Leiomyomas are common tumors that can originate from any location where smooth muscle exists[4]. While the most common site is the uterus, they have also been described in other locations[2, 21]. The retroperitoneum is a rare site for leiomyomas [1, 5, 12, 26], typically occurring in the pelvis but occasionally extending into the upper retroperitoneum [6].
In a 2008 review by Poliquin et al. [21], 105 cases of retroperitoneal leiomyomas were identified. In 2020 Sabrine et al [23], reported approximately 100 cases in English literature to date. Since then, no other systematic review of the literature has been conducted. A recent PubMed search using the terms “intraperitoneal” and “leiomyoma” revealed additional cases [3, 7, 9, 10, 14, 15, 17, 18, 20, 24, 25, 27–30].
More than 40% of women with retroperitoneal leiomyomas have a history of hysterectomy for uterine leiomyoma treatment or have concurrent uterine leiomyomas [21]. The etiology and pathogenesis of retroperitoneal leiomyomas are not well understood. It is unclear whether these lesions are metastatic or synchronous primary lesions and whether they arise from hormonally sensitive smooth muscle cells or embryonal remnants of the Müllerian or Wolffian ducts. Estrogen is believed to play a significant role in the development and maintenance of uterine leiomyomas, as evidenced by their frequent occurrence during reproductive age, reduction in size after menopause, and growth during pregnancy [1, 5]. Stutterecker et al. [26] speculated that embryonal remnants or local vessel musculature could be the tumor’s origin. Zaitoon et al. [31] proposed the theory of parasitic leiomyomas, where uterine leiomyomas that adhere to surrounding structures acquire an auxiliary blood supply and detach from the uterus. A newer type of parasitic myoma, called iatrogenic parasitic myoma, has been associated with the use of electric tissue morcellators in laparoscopy, which can produce small myoma fragments that implant on peritoneal and omental tissue and grow [8]. Several hypotheses have been proposed to explain the metastasizing theory [12]. One suggests benign tumors spread hematogenously, another proposes surgically induced hematogenous spread with tumor gaining venous access from surgical trauma (such as hysterectomy or myomectomy), and the last suggests a low-grade metastatic leiomyosarcoma. Retroperitoneal leiomyoma can also be isolated, without concurrent uterine leiomyoma or history of leiomyoma treatment, suggesting they could also represent primary extrauterine leiomyomatosis [11].
Retroperitoneal leiomyomas can be asymptomatic even if they are large and may be detected incidentally during routine check-ups or autopsies [5, 6]. Common symptoms include abdominal discomfort, fatigue, urinary symptoms, weight loss, back pain and pelvic pain. Almost 90% of patients with retroperitoneal leiomyomas present with a palpable pelvic mass [21].
Despite modern imaging techniques, retroperitoneal leiomyomas are often misdiagnosed as malignant tumors due to their unusual location. MRI is the most reliable technique for evaluating retroperitoneal masses, while ultrasound and CT scans can also be helpful. The differential diagnosis includes common benign and malignant neoplasms of neurogenic origin (such as paraganglioma, schwannoma, ganglioneuroma, extra-renal pheochromocytoma), as well as teratoma, desmoid tumor, hemangioma, extra-adrenal angiomyolipoma, sarcoma, lymphoma, and metastatic tumors. Most reported cases of retroperitoneal leiomyomas were not correctly diagnosed preoperatively, with the definitive diagnosis requiring pathological examination. US-guided percutaneous biopsy of the mass is useful for determining its histologic composition preoperatively, though sometimes a complete resection of the mass is necessary for a full histologic examination [16].
In current scientific literature, the only treatment for smooth muscle tumors of the retroperitoneum, whether benign or malignant, is surgical resection [11]. Most of the previous reported cases of retroperitoneal leiomyomata were treated by laparotomy but the laparoscopic approach is also feasible and provides many advantages for the patient, even for obese women. Surgical excision is invasive and requires familiarity and experience with this area. The goals of resection are to rule out any malignancy and offer pain or symptom relief if present [1]. The prognosis is good. Indeed, Poliquin et al. [21] found that surgery was mostly curative with only five reported cases of recurrence out of the 105 cases. Although surgery is currently the only recommended treatment, two cases have reported the use of medical treatment in conjunction with surgery. Kortekaas et al. [13] used ulipristal acetate after a relapse one-year post-surgery. The hypothesis is that ulipristal acetate and more recently relugolix, blocks the progesterone receptors in lesions, thereby restricting their growth. It can be used to reduce the size of fibroids before surgery or as a long-term treatment for patients who are not candidates for surgery. This should be balanced with the potential significant side effects, mostly hepatic. The patient showed multiple nodules in the lungs, omentum, mesenterium and retroperitoneum. With ulipristal acetate treatment (16 month total), the nodules decreased and the patient remained asymptomatic 8 month after the treatment ended. Rodriguez et al. [22] started treatment with aromatase inhibitors following incomplete surgical resection, leading to the patient remaining healthy and the residual tumor stable 23 months later.