Case 2 -

Multiple Gastrointestinal Stromal Tumors, Histologically Benign, Arising in a Patient with Type 1 Neurofibromatosis John R. Goldblum Cleveland Clinic and Cleveland Clinic Lerner College of Medicine Cleveland , Ohio

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Clinical History: 58-year-old male with multiple neurofibromas, including several plexiform neurofibromas, who presented with a 4 cm small intestinal mass. He also appeared to have two small, less than 1 cm masses in the wall of the small bowel several cms away from the main mass. This is a representative section of the 4 cm mass. Case 2 - Slide 1 Click to view with ImageScope Click to view with a Web-Based Viewer Case 2 - Figure 1 - Low-magnification view of the small bowel tumor. The tumor is composed of fairly bland-appearing spindled cells arranged into nests and short intersecting fascicles. Some areas of the tumor showed prominent vascular spaces. Case 2 - Figure 2 - High-magnification view of bland-appearing spindled cells arranged into a vaguely storiform growth pattern. The cells have abundant eosinophilic fibrillar cytoplasm. Cytologic atypia is not conspicuous. Case 2 - Figure 3 - High-magnification view of bland spindled cells with conspicuous collagen globules between the cells. Mitotic figures are not seen. Case 2 - Figure 4 - Separate from the main tumor, there was a grossly inconspicuous but histologically apparent spindle cell proliferation that interdigitated with the surrounding more densely eosinophilic muscle fibers of the muscularis propria. Case 2 - Figure 5 - Higher magnification view of separate spindle cell proliferation interdigitating among the muscle fibers of the muscularis propria. This spindle cell proliferation is histologically similar to that seen in the other figures from the main tumor. Ancillary Findings: Immunohistochemistry - strongly CD117-positive; strongly CD34-positive; focal SMA-positive; desmin negative, S100 protein negative Molecular studies - No evidence of KIT or PDGFRA mutation Diagnosis: Multiple gastrointestinal stromal tumors, histologically benign, arising in a patient with type 1 neurofibromatosis. Discussion Perhaps no other area in surgical pathology has caused as much confusion and controversy as has the subject of gastrointestinal stromal tumors (GIST). These tumors are a heterogeneous group of neoplasms, and prediction of clinical behavior requires a multiparametric evaluation of gross and microscopic features. However, the same criteria for malignancy do not apply to stromal tumors from different sites within the gastrointestinal tract, and the relative importance of each of these features is somewhat controversial. In the past, cellular spindle cell neoplasms of the gastrointestinal tract have often been referred to as "leiomyoma" or "leiomyosarcoma" based upon their purported morphologic resemblance to smooth muscle tumors in other anatomic sites. With the advent of electron microscopy and immunohistochemistry, an enormous number of publications attempted to analyze the characteristics of the constituent cells of these tumors. While a number of studies suggested that these tumors were either derived from or differentiated towards smooth muscle, an equal number of studies suggested neural derivation or differentiation. As discussed below, more recent evidence suggests that the vast majority (if not all) of these tumors are either derived from or differentiate toward interstitial cells of Cajal (ICC). The non-committal term "gastrointestinal stromal tumor" has been adopted to describe this heterogeneous group of neoplasms and is currently the term of preference. Although a wide array of mesenchymal tumors may arise within the gastrointestinal tract, GIST are by far the most common mesenchymal tumors of this site. Molecular Genetic Features of GIST The recent intense interest in GIST stems from the fact that much has been learned about the molecular pathogenesis of this heterogeneous group of tumors, and in fact, this tumor serves as the paradigm for targeted therapy based upon a detectable molecular genetic alteration. Recent studies have established that activating mutations of the KIT gene are present in up to 95% of GIST and likely play a fundamental role in the development of these tumors. [1, 2, 3, 4, 5, 6, 7, 8] Activating mutations of the KIT gene, most commonly occurring in the juxtamembrane domain in exon 11, result in ligand-independent activation of the tyrosine kinase activity, and this event appears to be central in the pathogenesis of these tumors. Much less commonly, mutations in exon 9 (extracellular domain) and exon 13 (kinase domain) can be detected. [2, 3] Mutations in the KIT gene are not found in other tumors that may potentially be confused with GIST, including leiomyosarcomas, fibromatoses and peripheral nerve sheath tumors. The central role of KIT mutations in the pathogenesis of GIST is underscored by the therapeutic success of the KIT inhibitor, imatinib. [9, 10, 11] It has been found that most patients with metastatic GIST show a major clinical response after treatment with this therapeutic agent. In fact, KIT positivity by immunohistochemistry (CD117) has been considered the gold standard for the diagnosis of GIST, and, at least for the initial clinical trials, immunoreactivity for this antigen has been used as an eligibility criterion for Imatinib therapy (discussed further below). Most recently, however, it has become apparent that not all GIST have mutations