If a mass within or adjacent to a cesarean section scar is found in female patients of reproductive age, what is the first consideration? For many doctors, especially gynecologists, abdominal wall endometriosis (AWE) might be the first clinical diagnosis. However, other diseases, such as aggressive fibromatosis, can also occur in similar demographics and locations within the body. Aggressive fibromatosis (AF), also called desmoid-type fibromatosis (DF), is a benign monoclonal fibroblastic proliferation that arises in the deep soft tissues (1). AF is characterized by infiltrative growth and a tendency toward local recurrence but an inability to metastasize (1). AF can be further subdivided into extra-abdominal, abdominal, and intra-abdominal types (2). Both abdominal AF and AWE are inclined to occur in young women with a history of cesarean section and can appear as solitary masses with infiltrative margins, which can render pre-operative clinical diagnosis difficult (2–5). Although there are a few reports in the literature about misdiagnosis of AF as AWE, few of them have discussed the differences between the two diseases in detail. Here, we report two cases of abdominal AF that mimicked AWE and summarize differences between the two entities through literature review in order to guide proper pre-operative diagnosis and foster strong surgical outcome.

A 39-year-old woman presented with an abdominal wall mass for 5 months. This gravida 4 para 2 patient had undergone cesarean section operations 12 and 3 years earlier. The abdominal wall mass increased during menstruation and decreased after menstruation. Physical examination revealed a 6 × 5-cm, fixed and firm abdominal mass on the right side of the scar. Ultrasound showed a 6.0 × 3.2 × 1.7-cm hypoechoic irregular mass in the muscular layer with a fuzzy boundary and hemogeneous internal echo. Color Doppler ultrasonography showed several strip blood flow signals inside with the peak systolic velocity (PSV) of 15.5 cm/s and resistance index (RI) of 0.77.

The clinical findings suggested the mass might be AWE. A wide local excision of the mass was performed. The patient provided written informed consent. At surgery, the lesion was found in the rectus abdominis under the fascia. The lesion was resected along the outer edge of the lesion and did not enter the abdominal cavity. The cut section showed areas of beige color. It was firm and solid. Histopathology confirmed desmoid-type fibromatosis. All margins were negative. Immunohistochemical findings were positive for SMA, β-catenin and CD34 (vessel), whereas desmin, Caldesmon and S-100 were negative. Fewer than 3% of cells were Ki67-positive. The patient is on follow-up with no clinical signs of recurrence after 4 years.

Aggressive fibromatosis (AF) is a distinct rare entity with an incidence of five to six cases per 1 million of the population per annum and a peak age of 30–40 years (2). The exact etiology is not fully understood. Documented etiological factors are surgical trauma, genetic factors (e.g., familial adenomatous polyposis, FAP), hormonal influences, and pregnancy (1, 2). Abdominal AF can occur in any part of the abdominal wall, mainly in the lower abdomen. It usually presents as a painless, solitary, and fixed mass in the deep layer (2). Abdominal AF usually involves muscle or aponeurosis and presents as a single lesion but rarely can be multifocal (6). At MRI, the lesions appear as soft-tissue masses with heterogeneous internal signal on all sequences with avid enhancement, reflecting their proportionate cellular and fibrous contents (2, 6, 7). One hallmark feature is the presence of linear or sheet intra-lesional hypointense bands on T2-weighted images (6, 8). The fascial tail, described as the linear extension of the tumor along the fascial planes, is also a highly suggestive feature (7, 8). Macroscopically, the section of AF mass is beige, swirling, and firm, and it often infiltrates the adjacent muscles and aponeurosis (1). Histologically, AF is characterized by a fibromatous, benign proliferation of well-differentiated fibroblast and tentacle-like spiculated extensions with infiltrative growth (1). Approximately 85–90% of AF has nuclear positivity for ß-catenin, which is helpful in establishing the diagnosis (1).

