to 16 years studied by User et al. 
, torsion was more common among masses smaller than 6 cm and of cystic nature. These observations might be explained by the shallow pelvis of young girls, which hinders the mobilization of large masses and the greater gravitational force of solid portions found in neoplasms 
. Torsion of normal adnexa without any underlying condition occurs more frequently in pediatric patients compared to adults 
. A postulated mechanism for this condition is the increased mobility of adnexa due to the greater length and laxity of ligaments in some girls 
. Ovarian torsion is rare after pelvic inflammatory disease (PID) and endometriosis probably because of the presence of adhesions that restrict ovarian mobility 
Clinical manifestations of ovarian torsion are often non-specific, mimicking other clinical entities of the genitourinary and gastrointestinal system. Most patients present with acute, unilateral, severe, lower abdominal pain, palpable mass, nausea/vomiting, fever and dysuria 
. As mentioned above, 16% of pediatric ovarian torsion cases occur in infants. Ovarian abnormalities on prenatal imaging, abdominal mass and feeding intolerance have been described as the characteristic presenting clinical features in this age group 
Ultrasonography (US) is the modality of choice for evaluating suspected ovarian torsion 
. The most common sonographic finding of ovarian torsion in the pediatric population is the unilateral ovarian enlargement considered to be secondary to stromal edema and hemorrhage 
. Sintim-Damoa et al. 
suggest that torsion should be suspected when the ovarian volume is at least 3 times greater than expected according to the girl’s age. When considering whether an ovary is enlarged or not, comparing the size of the affected ovary with the size of the contralateral ovary is of great importance 
. Other sonographic findings include peripherally displaced or even not visible follicles, medialization of the ovary, deviation of the uterus off the midline toward the twisted ovary, free pelvic fluid, a “whirlpool sign” of swirling vessels, coexistence of an adnexal mass and absent flow on Doppler examination 
. However, the absence of Doppler flow cannot establish the absolute diagnosis because it can also be observed in normal, non-torsed ovaries in the pediatric population. In addition, present arterial flow cannot exclude the likelihood of ovarian torsion due to the dual ovarian blood supply from both the ovarian and uterine arteries and because arterial occlusion occurs after lymphatic and venous obstruction 
Computed tomography (CT) and MRI can be used as secondary imaging modalities for the evaluation of ovarian torsion. Both methods may show an enlarged torsed ovary with cystic appearance, possibly dislocated to the midline, while the uterus may be deviated toward the twisted ovary 
. The MRI protocol should include fat suppression techniques to distinguish fat from hemorrhage and contrast-enhanced fat-suppressed T1-weighted images to help depict low enhancement of the ovary, which suggests infarction or necrosis 
In children and adolescents, ovarian torsion typically occurs acutely and constitutes a surgical emergency since it may result in impairment or loss of ovarian function due to persistent ischemia in cases of delayed diagnosis and intervention. Hence, it should be promptly detected and is treated with emergent, laparoscopic, surgical detorsion and visual inspection for viability in an attempt to salvage ovarian tissue 
5. Tubo-Ovarian Abscess
Tubo-ovarian abscess (TOA) is a complex, infectious, adnexal mass that results from PID and consequently affects sexually active patients usually aged 15–25 years . It can manifest with abdominal pain, pelvic palpable mass, fever, nausea/vomiting, vaginal discharge or abnormal bleeding and cervical motion tenderness during bimanual examination. Further evaluation may show leukocytosis and bacterial growth in cervical, urine and blood cultures . On imaging, TOA appears as a complex fluid-filled lesion with thick walls and septa . US reveals an adnexal mass with mixed or ground-glass echogenicity . On MRI, the fluid of TOA exhibits variable signal intensity on T1-weighted images, reflecting the degree of proteinaceous and hemorrhagic contents and often appears hyperintense on T2-weighted images. The walls and septa typically show low T1 and T2 signal intensity and enhance avidly . T1-weighted imaging may demonstrate an internal hyperintense and enhancing rim, which is supposed to represent granulation tissue combined with hemorrhage . Restricted diffusion is observed on diffusion-weighted imaging (DWI) . Since TOA occurs in the context of PID, imaging can also depict adnexal edema and peritoneal fibrosis and adhesions, which appear as mesh-like linear strands with low T1 signal intensity and enhancement . The combination of the aforementioned clinical and radiological characteristics render the diagnosis of TOA relatively straightforward. TOA is typically treated with antibiotics, although surgery may be unavoidable in some cases .
Lymphangiomas are rare benign malformations of the lymphatic system . Their localization in the ovary is uncommon . In fact, only 24 cases of ovarian lymphangioma have been reported in the literature during the last 70 years, the majority of which concerned adult patients . Pani et al.  reported a case of a 16-year-old girl that presented with a longstanding, painless, abdominal distension and a non-tender, mobile, palpable mass of the entire abdomen. Tumor markers were negative. CT and MRI depicted a large, multiseptated, fluid-filled, cystic mass with well-defined and non-enhanced walls that measured 40 cm in the longitudinal axis and did not infiltrate into the surrounding tissues. The lesion was surgically removed, and the final diagnosis after histopathological examination was lymphangioma arising from the left ovary. The postoperative course of the patient was uneventful. This case highlights that lymphangiomas can occur in the pediatric population and should be included in the differential diagnosis of ovarian masses in this age group.