Right inguinal canal endometriosis mimicking a groin mass: a rare case report of round-ligament involvement

A 31-year-old G0P0 woman presented with a severe right groin pain and swelling that had been gradually increasing over the past 4 months. The swelling was not influenced by menstruation. She reported regular menstrual cycles and denied any gynecological symptoms suggestive of endometriosis. Her gynecological history was unremarkable, with no previous hormonal therapy or contraception use, and no prior abdominal or pelvic surgeries or gynecological interventions. Her past surgical history included only a tonsillectomy and polyp removal. There was a negative family history of endometriosis. On physical examination, her vital signs were normal. Inspection revealed a non-tender right inguinal mass with unremarkable overlying skin and no cough impulse. The remainder of the abdominal and pelvic examination was normal. Pelvic MRI demonstrated an irregular nodular soft-tissue lesion within the right inguinal canal, hypointense with a tiny punctate hemorrhagic focus and showing progressive post-contrast enhancement on T2 and T1 (Fig. 1 ). Although the lesion appears dorsal to the deep inguinal ring on MRI, it remains anatomically intracanalar. The extraperitoneal segment of the round ligament traverses the deep inguinal ring and courses within the inguinal canal before exiting at the superficial ring. Lesions arising from this segment may appear posterior or dorsal on cross-sectional imaging, particularly when adherent to the posterior canal wall, yet still lie within the canal. This anatomic relationship explains both the MRI appearance and the lesion’s accessibility on physical examination as a palpable groin mass. Differential diagnoses included an endometriotic nodule and, less likely, a benign non-endometriotic fibrous lesion of the inguinal canal; the presence of a punctate hemorrhagic focus and progressive contrast enhancement on MRI favored endometriosis rather than an inert fibrous mass. The patient underwent right inguinal canal exploration (Fig. 2 ), which revealed a mass adherent to the round ligament and the canal floor. Under general anesthesia, a standard right inguinal incision measuring 7 cm was performed. Exploration of the inguinal canal revealed a firm nodular lesion adherent to the extraperitoneal portion of the round ligament and the inguinal canal floor. Complete excision of the mass measuring 3.5 × 2 × 1 cm was achieved (Fig. 3 ), without formal resection of the canal wall, although limited dissection resulted in localized weakening of the posterior wall. The inguinal canal floor was therefore reinforced with a synthetic mesh to reduce the risk of postoperative hernia formation. She tolerated the procedure well and was discharged in good condition. The excised mass was sent for histopathological examination and revealed benign mesothelium-lined vascularized fibrous tissue with focal benign endometrial glands and stroma that show focal hemorrhage, suggesting endometriosis of the right round ligament. Postoperative follow-up consisted of outpatient clinical evaluations at 2 weeks and 3 months after surgery, during which physical examination revealed complete resolution of symptoms, a well-healed surgical site, and no evidence of inguinal canal weakness, hernia formation, or local recurrence. Figure 1. Pelvic MRI demonstrating an irregular nodular lesion (blue arrow) arising from the extraperitoneal segment of the right round ligament, located within the inguinal canal. Although the lesion appears dorsal to the deep inguinal ring on axial imaging, its relationship to the round ligament confirms its intracanalar location. Figure 2. Open right inguinal canal exploration. Figure 3. Excision of the mass from the right inguinal ligament measuring 3.5 × 2 × 1 cm. Pelvic MRI demonstrating an irregular nodular lesion (blue arrow) arising from the extraperitoneal segment of the right round ligament, located within the inguinal canal. Although the lesion appears dorsal to the deep inguinal ring on axial imaging, its relationship to the round ligament confirms its intracanalar location. Open right inguinal canal exploration. Excision of the mass from the right inguinal ligament measuring 3.5 × 2 × 1 cm.

