Gastrointestinal stromal tumors: an ultrastructural study

Management of Gastrointestinal Stromal Tumors

Gastrointestinal stromal tumors had the reputation for poor outcomes because of their lack of response to nonsurgical interventions. The discovery of gain-of-function mutations involving receptor tyrosine kinase growth factor receptors altered the biological understanding and management. Beginning in 2000, management of these tumors has changed dramatically because of the availability of tyrosine kinase inhibitors. The role of surgery continues to be refined. This article reviews how surgery and systemic therapy are being used, incorporating definitions of risk. Decisions on how to treat a patient is based on the risk of progression, pathologic characteristics, and tumor location.

Carney triad, SDH-deficient tumors, and Sdhb+/- mice share abnormal mitochondria

Carney triad (CTr) describes the association of paragangliomas (PGL), pulmonary chondromas, and gastrointestinal (GI) stromal tumors (GISTs) with a variety of other lesions, including pheochromocytomas and adrenocortical tumors. The gene(s) that cause CTr remain(s) unknown. PGL and GISTs may be caused by loss-of-function mutations in succinate dehydrogenase (SDH) (a condition known as Carney-Stratakis syndrome (CSS)). Mitochondrial structure and function are abnormal in tissues that carry SDH defects, but they have not been studied in CTr. For the present study, we examined mitochondrial structure in human tumors and GI tissue (GIT) of mice with SDH deficiency. Tissues from 16 CTr tumors (n=12), those with isolated GIST (n=1), and those with CSS caused by SDHC (n=1) and SDHD (n=2) mutations were studied by electron microscopy (EM). Samples of GIT from mice with a heterozygous deletion in Sdhb (Sdhb(+) (/-), n=4) were also studied by EM. CTr patients presented with mostly epithelioid GISTs that were characterized by plump cells containing a centrally located, round nucleus and prominent nucleoli; these changes were almost identical to those seen in the GISTs of patients with SDH. In tumor cells from patients, regardless of diagnosis or tumor type, cytoplasm contained an increased number of mitochondria with a 'hypoxic' phenotype: mitochondria were devoid of cristae, exhibited structural abnormalities, and were of variable size. Occasionally, mitochondria were small and round; rarely, they were thin and elongated with tubular cristae. Many mitochondria exhibited amorphous fluffy material with membranous whorls or cystic structures. A similar mitochondrial hypoxic phenotype was seen in Sdhb(+) (/-) mice. We concluded that tissues from SDH-deficient tumors, those from mouse GIT, and those from CTr tumors shared identical abnormalities in mitochondrial structure and other features. Thus, the still-elusive CTr defect(s) is(are) likely to affect mitochondrial function, just like germline SDH-deficiency does.

Primary cilia in gastric gastrointestinal stromal tumours (GISTs): an ultrastructural study

Gastrointestinal stromal tumours (GISTs) are the most common mesenchymal (non-epithelial) neoplasms of the human gastrointestinal (GI) tract. They are thought to derive from interstitial cells of Cajal (ICCs) or an ICC progenitor based on immunophenotypical and ultrastructural similarities. Because ICCs show primary cilium, our hypothesis is based on the possibility that some of these neoplastic cells could also present it. To determine this, an exhaustive ultrastructural study has been developed on four gastric GISTs. Previous studies had demonstrated considerable variability in tumour cells with two dominating phenotypes, spindly and epithelioid. In addition to these two types, we have found another cell type reminiscent of adult ICCs with a voluminous nucleus surrounded by narrow perinuclear cytoplasm with long slender cytoplasmic processes. We have also noted the presence of small undifferentiated cells. In this study, we report for the first time the presence of primary cilia (PCs) in spindle and epithelioid tumour cells, an ultrastructural feature we consider of special interest that has hitherto been ignored in the literature dealing with the ultrastructure of GISTs. We also point out the frequent occurrence of multivesicular bodies (MVBs). The ultrastructural findings described in gastric GISTs in this study appear to be relevant considering the critical roles played by PCs and MVBs recently demonstrated in tumourigenic processes.

