The term "fibromatosis" covers a broad spectrum of benign fibrous tissue proliferations, the biological behavior of which is similar to both benign fibrous lesions and fibrosarcoma. Like fibrosarcoma, fibromatosis is characterized by infiltrative growth and a tendency towards recurrence; however, unlike sarcoma, it never develops metastasis [1]. Therefore, the most important strategy is to prevent direct invasion into adjacent tissues. Extraabdominal fibromatosis principally originates from the connective tissue of muscles and the overlying fascia or aponeurosis. It may occur in a variety of anatomical locations, including the muscles of the shoulder, the chest wall and back, thigh, and head and neck. However, solitary occurrence is rare in retroperitoneal space [1,2]. Here, we report on a case of extraabdominal fibromatosis in the retroperitoneum. Resection was successfully performed, and the patient has been tumor-free for two years after surgery.

A 43-year-old woman with a history of schizophrenia since 1982, and a history of hospitalization to help the patient to acquire social communication abilities, at the age of 23 presented with slight pain on her left flank and back. In 1995, she was treated with ureterolysis and ureteroureterostomy because of left-sided ureteral obstruction. Histological evaluation of the biopsy revealed benign fibrous tissue proliferations; however, no further evaluation and surgical excision was planned as her mental state was deteriorating. She was put on regular follow-up with computed tomography (CT) scans. In May 2002, she was referred from the psychiatric hospital to our Department of Urology, as the tumor tended to grow. CT scan with contrast enhancement revealed a tumor located between iliocostalis and psoas muscles in retroperitoneal space. The peripheral part of the tumor was enhanced, while the central part did not. The left paravertebral muscles around the tumor showed atrophy. The medial margin of the tumor was deformed by a left transverse process of the second lumber spine, suggesting invasive behavior (Figure (Figure1).1). Coronal magnetic resonance imaging (MRI) demonstrated the tumor to be located beneath the left kidney. The central part of the tumor was found to be of iso/low intensity in the T1-weighted phase and of heterogeneously high intensity in the T2-weighted phase. The marginal part showed very low intensity in both phases (Figure (Figure2).2). In May 2002, needle biopsy was performed and revealed that the tumor consisted of well-proliferated spindle cells rich in collagen fibers, an observation that was inconsistent with the histological pictures made in 1995. In June 2002, resection was performed using a paraspinal approach. Although the tumor strongly adhered to adjacent tissues, including the urinary tract and peritoneum, it was marginally resected, including paravertebral muscles and part of the spine. As the kidney was less affected by the tumor, ablation posed no problem. On gross examination, the cut surface appeared homogeneously gray and glossy (Figure (Figure3).3). Histologically, a uniform proliferation of spindle cells with a moderate amount of collagen fibers led to a diagnosis of extraabdominal fibromatosis in the retroperitoneal space (Figure (Figure4).4). No adjuvant treatment was given and during the two years of follow-up, the patient has remained asymptomatic, with no restrictions of daily living. There were no clues as to recurrence of the tumor in computed tomography.

The authors declare that they have no competing interests.

AKik and AKid performed the operation, are responsible for the clinical work and helped with the preparation of the manuscript. TK is the orthopedic consultant and helped with the preparation and editing of the manuscript. TH coordinated and drafted the manuscript. All authors read and approved the final manuscript.