Surgery is no longer the first-line treatment of AF because of the variable and unpredictable clinical course. Currently, a conservative wait-and-see policy for 1–2 years is the front-line approach to newly diagnosed patients, irrespective of clinical symptoms (1). In cases that progress, anti-hormonal therapy or surgical resection might be an option (1). Pharmacological options include anti-hormonal therapies such as tamoxifen, non-steroidal anti-inflammatory drugs (NSAIDs), and low-dose chemotherapy (1). It is necessary to remove the tumor with a margin of at least 2–3 cm to make sure a negative margin (9).

The clinical course of AF is variable and often unpredictable. Spontaneous regressions are observed in 20–30% of cases (1). The risk of progression during pregnancy is as high as 40–50% (1). The rate of local recurrence rate of abdominal AF after surgery ranges from <10 to 40% (1, 2).

The differences in clinical characteristics are summarized in Table 1. The two aforementioned cases were suspected for AWE because changes in size that correlate with the menstruation cycle are highly suggestive. However, the two patients did not complain of cyclic pain. A diagnosis of AWE should be scrutinized closely if the patient does not complain of cyclic pain. To our knowledge, this is the first report of periodic changes in size of abdominal AF.

In addition to cyclic pain, there are other clinical features for differentiation. The more common position for AWE is the adipose layer, while abdominal AF usually involves muscle or aponeurosis (6, 8). On MRI, the appearances of AF depend on the proportion of cellular and fibrous contents whereas those of AWE depend on lesion chronicity (2, 6, 7). The most common diagnosis of abdominal wall lesions exhibiting a high T1 signal that does not decrease with fat saturation signal is endometriosis (8). The section of the AWE mass is usually yellowish with areas of hemorrhage or chocolate-like micro-cysts (4), unlike AF. Thus, if the typical section is not found upon the removal of AWE, other diseases should be considered. A pathological examination of frozen samples should be made to make sure the proper resection range is used. Occurrence or progression during pregnancy or coexisting FAP are also suggestive of AF.

AF oncogenesis is associated with estrogen hormonal stimulus (2). To determine the causes of the periodic symptoms in our cases, the levels of expressions of estrogen receptors (ERs) and progesterone receptors (PRs) were here verified. The immunohistochemical testing showed negative results for both ER and PR in the two cases (Figure 2).

Several differential diagnoses need to be considered, such as disseminated peritoneal leiomyomatosis (DPL), parasitic leiomyoma, abdominal wall metastase and soft tissue sarcomas. DPL is defined as the presence of multiple peritoneal/sub-peritoneal nodules of various sizes composed of bland smooth muscle cells. In one of the largest cohorts of DPL, 29% of DPL and 60% of malignant DPL had abdominal wall involvement (11). Parasitic leiomyoma is a rare complication of power morcellation following laparoscopic myomectomy or hysterectomy, in which the fragment of myoma may be trapped somewhere along the trocar tract in the abdominal wall (12). Abdominal wall metastasis comprise tumors that reach the abdominal wall by implantation, direct invasion and metastasis. Imaging appearances are usually non-specific, often resembling other sites of primary disease (8). Implantation cancers have been reported to occur in 1.18% of patients with gynecological malignancies after laparoscopy (13). Soft tissue sarcomas usually occur later in life, with a median age at presentation of ~50 years. Fixation to underlying structures is suggestive of a soft-tissue sarcoma (14). Therefore, patients with abdominal wall masses should be referred to specialist centers benefiting from multidisciplinary teams experienced in the management of soft tissue tumors.

AF and AWE are important conditions that should be considered in the differential diagnosis of masses located at cesarean section scars. Cyclic pain is an important differential point. Imaging is a helpful tool for diagnosis but its value is limited. In cases without typical manifestation, fine-needle aspiration cytology can help to prevent mistakes. Furthermore, a multidisciplinary team should be recommended.

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

Written informed consent was obtained from the individual(s) for the publication of any potentially identifiable images or data included in this article.

HS, HL, QF, and JL diagnosed the patients. YW did the immunohistochemical test. XC wrote the manuscript. All authors revised the manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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