Endometriosis is a common condition, affecting more than 1 in 10 women worldwide. Estimates of its true incidence have shifted over time, and significant gaps remain in our understanding of its epidemiology, including potential differences across races and countries [ 1 – 3 ] . These uncertainties largely stem from the fact that diagnosing superficial endometriosis still requires laparoscopy, and even then, subtle lesions can easily be missed [ 1 ] . A substantial number of endometriotic lesions likely go unrecognized. For example, studies show that around 30% of appendices that appear completely normal during surgery still contain microscopic endometriosis [ 4 ] . Similarly, women with deep bowel endometriosis often have microscopic disease in distant bowel segments and within lymph nodes [ 1 ] . Currently, a definitive diagnosis requires surgical confirmation of endometrial-like tissue located outside the uterus. Lesions may also occur in women without symptoms and are present in up to half of those evaluated for infertility [ 5 ] . Diagnosis is frequently delayed because hallmark symptoms such as pelvic pain or infertility overlap with other gynecologic and non-gynecologic conditions. Although imaging can speed detection of certain types of endometriosis, progress toward a reliable, noninvasive blood test has been slow [ 5 ] . Treatment remains centered on two main approaches: surgical removal of lesions and medical therapy aimed at suppressing ovarian hormone production [ 5 ] . This case is presented for its educational value in highlighting an atypical, noncyclical presentation of round-ligament endometriosis, the diagnostic role of MRI in a challenging clinical scenario, and the surgical considerations necessary to achieve complete excision and prevent postoperative inguinal canal weakness. This case report has been reported in line with the SCARE checklist [ 6 ] . HIGHLIGHTS Inguinal endometriosis of the round ligament is a rare extrapelvic manifestation that often mimics common groin pathologies. MRI provides key diagnostic clues, particularly T2-hypointense fibrotic tissue with small hemorrhagic foci. Surgical exploration confirms the diagnosis and allows complete excision of the lesion with favorable short-term postoperative outcomes. Mesh-reinforced canal floor repair is effective in preventing postoperative inguinal canal floor weakness and hernia recurrence. Inguinal endometriosis should be considered in reproductive-age women with groin masses, even without cyclical symptoms or known pelvic endometriosis. Inguinal endometriosis of the round ligament is a rare extrapelvic manifestation that often mimics common groin pathologies. MRI provides key diagnostic clues, particularly T2-hypointense fibrotic tissue with small hemorrhagic foci. Surgical exploration confirms the diagnosis and allows complete excision of the lesion with favorable short-term postoperative outcomes. Mesh-reinforced canal floor repair is effective in preventing postoperative inguinal canal floor weakness and hernia recurrence. Inguinal endometriosis should be considered in reproductive-age women with groin masses, even without cyclical symptoms or known pelvic endometriosis. Inguinal endometriosis of the round ligament is a rare extrapelvic manifestation that often mimics common groin pathologies. MRI provides key diagnostic clues, particularly T2-hypointense fibrotic tissue with small hemorrhagic foci. Surgical exploration confirms the diagnosis and allows complete excision of the lesion with favorable short-term postoperative outcomes. Mesh-reinforced canal floor repair is effective in preventing postoperative inguinal canal floor weakness and hernia recurrence. Inguinal endometriosis should be considered in reproductive-age women with groin masses, even without cyclical symptoms or known pelvic endometriosis.

The most widely recognized explanation of endometriosis is that endometrial tissue is implanted in the peritoneal cavity by retrograde menstruation. The first theory describing the origin of endometriosis is the retrograde menstruation theory. According to this idea, endometriosis develops when sloughed endometrial cells and debris after menstruation travel retrogradely down the fallopian tubes and enter the pelvic cavity. 76–90% of women having patent fallopian tubes experience retrograde menstruation, albeit not all of these women have endometriosis. Endometrial cell resorption into the abdominal wall during menstrual flow is a common occurrence in 90% of menstrual females with patent fallopian tubes, even though it is only seen in those with hormonal or immunological issues [ 7 , 8 ] . Endometriosis involving the round ligament and extrapelvic inguinal structures is uncommon but well-described in surgical and gynecologic literature. Cases may present as a painful or painless groin mass and are frequently mistaken for inguinal hernia, lipoma, lymphadenopathy, or other soft-tissue lesions. The entity most often affects women of reproductive age and shows a right-side predominance in many series; several reviews and case series report a higher frequency on the right side and note that a substantial proportion are associated with a groin hernia [ 9 , 10 ] . Our patient’s presentation had several features that are concordant with previously reported inguinal/round-ligament endometriosis but also underscore common diagnostic pitfalls. Like many published cases, the lesion presented as an isolated, slowly enlarging groin mass without clear prior gynecologic symptoms or a history of pelvic surgery. Although cyclical (catamenial) variation of pain or mass size is a helpful clinical clue, it is not universally present; several series report cases in which the mass was noncyclical or asymptomatic in the pelvic domain, as in our patient. This absence of classic gynecologic symptoms can delay the correct preoperative diagnosis [ 9 , 11 ] Imaging, particularly MRI, can substantially increase preoperative suspicion by demonstrating features suggestive of endometriosis. Typical MRI findings for endometriotic nodules include areas of low T2 signal intensity (fibrotic component) together with tiny high-signal foci on T1 (hemorrhagic glandular foci) or fat-suppressed T1 sequences indicative of blood products [ 12 ] . Round-ligament and canal-of-Nuck lesions frequently appear as nodular masses centered on the ligament and may show progressive post-contrast enhancement if fibrotic and vascularized [ 12 ] . In our case, the lesion’s T2 hypointensity with a punctate hemorrhagic focus and contrast enhancement matched commonly described imaging