Invasive fibromatosis or ligament-like fibromatosis is the abnormal proliferation of clonal fibroblasts and/or myofibroblasts that occur in deep soft tissues, and has the characteristics of infiltrating growth into surrounding soft tissues and high local recurrence tendency. The etiology of the disease is currently controversial. The literature puts forward three hypotheses : (1) Injury factors, pregnancy, childbirth, abdominal wall muscle fiber injury and bleeding, and hematoma formation provide conditions for tumor occurrence; (2) Endocrine disorders, taking estrogen drugs; (3) Chromosomal abnormalities, that is, the Y chromosome corresponding to chromosome 8 / 20 has a triploid. According to the WHO classification of soft tissue tumors in 2013, it belongs to intermediate local invasive tumors [1–3]. The peak age of onset is between 10 and 40 years old, and there is no obvious gender difference. According to the location of the tumor, it can be divided into three types: abdominal wall fibromatosis, extra-abdominal fibromatosis, intra-abdominal and mesenteric fibromatosis. Extra-abdominal ligamentoid fibromatosis mainly affects the skeletal muscle system of the neck, shoulders, and limbs, and a small number of cases involve the intracranial, thoracic cavity, breast, thyroid, liver, etc. [2–4]. Primary gastric invasive fibromatosis is rare, and the clinical manifestations are upper abdominal discomfort. This study found that the age of onset was 28 to 46 years old, with an average of 37.3 years old. The tumor is located in the submucosa and grows invasively. Key points of histopathological diagnosis: (1) The tumor has no envelope and has the characteristics of strong invasion and destruction of surrounding tissues without causing tissue necrosis; (2) The tumor cell nucleus is elongated, spindle-shaped or rod-shaped, and the cell morphology is relatively consistent, no atypia, mitosis is rare, cell proliferation index is low; (3) The density of tumor cells that invade the smooth muscle tissues of the gastric wall is high, and the density of tumor cells that invade the fat tissues outside the serosal membrane is low; (4) Directional destruction invades smooth muscles and blood vessels and nerve tissue; (5) The tumor tissue is rich in collagen fibers, less inflammatory cell infiltration, and there may be lymphocyte aggregation, about 0.5-1.0 mm in size.
Differential diagnosis: (1) Inflammatory fibrous polyps are interstitial hyperplasia composed of spindle cells, small blood vessels and inflammatory cells, especially eosinophils. The age of onset is older, usually 60 to 75 years old, and it is a sessile polypoid mass with an average diameter of 1.5 cm. Histologically, inflammatory fibrous polyps are composed of loose connective tissue, and are mixed with more inflammatory cells and proliferating thin-walled blood vessels, regional edema or mucus-like background. Inflammatory cells are mainly lymphocytes and eosinophils, and sometimes eosinophils are the main component of inflammation, often surrounding blood vessels. Sometimes proliferating mesenchymal cells surround the small and medium-sized blood vessels, forming a concentric structure. The immunophenotype is positive for vimentin and CD34, focally expressing SMA, and negative for β-catenin and ALK . (2) Gastrointestinal stromal tumor (GIST): GIST is the most common mesenchymal tumor of the gastrointestinal tract, which usually occurs in the elderly, with a median age ranging from 60 to 65 years. Gastric GIST can occur in any part of the stomach, from the smallest mural nodules to large complex masses with intracavity and extracavity. The histology of gastric GIST is diverse, most of which are spindle cell type, a few are epithelioid cell type, and mixed type of spindle cell epithelioid cell, and there are a few special morphology, showing sarcomatoid features, accompanied by a large number of nuclear atypia and mitosis, which are pleomorphic cell type. Most gastric GISTs positively express CD117 and DOG1; some positively express CD34 and S1-00 proteins. The KIT or PDGFRA gene mutation can be detected by molecular method [10, 11]. (3) Invasive fibromatosis in the abdominal cavity involving the stomach: it can be distinguished by the combination of surgical findings, gross specimen examination and imaging examination. Histologically, invasive fibromatosis has the characteristics of directional invasion and destruction of smooth muscle, blood vessels and nerve tissues, which can exclude gastric involvement in abdominal invasive fibromatosis [12, 13]. (4) Gastric plexiform fibromyxoma: it is a rare gastric mesenchymal tumor with unique histological characteristics, which mostly occurs in the antrum of the stomach. Tumors grow in clusters or multiple nodules of varying sizes between the gastric wall, staggered with the smooth muscle of the gastric wall. The tumors are rich in small, thin-walled blood vessels, and the cells are rich in mucin-like or fibromycin-like stroma; the tumor cell nuclei are spindle-shaped or oval, nucleolus is not obvious, cytoplasm is slightly eosinophilic, and cell atypia is not obvious. The immunophenotypic spindle cells mainly express α-SMA and muscle-specific actin (MSA), while β-catenin is negative. Electron microscopy shows that the tumor has myofibroblast differentiation . (5) Schwannoma rarely occurs in the