Gastrointestinal stromal tumor with structures resembling intracytoplasmic lumina

Gastrointestinal stromal tumor (GIST) is the most common mesenchymal tumor of the gut. It is characterized by positive immunostaining for CD117, and bears mutations in the c-kit or PDGFRA genes. Its origin remains uncertain. GISTs mainly possess primitive smooth muscle or neuronal differentiation. Although an epithelioid pattern of GIST is a common finding on light microscopy, true epithelial differentiation has never been demonstrated by either immunohistochemistry or ultrastructural study. Here the authors report an epithelioid GIST of the stomach, immunopositive for CD117, DOG1.1, CD34, and PDGFRA, with slight cytoplasmic staining for epithelial membrane antigen. One heterozygous mutation on codon 842 of exon 18 of the PDGFRA gene was also found. Ultrastructurally, tumor cells had plentiful organelles, including some membrane-bound, dense-core granules and cytoplasmic vacuoles. Intermingled thin cellular processes were also found. Unusually, there were many structures resembling glandular epithelial intracellular lumina with processes. The processes, although resembling microvilli, did not have filament cores, while the lumina were either empty or contained some dense or flocculent content of uncertain nature. True intracellular lumina are very rare in GIST and the authors present findings related to this issue, with a discussion on their nature, origin, and significance.

Gastrointestinal stromal tumor: an ultrastructural investigation on regional differences with considerations on their histogenesis

Gastrointestinal stromal tumor (GIST) is the most frequent spindle cell tumor in the gastrointestinal tract and may arise from esophagus to rectum. The stomach is the most frequent site, followed by small intestine, rectum, and esophagus. There have been some regional differences reported in their histopathologic and clinical presentations. The purpose of this study is to compare ultrastructural features of GIST, according to its anatomic site, in order to provide additional data to support the current concept of its histogenesis. Fifty-four GISTs (27 from stomach, 23 from small intestine, and 4 from rectum) were included in the study. Histopathologically, gastric GISTs tended to be more frequently epithelioid, particularly those in children, while small intestinal GISTs (SISTs) were mostly spindly in all but three cases. All four of the rectal GISTs were spindly. Ultrastructurally, there seem to be considerable regional differences. In the majority of gastric GISTs, in both epithelioid and spindle types, tumor cells exhibited focal features of myoid differentiation evidenced by the presence of incomplete external lamina (EL) and/or focal accumulations of thin fibers with interrupted electron densities consistent with actin filaments. However, features of myoid differentiation were exceptional for SISTs and rectal GISTs, being present in only one example in each. Some gastric GISTs, particularly those having an epithelioid appearance, showed cell borders luxuriously decorated by long filopods (anemone cell features). Anemone cell features were also present in spindle cell types of gastric GISTs as well as SISTs, albeit it was simpler and less luxuriant. Skeinoid fibers were present in the majority of SISTs and rectal GISTs, but absent in all gastric GISTs except one. These differences appeared to be too significant to propose a uniform histogenesis for all GISTs. Nevertheless, on closer analysis, certain features could be identified to explain a line of differentiation in all GISTs ranging from (1) polygonal uncommitted epithelioid mesenchymal cells with cell borders decorated by luxuriant fimbria, to (2) spindly tumor cells with less prominent fimbria, or (3) cells with or without features of minimum myoid differentiation characterized by the focal presence of cytoplasmic actin fibers or incomplete EL or skeinoid fibers, which might represent an altered product of EL protein. These findings led the author to speculate that the probable primordial cells of GIST may be the primitive mesenchymal cells, which have the potential to differentiate into myoid cells. In this regard, it is important to note that the putative primordial cell of GIST, interstitial cells of Cajal (ICC), and intestinal smooth muscle cells have been shown to develop from the common progenitor cells of the primitive gut, and c-Kit plays a crucial role in the determination of their fate to differentiate to muscle cells or ICC. The author concludes that all GISTs derive from stem cells in the gut retaining some of the differentiation potential seen in primitive gut cells. One of the likely candidates for such cells in the intestinal musculature is ICC-DMP (interstitial cells of Cajal associated with deep muscular plexus) identified as ICC having smooth muscle features identified exclusively by electron microscopy. These cells have been shown to have some of the features of muscle cells by the presence of external lamina and less well-organized cytoplasmic filaments; they also express CD117 in the cytoplasm. Furthermore, recent studies demonstrated the presence of so-called progenitor cells of ICC, similar to ICC-DMP in appearance, expressing insulin-like growth factor and CD34, indicating their stem cell nature. The author proposes that all GISTs develop from the common progenitor cells similar to primitive gut cells, which may differentiate into tumor cells with more myoid features in the stomach (similar to so-called ICC-DMP) as well as spindle cells with less myoid features (similar to ICC-MP [interstitial cells of Cajal associated with the myenteric plexus] in the small intestine and rectum). ICC-DMP have been recruited in the group of ICC by electron microscopic technique alone without methylene blue stain and it is questionable whether they are part of ICC depicted by the ICC network originally shown by Dr. Cajal more than century ago. Recent discovery of their expression of insulin-like growth factors may indicate that they represent persisting primitive gut cells (gut stem cells), which may serve as the progenitor cells to GIST. It is also pointed out that in this era of ICC and GIST pandemonium, a minority of intestinal stromal tumors with mature smooth muscle features have been totally ignored; these now appear to belong to GISTs, representing the best differentiated example among the tumors developing from the same progenitor cells.