features and helped prioritize endometriosis in the differential diagnosis. Radiologic recognition is important because it may change surgical planning (anticipating need for complete excision of the extraperitoneal round ligament and possible gynecologic referral) [ 12 ] . From a surgical perspective, MRI is superior to computed tomography (CT) scan for the evaluation of suspected inguinal endometriosis. CT may demonstrate a nonspecific soft-tissue mass but lacks sensitivity for identifying hemorrhagic foci and fibrotic components, often leading to misdiagnosis as hernia, lymphadenopathy, or lipoma. In contrast, MRI provides superior soft-tissue characterization, allowing detection of T2-hypointense fibrotic tissue with punctate hemorrhagic foci and progressive contrast enhancement features that are highly suggestive of endometriosis [ 12 ] . This improved tissue characterization aids preoperative diagnosis, surgical planning, and anticipation of the need for complete excision of the round ligament and canal repair. Definitive diagnosis remains histopathological. The characteristic finding is endometrial-type glands and stroma outside the uterus; immunohistochemical stains such as CD10 (stromal marker) and estrogen/progesterone receptors can be particularly helpful in small or fibrotic lesions where glandular elements are sparse. CD10 is commonly used to highlight endometrial stroma and can detect occult stromal components that might be inconspicuous on routine H&E [ 13 ] . In our case, histopathological examination with H&E staining demonstrated ectopic endometrial glands and stroma with focal hemorrhage within fibrous tissue, establishing the diagnosis of round-ligament endometriosis; immunohistochemical studies were not required due to the characteristic morphology. Surgical excision is the mainstay of treatment for inguinal/round-ligament endometriosis. Reported surgical strategies include wide local excision of the lesion together with removal of the affected portion of the round ligament and repair of any hernia defect. Tension-free mesh repair is frequently employed when a true hernia or a weakened canal floor is present; several recent case reports and series report good short-term outcomes with combined excision and mesh reinforcement [ 14 , 15 ] . Where pelvic endometriosis is suspected or when intra-abdominal disease is present, multidisciplinary management with gynecology is recommended to address synchronous pelvic disease and consider adjuvant hormonal therapy when indicated [ 14 , 15 ] . Our approach, complete excision of the mass with mesh-reinforced repair of the canal floor, follows the principles described in the literature and was uneventful. In addition, it is less likely in our case to have endometriosis deposits after inserting the mesh because the origin of the endometriosis, which is the round ligament migration, is excised completely, and there is a minimal risk of recurrence. Medical management of round-ligament endometriosis mirrors the hormonal approaches used for pelvic disease, but published reports consistently show limited effectiveness for isolated inguinal lesions. Agents such as combined oral contraceptives, progestins, and GnRH agonists may reduce cyclical pain or suppress lesion activity, yet they do not eliminate extrapelvic nodules, which are largely fibrotic and hormonally less responsive. Several case series emphasize that medical therapy is most useful as an adjunct, particularly when there is coexisting pelvic endometriosis or when postoperative hormonal suppression is desired to reduce the risk of recurrence [ 16 , 17 ] . Overall, peer-reviewed evidence supports that while hormonal therapy can provide symptom relief, definitive treatment of round-ligament endometriosis remains surgical, with medical therapy reserved for symptom modulation or for patients who are not surgical candidates. Prognosis after complete excision is generally favorable, although long-term data are limited. Recurrence appears uncommon when the lesion and involved segment of the round ligament are completely removed; however, if residual disease remains or if pelvic endometriosis is unrecognized and untreated, recurrence or persistent symptoms are possible. There are rare reports of malignant transformation (adenocarcinoma) arising from extrapelvic endometriosis, including the round ligament, so pathological examination should include careful assessment for atypia, especially in older patients or those with rapidly enlarging lesions [ 9 ] . The differential diagnosis of groin masses in women is broad and includes inguinal or femoral hernia, lymphadenopathy, lipoma, abscess, vascular abnormalities, and soft-tissue tumors [ 18 ] . Inguinal endometriosis may clinically mimic these conditions, particularly when cyclical symptoms are absent. Imaging plays a key role in differentiation: MRI findings such as T2-hypointense fibrotic tissue with punctate hemorrhagic foci or T1 hyperintensity are suggestive of endometriosis and help distinguish it from lipoma or lymphadenopathy. Definitive differentiation, however, relies on surgical exploration and histopathological confirmation demonstrating ectopic endometrial glands and stroma. Endometriosis involving the round ligament is a rare manifestation of extrapelvic disease. Published literature consists predominantly of isolated case reports and small case series, with inguinal or round-ligament involvement accounting for a small fraction of extrapelvic endometriosis cases. Reviews consistently report a right-sided predominance and frequent preoperative misdiagnosis as inguinal hernia or other groin pathologies. Moreover, a significant proportion of reported cases are associated with cyclical pain or known pelvic endometriosis, making the present case, characterized by an isolated, noncyclical groin mass without prior pelvic disease, an uncommon clinical presentation. Furthermore, it is important to highlight that right round ligament endometriosis, as in our case maybe more prominent than on the left side because the left adnexal element is covered by the sigmoid colon.

Round-ligament/inguinal endometriosis is an uncommon but important differential diagnosis for groin masses in reproductive-age women. Preoperative MRI can provide suggestive features that guide surgical planning; definitive diagnosis requires histopathology and may be aided by CD10 and hormone-receptor staining. Complete surgical excision of the lesion and repair of the canal floor yields good outcomes; multidisciplinary gynecologic input should be considered when pelvic disease is suspected.