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Parasitic leiomyoma in the anterior abdominal wall

Leiomyomas are the most common benign genital tract tumours, affecting 20% of women of reproductive age, while extrauterine leiomyomas are extremely rare. This entity of leiomyoma is named parasitic leiomyoma and was first described by Paul E. Steiner from the University of Chicago in 1939 with no more than 200 cases have been documented. It has been proposed that seeding of myomatous tissues during previous gynaecological surgery causes uterine mass to adhere to adjacent organs. Subsequently, it develops its own blood supply and gradually loses attachment from the organ of origin, thus becoming a ‘parasite’ in the new location. A 52-year-old Chinese lady with history of open appendectomy and open myomectomy at the ages of 23 and 35 years presented with an incidental ultrasound (ultrasonography) finding of an intra-abdominal mass during her work-up for deranged liver function test. Ultrasonography reported a 12.3-cm heterogenous hypoechoic solid mass in the right iliac fossa. Computed tomography (CT) scan showed a 11.3 × 7.5 × 13.6 cm soft tissue mass at the right lower abdomen, with mild contrast enhancement at the periphery of the lesion and delayed central enhancement (Fig. 1a,b). The mass displaced the small bowel medially and caecum superiorly. The most likely differential diagnosis was a mesenteric desmoid tumour. She was asymptomatic with no pain or obstructive symptoms. Physical examination showed a 10-cm mobile intra-abdominal globular mass underneath the gridiron scar. Alpha fetoprotein, Ca 125 and carcinoembryonic antigen were all normal. The clinical suspicion was a desmoid tumour originating from the gridiron scar. Midline laparotomy showed a mobile, well-circumscribed and encapsulated tumour at the right iliac fossa arising beneath the gridiron scar, with no attachment to any intra-abdominal organs or mesentery. It had a narrow pedicle with feeding vessels from the peritoneum (Fig. 2). The tumour was excised along its stalk, and the specimen was cut open (Fig. 3). Microscopic examination showed a well-circumscribed smooth-muscle tumour that stained positive for actin and desmin. The final pathology was a leiomyoma. Due to discrepancy between the preoperative and final pathological diagnosis, the radiologist has reviewed the CT and agreed that, retrospectively, the tumour was less likely to originate from the mesentery in the presence of bowel displacement. There was no obvious fat plane between the anterior abdominal wall and the mass, thus making an abdominal wall tumour the more probable diagnosis. However, this would not alter their priority of differentials as abdominal wall desmoid tumours are far more common. Differentiating desmoid tumours from parasitic leiomyomas on CT has been challenging due to their imaging similarities. Most desmoid tumours are well-circumscribed masses, although some may appear more aggressive with illdefined margins. They are relatively homogeneously or focally hyper-attenuating when compared with soft tissue on noncontrast scan. Meanwhile, parasitic leiomyomas are seen as soft tissue density lesions and exhibit coarse peripheral or central calcification. Magnetic resonance imaging would be the imaging of choice by providing specific signal intensity and improving the anatomical relationship with surrounding structures. Parasitic leiomyoma occurs predominantly in women of reproductive age who have undergone previous myomectomy or hysterectomy. This late complication can occur in 0.2–1.2% of myomectomies. Higher risk has been reported in laparoscopic operations compared to open because laparoscopic retrieval of excised leiomyoma is difficult, and morcellation may be required. A