Advances in pathological diagnosis of gastrointestinal stromal tumors

Gastrointestinal stromal tumors (GIST) are the most common mesenchymal tumors of the gastrointestinal tract. The pathogenesis of GIST may be associated with the mutations of oncogenic c-kit or platelet-derived growth factor receptor alpha (PDGFRA). The pathological diagnosis of GIST needs a combined approach of macropathology, histopathology, immunohistochemistry and gene testing. Preoperative diagnosis is very important for patients with GIST, especially for those needing individualized treatment. In this article, we will review the recent advances in pathological diagnosis of GIST and the consensus criteria for risk stratification of GIST. Besides, we will also summarize the molecular biological parameters used for evaluation of the biological behavior of GIST.

Duodenal epithelioid gastrointestinal stromal tumor with prominent granular cell features

Gastrointestinal stromal tumors are a group of mesenchymal tumors arising from the wall of the gastrointestinal tract that are characterized by activating mutations in KIT or PDGFRA. Their proper recognition is important clinically because of their potential responsiveness to targeted therapies. We report a case of duodenal gastrointestinal stromal tumor with a highly unusual epithelioid morphology that had an appearance reminiscent of a steroid producing neoplasm, such as an adrenal cortical neoplasm or, alternatively, a renal cell carcinoma variant. The recognition of the current tumor as a duodenal gastrointestinal stromal tumor was prompted by its apparent location in the duodenal wall. Ancillary immunohistochemical and molecular sequence analyses were necessary to confirm the diagnosis as a gastrointestinal stromal tumor. The current case illustrates the importance of considering gastrointestinal stromal tumor in the differential diagnosis of any epithelioid tumors in the gastrointestinal tract or the abdominal-pelvic cavity.

Gastrointestinal stromal tumors: pathology and prognosis at different sites

Gastrointestinal (GI) stromal tumors (GISTs) are the most common mesenchymal tumors specific to the GI tract, generally defined as KIT (CD117)-positive tumors with a characteristic set of histologic features. These tumors, derived from Cajal cells or their precursors, most commonly occur at the age >50 years in the stomach (60%), jejunum and ileum (30%), duodenum (4-5%), rectum (4%), colon and appendix (1-2%), and esophagus (<1%), and rarely as apparent primary extragastrointestinal tumors in the vicinity of stomach or intestines. Their overall incidence has been estimated as 10 to 20 per million, including incidental minimal tumors. GISTs are rare in children (<1%) and almost exclusively occur in stomach. They are common in patients with neurofibromatosis 1, who have a predisposition to (multiple) small intestinal GISTs. GISTs contain a spectrum from minute indolent tumors to sarcomas at all sites of occurrence. Their gross patterns are diverse, including nodular, cystic, and diverticular tumors. External involvement of pancreas and liver can simulate primary tumor in these organs. In general, gastric tumors have a more favorable prognosis than the intestinal ones with similar parameters. Gastric GISTs < or =10 cm and < or =5 mitoses per 50 HPFs have a low risk for metastasis, whereas those with >5 per 50 HPFs and >5 cm in diameter have a high risk for metastasis. In contrast, all intestinal GISTs >5 cm independent of mitotic rate have at least moderate risk for metastases, and all >5 mitoses per 50 HPFs have a high risk for metastases. Intestinal GISTs < or =5 cm with < or =5 mitoses per 50 HPFs have a low risk for metastases. Gastric GISTs can be divided into histologic subgroups including 4 spindle cell and 4 epithelioid variants. Intestinal GISTs are a histologically more homogeneous group and often contain distinctive extracellular collagen globules, skeinoid fibers. Immunohistochemical demonstration of KIT, CD34, or protein kinase theta positivity helps to properly identify these tumors.