in the KIT gene or, for that matter, stain for CD117. Rather, activating mutations of the platelet derived growth factor receptor alpha (PDGFRA) gene appears to be an alternative mechanism critical in GIST pathogenesis. [12, 13] In a recent study by Heinrich et al evaluating the genotype of 127 metastatic GIST, [14] 112 cases were found to harbor mutations in the KIT gene (88.2%), including 23 cases with mutations in exon 9 and 85 cases with mutations in exon 11. Six tumors (4.7%) were found to harbor PDGFRA mutations, none of which harbored mutations in the KIT gene, confirming that these are mutually exclusive mutational events. Importantly, patients with exon 11 KIT mutations showed a partial response to imatinib therapy in 83.5%, whereas patients with mutations in exon 9 of the KIT gene and those who harbored no mutations in either KIT or PDGFRA showed a partial response in 47.8% and 0%, respectively, suggesting differences in downstream signaling mechanisms which influence the response to imatinib therapy. Medeiros and colleagues [15] studied 25 GIST-like tumors that did not express KIT by immunohistochemistry (CD117), which accounted for approximately 4% of all of the GIST they evaluated. Of these 25 CD117-negative GIST-like tumors, 4 tumors harbored mutations in the KIT gene (16%), including 3 with mutations of exon 11 and 1 with an exon 9 mutation. Interestingly, the vast majority of CD117-negative tumors (18/25; 72%) had detectable mutations in the PDGFRA gene, most commonly in exon 18 (15 of 18 cases). Only 3 of these tumors did not have a mutation in either KIT or PDGFRA. No tumor had mutations of both KIT and PDGFRA, again confirming that these are mutually exclusive events. This group of CD117-negative GIST more commonly occurred in males, had a predominant epithelioid morphology and more commonly arose in extra-gastrointestinal sites, usually the omentum. Immunohistochemical Features of GIST The literature on the immunohistochemical features of GIST has evolved over the past 20 years. Earlier studies focused on the expression of myoid and neural antigens, and the results of these studies were often conflicting. CD34, a hematopoietic stem cell marker, was found to be expressed in the majority (approximately 70%) of GIST from all sites. [16, 17, 18, 19] However, as previously mentioned, the product of the KIT gene as detected by CD117 is clearly the most sensitive marker of GIST from all sites. [20, 21, 22, 23] Although some authors have equated immunoreactivity for CD117 as definitional of GIST, it is clear that not all GIST express this antigen, as shown in the study by Medeiros et al. [15] Overall, approximately 95% of GIST express this antigen. As previously mentioned, many of these CD117-negative GIST harbor mutations in the PDGFRA gene. Thus, Medeiros and colleagues emphasized that patients with CD117-negative GIST should not necessarily be denied therapy with imatinib. Myoid markers may be found in a subset of GIST. Up to 20-30% of these tumors stain for smooth muscle actin, either focally or diffusely; expression of this antigen seems to be reciprocally related to CD34 expression. [6] Desmin is found in less than 10% of these tumors, and staining is typically limited to scattered tumor cells with more prominent staining in epithelioid GIST. Heavy caldesmon and actin-binding cytoskeleton-associated protein are also detected in a subset of these tumors, supporting at least partial smooth muscle differentiation. [24] Neural antigens may also be detected in some of these tumors; S100 protein is found in up to 10% of GIST, typically with focal immunoreactivity. These tumors are usually negative for neurofilament protein and GFAP. Finally, although scattered tumor cells may stain for cytokeratins, particularly malignant epithelioid GIST, the co-expression of CD117 should prevent confusion with a carcinoma. Prediction of Clinical Behavior of GIST The prediction of clinical behavior of GIST is notoriously difficult. In fact, the recent article entitled, "The diagnosis of gastrointestinal stromal tumors: a consensus approach," published in Human Pathology by an esteemed group of pathologists, [25] states that, "Criteria for distinguishing benign from malignant GISTs, or at least to identify those lesions more likely to metastasize, have been sought, analyzed and disputed for many years. Many parameters have been proposed, but the morphologic features that have gained greatest acceptance as being predictive of outcome are the mitotic rate and tumor size." In this publication, Fletcher and colleagues proposed an easy-to-use approach for defining relative risk of aggressive behavior in GIST, utilizing only tumor size and mitotic counts as parameters. They categorize these tumors as "very low risk," "low risk," "intermediate risk" and "high risk," as noted in the table below. Obviously, these authors went to great lengths to avoid designating a tumor as benign and, conversely, they went to equal lengths to avoid calling any malignant, opting for "high risk." In the opinion of this author, although this approach has its appeal given the relative reproducibility of tumor size and mitotic counts as parameters, it is also believed to be an oversimplification of a complex group of tumors. The major reason for this objection is that this approach groups GIST from all sites together. In fact, if one gathers experience in evaluating GIST from different sites, one quickly comes to the conclusion that these tumors are