stomach, and its frequency is similar to that of gastric leiomyoma. The tumor often occurs in the elderly, and generally forms a mass of 2 cm to 5 cm in the stomach wall. The clinical manifestations, including general features, are similar to those of GlST. The biological behavior of the tumor is benign and recurrence is rare. Tumor cells are composed of spindle-shaped cells, arranged in a cross-bundle shape, and the nuclei can be arranged in a fence shape. Different amount of collagen fibers can be seen between the cells. Immunophenotypic tumor cells strongly express S-100 protein diffusely, and some cases express CD34; they do not express CD117, DOG1, ALK, β-catenin, desmin, SMA . (6) Inflammatory myofibroblastic tumors: mostly occur in children under 10 years of age, mainly composed of proliferating obese spindle-shaped fibroblasts or myofibroblasts, with a large number of inflammatory cell infiltration in the interstitium, mostly mature plasma cells, lymphocytes and eosinophils, a few are neutrophils, and regional lymphocytes aggregate. In addition to the spindle cells, round-shaped histiocyte-like cells can be seen in the lesions. In some cases, some irregular, polygonal or odd-shaped cells can be seen. Eosinophilic or basophilic inclusions can be seen in the nucleus, similar to ganglion cells or R-S cells. The immunophenotype is positive for vimentin and SMA, and desmin and MSA are focally positive. 50% of the cases are positive for ALK, and CD117. DOG1, β-catenin, and S-100 protein are not expressed . (7) Synovial sarcoma: it is a malignant mesenchymal tumor showing epithelioid differentiation to varying degrees, with characteristic chromosomal translocation t(X;18)(p11;q11) and the resulting SS18-SSX gene fusion. Mainly occurring in the limbs of young people, gastric synovial sarcoma is very rare, and 24 cases have been reported in the English literature . A typical synovial sarcoma is composed of cancer-like epithelioid cells and fibrosarcoma-like spindle cells mixed in different proportions to form a biphasic structure. Gross examination, usually occurs in the gastric body and bottom of the stomach, and occasionally occurs in the gastric antrum or gastroesophageal junction. The tumor is 0.8–16.0 cm in diameter, with multiple or single nodules, uneven surface, and ulcer formation. The section is grayish white, grayish red, fish like, with or without necrosis. Histologically, 22 cases of gastric synovial sarcoma were reported in the literature as monophasic type and 2 cases were biphasic type. Immunohistochemistry shows positive expression of CK and EMA, molecular detection of SYT-SST gene fusion. (8) Myofibroma/myofibromatosis: refers to benign tumors formed by muscle-like cells with contractile function arranged around thin-walled blood vessels. The isolated ones are called myofibromatosis, and the multiple ones are myofibromatosis. It can occur from newborns to the elderly, but many are seen at birth and in infants and children under 2 years of age. Men are more common. It mainly occurs in the head and neck, trunk and limbs, as well as skeletal muscle and aponeurosis. 15–20% occurs in internal organs, such as lung, heart, gastrointestinal tract, liver, kidney, pancreas, etc. Myofibroblasts are spindle-shaped, with pale pink cytoplasm, long spindle-shaped nucleus, vacuolated chromatin, with 1 to 2 small nucleoli, and the cell atypia and pleomorphism are not obvious. Round and polygonal cells with slightly larger nuclei can also be seen. The tumor cells are arranged around the irregularly branched perivascular tumor-like thin-walled blood vessels, intersecting with perihemangioma, and the tumor is an image of hemangiopericytoma. Calcification, hyaline degeneration, necrosis and apoptosis are common. Generally, there are not many lymphocytes and plasma cells. Immunophenotype: positive expression of vimentin and SMA, stronger positive for whole actin HHF35, negative for CK, EMA, ALK, β-catenin, and S-100 protein [18, 19]. (9) Gastric malignant solitary fibrous tumor (SFT): the tumor is a non-fixed tissue structure, and its histological characteristics are the irregular distribution of areas with few tumor cells and areas with rich tumor cells. There are more dense scar-like collagen fiber deposits and branched perivascular tumor-like vascular separation between the two (hemangiopericytoma-like areas). Gastric malignant SFT tumor cells are abundant, and the tumor cells are at least moderately to severely atypia, with tumor tissue necrosis. Lymph node metastasis occurred in gastric malignant SFT. Immunophenotype: tumor cells CD99, CD34, Bcl-2, vimentin positive expression; CKpan, EMA, SMA, S-100 protein, desmin focal weak positive expression, no express of ALK, β-catenin, CD68, CD163, CD21, CD23, CD117, DOG1 . Other tumors that require differential diagnosis include spindle cell hyperplastic nodules, leiomyosarcoma, and epithelioid sarcoma after gastric surgery.
Treatment and prognosis: Invasive fibromatosis is a locally aggressive tumor, and surgical resection is the main method of treatment. Because the disease has a very high recurrence rate, no distant metastasis and low fatality rate, close follow-up is often used in clinical practice. Sometimes the pathology and clinical manifestations are special and must be differentiated from other spindle cell tumors and tumor-like lesions.