Comparative ultrastructural analysis and KIT/PDGFRA genotype in 125 gastrointestinal stromal tumors

GISTs are the most common mesenchymal neoplasms of the digestive tract and are thought to originate from or differentiate toward the interstitial cell of Cajal lineage. Almost all GISTs express KIT protein and the majority show activating mutations in either KIT or PDGFRA proto-oncogenes. Ultrastructurally, these tumors have been shown to have either a smooth muscle, neuronal, dual, or null phenotype. The objective of this study was to investigate the relationship between ultrastructural features and genotype in a large series of 125 histologically confirmed and CD117 positive GISTs. PCR analysis for the presence of KIT exon 9, 11, 13, and 17 and PDGFRA exon 12 and 18 mutations was performed. There were 62 (50%) tumors located in the stomach and 45 (36%) in the small bowel. Overall, KIT mutations were detected in 93 (75%) patients: 86 (69%) in exon 11, and 7 (6%) in exon 9. A PDGFRA mutation was detected in 7 (6%) cases and 25 (19%) cases had no mutation. Ultrastructurally, skeinoid fibers were seen in 55 (44%) cases and were more common in small bowel than stomach GISTs, and occurred in only in 1 of 16 patients with an ITD (KIT) exon 11 or PDGFRA mutation. Focal actin microfilaments were identified in 82 (65%) cases and did not correlate with location or mutation type. Rare neurosecretory-type granules (NS-G) were seen in 34 (27%) of cases, but were seen in most of the cells in only 5 (4%) cases. GISTs showing both NS-G and microtubules were associated with KIT exon 11 genotype and spindle cell morphology. PDGFRA mutated cases were associated with gastric location, predominantly epithelioid morphology and lacked NS-G.

Gastrointestinal stromal tumors: review on morphology, molecular pathology, prognosis, and differential diagnosis

CONTEXT

Gastrointestinal stromal tumors (GISTs) are specific, generally Kit (CD117)-positive, mesenchymal tumors of the gastrointestinal tract encompassing a majority of tumors previously considered gastrointestinal smooth muscle tumors. They are believed to originate from interstitial cells of Cajal or related stem cells.

OBJECTIVE

To review current clinicopathologically relevant information on GIST.

DATA SOURCES

Literature in Medline and authors' own experience.

CONCLUSIONS

GISTs usually occur in older adults (median age 55-60 years) and rarely in children in the second decade (<1%) throughout the gastrointestinal tract: 60% in stomach, 35% in small intestine, and less than 5% in rectum, esophagus, omentum, and mesentery; most GISTs in the latter 2 sites are metastatic. Five percent of GISTs occur in patients with neurofibromatosis type 1 syndrome (multiple small intestinal tumors) and in Carney triad (gastric epithelioid GISTs in young females). Familial GISTs occur in patients with inheritable germline Kit or platelet-derived growth factor receptor alpha (PDGFRA) mutations. Histologically GISTs vary from spindle cell tumors to epithelioid and pleomorphic tumors. Most GISTs (95%) express Kit (CD117), CD34 (70%), and heavy caldesmon (80%), whereas 25% are positive for smooth muscle actin and less than 5% for desmin. Tumor size and mitotic activity are best predictive prognostic features; small intestinal tumors behave more aggressively than gastric tumors with similar parameters. Mutually exclusive gain-of-function Kit or PDGFRA mutations occur in a majority of GISTs representing in-frame deletions, point mutations, duplications and insertions. Mutations in Kit juxtamembrane domain (exon 11) are the most common in GISTs of all sites, whereas rare Kit extracellular domain (exon 9) Ala502-Tyr503 duplication is specific for intestinal GISTs. Mutations in PDGFRA have been identified in juxtamembrane (exon 12) and tyrosine kinase domains (exons 14 and 18), nearly exclusively in gastric GISTs, mostly in epithelioid variants. Some Kit and PDGFRA mutations have a prognostic value. Kit/PDGFRA tyrosine kinase inhibitor imatinib has been successfully used in the treatment of metastatic GISTs for more than 5 years. However, primary and acquired secondary resistance linked to certain types of Kit and PDGFRA mutations is limiting long-term success necessitating the use of alternative treatments.