quite distinct histologically, and numerous clinical studies have shown they are also distinct biologically. In my opinion, this would be akin to grouping gastric adenocarcinomas with colorectal adenocarcinomas in a single study evaluating their clinical behavior, something which presumably would never be published. Although the "Fletcher classification" approach seems reasonable and likely will catch on among pathologists, the problem still remains that these tumors are quite distinct from one another in different parts of the gastrointestinal tract. Thus, for the remainder of this discussion, I will admit my profound bias and provide you with information that I think is important in understanding the histologic and clinical features of this heterogeneous group of tumors, separated by anatomic site. Table 1: Proposed Approach for Defining Risk of Aggressive Behavior in GIST Tumor Size Mitotic Counts Very Low Risk <2 cm <5/50 HPF Low Risk 2-5 cm <5/50 HPF Intermediate Risk <5 cm 6-10/50 HPF 5-10 cm <5/50 HPF High Risk >5 cm >5/50 HPF >10 cm Any mitotic rate >Any size >10/50 HPF From Fletcher CDM, Berman JJ, Corless C, et al. Diagnosis of gastrointestinal stromal tumors: a consensus approach. Hum Pathol 2002;33:459-65. Type 1 Neurofibromatosis (NF1) and GIST The vast majority of GIST are sporadic, but a small percentage arise in patients with NF1. Although patients with NF1 develop a number of different gastrointestinal and abdominal tumors (e.g. neuromas, ampullary carcinoid, pheochromocytoma), GIST are thought to be the most common gastrointestinal neoplasm in these patients. [26] In recent years, the clinicopathologic and molecular genetic features of GIST arising in NF1 have been more fully delineated. [27, 28, 29, 30, 31] Recently, Miettinen and colleagues from the AFIP reported a detailed study of 45 cases of GIST arising in patients with NF1. [27] These cases were culled from a review of 3000 patients with GIST reviewed at the AFIP between 1970 and 1996 (1.6% of GIST). The study included 26 females and 19 males, whose ages ranged from 23 to 74 years (median 49 years). Most patients presented with gastrointestinal bleeding and/or anemia, but some presented with intestinal obstruction, a palpable mass on pelvic examination or as an incidental finding. Most of the tumors arose in the small bowel, but some also occurred in the stomach. Multiple tumors (usually throughout the small bowel) were found in 28 of 42 patients. Tumor size ranged from 0.4 to up to 29 cm (median 4 cm), and most were smaller than 5 cm. Histologically, the vast majority of tumors were composed of bland uniform spindled cells arranged in a vague paraganglioma-like growth pattern. Skeinoid fibers were usually a prominent feature. Most had low mitotic activity (<5 MF/50HPF), lacked necrosis and mucosal invasion. Only four tumors were overtly sarcomatous. Interesting, approximately one-half showed ICC hyperplasia in the adjacent myenteric plexus. By immunohistochemistry, all (31 of 31) stained for KIT (CD117) and most coexpressed CD34 (23/29). SMA was detected in 7 of 31 cases, but none expressed desmin. A minority (7 of 19) showed focal staining for S100 protein. Molecular analysis revealed wild-type KIT exons 9, 11, 13 and 17 and wild-type PDGFRA exons 12 and 18 in all 16 cases tested. Clinical follow-up revealed most patients to be alive without disease (20 of 36 patients). Seven patients died of unrelated causes, 2 died of peri/postoperative complications and 2 were alive with unknown tumor status. However, 5 patients developed metastatic disease and died as a direct result of their tumor. Of these, 3 patients had tumors >5 cm, mitotically active (>5 MF/50HPF), or both. Several other studies have evaluated the molecular features of NF1 related GIST. Kinoshita et al. [32] studied 21 GIST in 7 NF1 patients, and none were found to harbor KIT mutation. However, PDGFRA mutations were not evaluated in this study. In the recent study by Yantiss and colleagues, [28] one of 3 NF1 patients had KIT mutations -- this case revealed identical KIT exon 11 point mutations in 3 separate tumors from the same patient. Takazawa et al. found two different KIT mutations in separate tumors from one patient, and two different PDGFRA mutations in separate tumors from another patient. [31] The other 7 patients had neither KIT nor PDGFRA mutations found in their GIST. Clearly, patients with NF1 have an increased risk of developing GIST. Miettinen et al. estimated an approximately 45-fold increased risk over the general population. [27] NF1 patients developed GIST at a younger age than those with sporadic GIST. They are also more likely to develop tumors arising anywhere in the small bowel when compared to patients with sporadic GIST. There is a slight female predominance in NF1-related GIST, compared to a slight male predominance in sporadic tumors. Most NF1 patients with GIST have multiple tumors, a feature that is exceedingly uncommon in patients with sporadic GIST. Usually, the tumors are small, composed of bland spindled cells that are mitotically inactive and have a favorable clinical outcome. The relatively low frequency of KIT and PDGFRA mutations in NF1-related GIST suggests a different pathogenesis from sporadic GIST. References Hirota S, Isozaki K, Moriyama Y, et al. Gain-of-function mutations of c-kit in human gastrointestinal stromal tumors. Science 1998;279:577-80. Lasota J, Wozniak A, Sarlomo-Rikala M, et al. 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