Interstitial cells of Cajal (ICC) and gastrointestinal stromal tumor (GIST): facts, speculations, and myths

Interstitial cells of Cajal (ICC) is a peculiar cell network composed of cells having processes described by the eminent Spanish neuroanatomist of the 19th century, S. Ramon y Cajal. ICC became a fascinating subject to many investigators and it is estimated that there are over 100 publications yearly on the subject related to ICC, in the last three years. Now it is widely accepted that ICC are pace maker cells of the gut and probable progenitor cells of gastrointestinal stromal tumors (GIST). Lately, interstitial Cajal-like cells (ICLC) are being found in various organs and their physiological role is still to be defined. We have reviewed the literature trying to evaluate the validity of the current concept and found that there are a few salient points to be considered. 1) There has been some important departure in defining the identity of ICC from the original criteria of Cajal. In particular, ICC with myoid feafures in intestinal smooth muscle layers (ICC-DPM) do not seem to fit to the original description of interstitial cell network by Cajal. We have also pointed out that the current reports assigning a pace maker role to ICC vastly depend on the scientific data on "ICC with myoid features", not on "fibroblast-like ICC", which are more abundant and easier to identify. 2) There seem to be an overwhelming amount of data proving the relationship between ICC and GIST. Both are known to express c-Kit and the ultrastructural characteristics seen in GIST roughly parallel those of ICC including minimal myoid differentiation seen in the majority of GIST, supporting the current concept that GIST are ICC tumors. 3) According to the original description of Cajal, ICC was not limited to the gut, suggesting an existence of ICC in other organs. The list of organs reported to contain ICC (currently identified by immunohistochemistry and electron microscopy) is ever growing and further studies are needed to define their identity and pathophysiologic role. 4). Recent data concerning gut development suggest that both c-Kit expressing ICC (fibroblasts-like as well as muscle-like) and gut muscle cells derive from the common progenitor cells of the embryonic gut unifying the histogenetic concept of all GIST with heterogeneous cytomorphologic features. In this review we attempted to incorporate recent information on interstitial Cajal-like cells (ICLC) found in other organs to broaden our understanding of ICC in general in terms of their ultrastructure, physiology, and neoplasia.

Gastrointestinal stromal tumours: an update

Recently, there has been intense interest in the study of gastrointestinal stromal tumour (GIST); one might call it a virtual GIST revolution. This is due largely to the realization that most GISTs express KIT and harbour activating c-KIT (KIT) or platelet-derived growth factor receptor-alpha (PDGFRA) receptor tyrosine kinase mutations that can be targeted by small molecule pharmacological inhibitors. Pathologists have benefited greatly from this revolution, mainly in the form of an improved ability to classify GISTs and, even more recently, in understanding the molecular underpinnings that underlie many fascinating clinical and pathological correlations. It is the purpose of this review to summarize recent developments in GIST classification and the molecular pathogenesis of GIST.

Reactive nodular fibrous pseudotumors of the gastrointestinal tract: report of 8 cases

Eight cases of reactive nodular fibrous pseudotumor of the gastrointestinal tract are presented. The patients included 6 males and 2 females between the ages of 1 and 68 years (mean age 41.5 years). Three tumors involved the small intestine, and 5 of the investigated lesions were located in the large bowel. Of these, 2 originated in the sigmoid colon, 1 in the cecum, 1 in the appendix, and 1 in the large bowel not otherwise specified. The tumors' size varied from 3 to 10 cm in the greatest diameter (mean 6.2 cm). Histologically they were composed of stellate or spindle shaped cells resembling fibroblasts arranged haphazardly or in intersecting fascicles, embedded in a collagen-rich stroma, with sparse intralesional mononuclear cells frequently arranged in lymphoid aggregates. Immunohistochemically, the lesions were positive for vimentin (7/7), smooth muscle actin (8/8), muscle-specific actin (5/7), cytokeratins AE1/AE3 (6/7), and CAM 5.2 (1/7), and antigen CD68 (1/7). No case (0/8) reacted positively with antibody to CD117 (c-kit). Genetically no substitutions, deletions, or insertions occurred in exon 11 in all analyzed samples. Likewise, no deletions or insertions in part of exon 9 were observed. Ultrastructurally the tumor cells revealed features typical of myofibroblasts. According to the morphologic, immunohistochemical, and ultrastructural features mentioned above, especially to the positivity of low-molecular-weight cytokeratins, we propose this lesion to be related to a proliferation of multipotential subserosal cells rather than ordinary myofibroblasts or fibroblasts.

Ultrastructural studies of gastrointestinal stromal tumors

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumors in the gastrointestinal tract (GIT). Although interstitial cells of Cajal has been suggested as origin of this tumor, the cytological and ultrastructural features of GISTs are heterogeneous and unclear. A total 10 cases of normal gastrointestinal tissue (control), 13 GISTs of the stomach (8), small intestine (3), mesocolon (1) and liver (1), and 2 gastrointestinal autonomic nervous tumor (GANT) of small intestine were ultrastructurally studied. Normal interstitial cells of Cajal (ICC) were abundantly present around the myenteric plexuses or individually scattered through the wall of GIT. ICC was characterized by slender cytoplasmic processes, well-developed endoplasmic reticulum (ER), mitochondria, Golgi apparatus, caveolae and intermediate filaments. The GISTs and GANTs had overlapping ultrastructures. The most common and important ultrastructural features of GISTs were rich villous cytoplasmic processes, dispersed intermediate filaments and abundant SER, and those of GANTs were neurosecretory granules and skenoid fibers. Compared with ICC, the GISTs and GANTs had remarkably reduced caveolae and gap junctions. Our study suggested that ultrastructural analysis gives much information to investigate lineage differentiation of neoplastic cells and make a differential diagnosis of these tumors from other mesenchymal tumors and between GISTs and GANTs.

Undifferentiated mesenchymal neoplasm of the esophagus in a child: case report and comparison with gastrointestinal stromal tumor

Mesenchymal neoplasms of the esophagus are relatively uncommon in adults and exceedingly rare in children. Childhood tumors consist almost exclusively of smooth muscle tumors (leiomyomas). We report a case of an undifferentiated mesenchymal neoplasm occurring in the distal esophagus of a 15-year-old boy which is not a gastrointestinal stromal tumor. To our knowledge, this is the first reported case of such a neoplasm occurring in the esophagus of either an adult or child.

Reactive nodular fibrous pseudotumor of the gastrointestinal tract and mesentery: a clinicopathologic study of five cases

Although the majority of mesenchymal lesions of the gastrointestinal tract are neoplastic in nature, nonneoplastic reactive processes may involve the gastrointestinal tract and mesentery, causing diagnostic confusion with more aggressive neoplasms, such as fibromatosis or gastrointestinal stromal tumors. In this study, we report a series of fibroinflammatory lesions of the gastrointestinal tract that we think represent a relatively cohesive group of tumors and describe the clinical and pathologic features of this entity, which we have termed "reactive nodular fibrous pseudotumor." The tumors affected five patients (four male and one female patient) who ranged in age from 48 to 71 years (mean 56 years). Two patients presented with acute abdominal pain without a significant past medical history, two had incidental lesions discovered during evaluation for other medical conditions, and one was found to have an abdominal mass. Three patients had a history of abdominal surgery. The tumors were multiple in three patients and solitary in two patients. In four cases, at least one of the tumors involved the small intestine or colon, and the lesion was confined to the peripancreatic soft tissue in one case. The tumors were firm, tan-white, ranged in size from 4.3 to 6.5 cm in greatest dimension, and were grossly well circumscribed. All of the lesions were of low to moderate cellularity and composed of stellate or spindled fibroblasts arranged haphazardly or in intersecting fascicles. Three cases had microscopically infiltrative borders. The stroma was rich in collagen, which was wire-like, keloidal, or hyalinized. Intralesional mononuclear cells were sparse but were more numerous peripherally and frequently arranged in lymphoid aggregates. Immunohistochemical stains demonstrated that all of the tumors stained for vimentin, 80% stained for CD117 or muscle specific actin, 60% stained for smooth muscle actin or desmin, and none of the tumors stained for CD34, S-100 protein, or anaplastic lymphoma kinase-1. Follow-up information was available in all cases: four patients had no residual disease following surgical resection (mean follow-up 16.3 months) and one patient who had an incomplete surgical resection had stable disease at 26 months. In summary, we report a series of distinct intraabdominal fibroinflammatory pseudotumors that we have collectively termed "reactive nodular fibrous pseudotumors." These lesions are uncommon and may infiltrate the bowel wall, thereby mimicking primary bowel neoplasms or intraabdominal fibromatosis. Recognition of these nonneoplastic lesions is important, as they pursue a benign clinical course, but may be confused with other mesenchymal neoplasms that require more aggressive treatment.